Uninterruptible Power Supply

Uninterruptible power supply (UPS)

An uninterruptible power supply (UPS) is a device that allows a computer to keep running for at least a short time when the primary power source is lost. It also provides protection from power surges.

A UPS contains a battery that "kicks in" when the device senses a loss of power from the primary source. If you are using the computer when the UPS notifies you of the power loss, you have time to save any data you are working on and exit gracefully before the secondary power source (the battery) runs out. When all power runs out, any data in your computer's random access memory (RAM) is erased. When power surges occur, a UPS intercepts the surge so that it doesn't damage the computer.

UPS in the data center

Every UPS converts incoming AC to DC through a rectifier, and converts it back with an inverter. Batteries or flywheels store energy to use in a utility failure. A bypass circuit routes power around the rectifier and inverter, running the IT load on incoming utility or generator power.

While UPS systems are commonly called double-conversion, line-interactive and standby designs, these terms have been used inconsistently and manufacturers implement them differently: At least one system allows any of the three modes. The International ElectroTechnical Commission (IEC) adopted more technically descriptive terminology in IEC Std. 62040.

Voltage and frequency independent (VFI) UPS systems are called dual or double conversion because incoming AC is rectified to DC to keep batteries charged and drive the inverter. The inverter re-creates steady AC power to run the IT equipment.

When power fails the batteries drive the inverter, which continues to run the information technology (IT) load. When power is restored, either from the utility or a generator, the rectifier delivers direct current (DC) to the inverter and simultaneously recharges the batteries. The inverter runs full time. Utility input is completely isolated from the output, and bypass is only used for maintenance safety or if there's an internal electronics failure. Since there is no break in the power delivered to the IT equipment, vacuum fault interrupter (VFI) is generally considered the most robust form of UPS. Most systems synchronize the output frequency with the input, but that's not necessary, so it still qualifies as frequency independent.

Every power conversion incurs a loss, so the wasted energy has historically been considered the price of ultimate reliability. The newest VFI systems claim better than 96% efficiency at nearly all loads. 

Voltage independent (VI), or true line interactive UPSes have a controlled output voltage, but the same output frequency as the input. Frequency independence is rarely a concern with power in developed countries. Utility power feeds directly to the output and IT equipment, and the rectifier keeps the batteries charged. The inverter is paralleled with the output, compensating for voltage dips and acting as an active filter for voltage spikes and harmonics. Rectifier and inverter losses only occur when incoming power fluctuates. Flywheels and motor/generator sets also qualify as VI.

When incoming power fails, or the voltage goes out of range, the bypass quickly disconnects from the input and the battery drives the inverter. When input power is restored, the bypass re-engages the input, re-charges the batteries, and keeps output voltage constant. UPS vendors who use paralleled power sources claim no loss of reliability. The result is around 98% energy efficiency.

Voltage and frequency dependent (VFD), or standby UPS, is operationally similar to VI and is sometimes mistakenly called line interactive. In conventional VFD systems the inverter is turned off, so it can take as long as 10 to 12 milliseconds (ms) to start creating power. That break can crash servers, making legacy VFD UPSes a bad fit for data centers.

New VFD concepts have the inverter producing power within 2 ms after being activated. The bypass is normally engaged, just as with VI, so equipment operates directly from the utility or generator. Since the inverter isn't working until power fails, there is no voltage control or power consumed, enabling efficiencies as high as 99%. Power failure or voltage outside of range opens the bypass switch, disengaging input from output; the inverter starts operating from the batteries. The rectifier is only large enough to keep the batteries charged.

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INDUSTRIAL UPS AND DC POWER SYSTEM

INDUSTRIAL UPS AND DC POWER SYSTEM

I. Basic of AC/DC UPS Solution

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General principles of measuring current and voltage

General principles of measuring

Current and voltage

Contents:

1. General principles of measuring current and voltage
1.1 Instrument transformers
1.2 Current transformers operating principles
1.2.1 Measuring errors
1.2.2 Calculation of errors
1.2.3 Variation of errors with current
1.2.4 Saturation factor
1.2.5 Core dimensions
1.3 Voltage transformers operating principles
1.3.1 Measuring errors
1.3.2 Determination of errors
1.3.3 Calculation of the short-circuit impedance Zk
1.3.4 Variation of errors with voltage
1.3.5 Winding dimensions
1.3.6 Accuracy and burden capability

1.1 Instrument Transformers
The main tasks of instrument transformers are:
• To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments.
• To insulate the metering circuit from the primary high voltage system.
• To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages.

Instrument transformers are special types of transformers intended to measure currents and voltages. The common laws for transformers are valid.

Current transformers
For a short-circuited transformer the following is valid:

I₁/I₂=N₂/N₁

This equation gives current transformation in proportion to the primary and secondary turns.
A current transformer is ideally a short-circuited transformer where the secondary terminal voltage is zero and the magnetizing current is negligible.


Voltage transformers
For a transformer in no load the following is valid:

E₁/E₂=N₁/N₂

This equation gives voltage transformation in proportion to the primary and secondary turns.
A voltage transformer is ideally a transformer under no-load conditions where the load current is zero and the voltage drop is only caused by the magnetizing current and is thus negligible.            


1.2 Current transformers operating principles
A current transformer is, in many respects, different from other transformers. The primary is connected in series with the network, which means that the primary and secondary currents are stiff and completely unaffected by the secondary burden. The currents are the prime quantities and the voltage drops are only of interest regarding exciting current and measuring cores.

1.2.1 Measuring errors

If the exciting current could be neglected the transformer should reproduce the primary current without errors and the following equation should apply to the primary and secondary currents:

I_s = (N_p/N_s). I_p

In reality, however, it is not possible to neglect the exciting current.
Figure 1.2 shows a simplified equivalent current transformer diagram converted to the secondary side.

The diagram shows that not all the primary current passes through the secondary circuit. Part of it is consumed by the core, which means that the primary current is not reproduced exactly. The relation between the currents will in this case be:

I_s = (N_p/N_s). I_p - I_e

The error in the reproduction will appear both in amplitude and phase. The error in amplitude is called current or ratio error and the error in phase is called phase error or phase displacement.

Figure 1.3 shows a vector representation of the three currents in the equivalent diagram.
Figure 1.4 shows the area within the dashed lines on an enlarged scale.

In Figure 1.4 the secondary current has been chosen as the reference vector and given the dimension of 100%. Moreover, a system of coordinates with the axles divided into percent has been constructed with the origin of coordinates on the top of the reference vector. Since δ is a very small angle, the current error ε and the phase error δ could be directly read in percent on the axis ( δ = 1% = 1 centiradian = 34.4 minutes).

According to the definition, the current error is positive if the secondary current is too high, and the phase error is positive if the secondary current is leading the primary. Consequently, in Figure 1.4, the positive direction will be downwards on the ε axis and to the right on the δ axis.


1.2.2 Calculation of errors

The equivalent diagram in Figure 1.5 comprises all quantities necessary for error calculations. The primary internal voltage drop does not affect the exciting current, and the errors - and therefore the primary internal impedance - are not indicated in the diagram. The secondary internal impedance, however, must be taken into account, but only the winding resistance Ri. The leakage reactance is negligible where continuous ring cores and uniformly distributed secondary windings are concerned. The exiting impedance is represented by an inductive reactance in parallel with a resistance. Iµ and If are the reactive and loss components of the exiting current.

The error calculation is performed in the following four steps:
1. The secondary induced voltage Esi can be calculated from

Esi = Is * Z [V]

where
Z = the total secondary impedance

Z = sqr [(R_i + R_b)² +Xb²]

2. The inductive flux density necessary for inducing the voltage Esi can be calculated from

B = Esi / (pi * sqr (2) * f * A_j * N_s)

3. The exciting current, Iµ and If, necessary for producing the magnetic flux B. The magnetic data for the core material in question must be known. This information is obtained from an exciting curve showing the flux density in Gauss versus the magnetizing force H in ampere-turns/cm core length.

Both the reactive component Hµ and the loss component Hf must be given.
When H_µ and H_f are obtained from the curve, Iµ and If can be calculated from:

I_µ = H_µ * (L_j/N_s) [A]

I_f = H_f * (L_j/N_s) [A]

where magnetic path
L_j = length in cm
Ns = number of secondary turns
4. The vector diagram in Figure 1.4 is used for determining the errors. The vectors I_m and I_f, expressed as a percent of the secondary current Is, are constructed in the diagram shown in Figure 1.6. The directions of the two vectors are given by the phase angle between the induced voltage vector Esi and the reference vector Is.

The reactive component Iµ is 90 degrees out of phase with Esi and the loss component I_f is in phase with E_si.

1.2.3 Variation of errors with current
If the errors are calculated at two different currents and with the same burden it will appear that the errors are different for the two currents. The reason for this is the non-linear characteristic of the exciting curve. If a linear characteristic had been supposed, the errors would have remained constant. This is illustrated in Figure 1.7 and Figure 1.8. The dashed lines apply to the linear case.

Figure 1.8 shows that the error decreases when the current increases. This goes on until the current and the flux have reached a value (point 3) where the core starts to saturate. A further increase of current will result in a rapid increase of the error. At a certain current Ips (4) the error reaches a limit stated in the current transformer standards.

1.2.4 Saturation factor
Ips is called the instrument security current for a measuring transformer and accuracy limit current for a protective transformer. The ratio of Ips to the rated primary current Ipn is called the Instrument Security Factor (FS) and Accuracy Limit Factor (ALF) for the measuring transformer and the protective transformer respectively. These two saturation factors are practically the same, even if they are determined with different error limits.

If the primary current increases from Ipn to Ips, the induced voltage and the fluxm increase at
approximately the same proportion.

Because of the flat shape of the excitation curve in the saturated region, Bs could be looked upon as approximately constant and independent of the burden magnitude. Bn, however, is directly proportional to the burden impedance, which means that the formula above could be written

(FS)ALF ~ 1/B_n ~1/Z

The formula states that the saturation factor depends on the magnitude of the burden. This factor must therefore always be related to a certain burden. If the rated saturation factor (the saturation factor at rated burden) is given, the saturation factor for other burdens can be roughly estimated from:

ALF ~ ALF_n * (Z_n/Z)

where
ALFn = rated saturation factor
Zn = rated burden including secondary winding resistance
Z = actual burden including secondary winding resistance


1.2.5 Core dimensions
Designing a core for certain requirements is always a matter of determining the core area. Factors, which must be taken into account in this respect, are:
• Rated primary current (number of ampere-turns)
• Rated burden
• Secondary winding resistance
• Accuracy class
• Rated saturation factor
• Magnetic path length
The procedure when calculating a core with respect to accuracy is in principle as follows:

A core area is chosen. The errors are calculated within the relevant burden and current ranges. If the calculated errors are too big, the core area must be increased and a new calculation must be performed. This continues until the errors are within the limits. If the errors in the first calculation had been too small the core area would have had to be decreased.

The procedure when calculating a core with respect to a certain saturation factor, ALF, is much simpler:
The core area can be estimated from the following formula:

Aj ~ (K*ALF * I_sn*Z_n)/N_s

where
K = constant which depends on the core material (for cold rolled oriented steel K~25)
Isn = rated secondary current
Zn = rated burden including the secondary winding resistance.

NOTE! It is important for low ampere turns that the accuracy is controlled according to the class.

1.3 Voltage transformers operating principles
The following short introduction to voltage transformers concerns magnetic (inductive) voltage transformers. The content is, however, in general also applicable to capacitor voltage transformers as far as accuracy and measuring errors are concerned.

1.3.1 Measuring errors

If the voltage drops could be neglected, the transformer should reproduce the primary voltage without errors and the following equation should apply to the primary and secondary voltages:

Us = (Ns/Np) *Up

In reality, however, it is not possible to neglect the voltage drops in the winding resistances and the leakage reactances. The primary voltage is therefore not reproduced exactly. The equation between the voltages will in this case be:

Us = (Ns/Np)*Up –∆U

where
∆U = voltage drop

The error in the reproduction will appear both in amplitude and phase. The error in amplitude is called voltage error or ratio error, and the error in phase is called phase error or phase displacement.

Figure 1.10 shows a vector representation of the three voltages. Figure 1.11 shows the area within the dashed lines on an enlarged scale. In Figure 1.11 the secondary voltage has been chosen as the reference vector and given the dimension of 100%. Moreover a system of coordinates with the axis divided into percent has been created with origin of coordinates on the top of the reference vector. Since δ is a very small angle the voltage error ε and the phase error δ could be directly read in percent on the axis (ε = 1% = 1 centiradian = 34.4 minutes).
According to the definition, the voltage error is positive if the secondary voltage is too high, and the phase error is positive if the secondary voltage is leading the primary. Consequently, the positive direction will be downwards on the e axis and to the right on the δ axis.

1.3.2 Determination of errors

Figure 1.12 shows an equivalent voltage transformer diagram converted to the secondary side.
The impedance Zp represents the resistance and leakage reactance of the primary, Zs represents the corresponding quantities of the secondary. It is practical to look upon the total voltage drop as the sum of a no-load voltage drop caused by Is. The diagram in Figure 1.12 is therefore divided into a no-load diagram shown by Figure 1.13 and a load diagram shown in Figure 1.14.

The no-load voltage drop is, in general, very small and moreover it is always of the same magnitude for a certain design. For these reasons, the no-load voltage drop will be given little attention in the future. The attention will be turned to Figure 1.14 and the load voltage drop ∆Ub

The vector diagram in Figure 1.11 is used for determining the errors. The two vectors ∆Ur and ∆Ux are constructed in the diagram shown by Figure 1.15.
The direction of the two vectors is given by the phase angle between the load current vector Is and the reference vector Us

The resistive component ∆Ur is in phase with Is and the reactive component ∆Ux is 90º out of phase with Is.

1.3.3 Calculation of the short-circuit impedance Zk
Figure 1.16 shows, in principle, how the windings are built up. All quantities, which are of interest concerning Zk, are given in the figure.

The two components Rk and Xk composing Zk are calculated in the following way.

 1.3.4 Variation of errors with voltage
The errors vary if the voltage is changed. This variation depends on the non-linear characteristic of the exciting curve which means that the variation will appear in the no-load errors. The error contribution from the load current will not be affected at all by a voltage change.

The variation of errors is small even if the voltage varies with wide limits. Typical error curves are shown in Figure 1.17.

 1.3.5 Winding dimensions

Designing a transformer for certain requirements is always a matter of determining the cross-sectional area of the winding conductors. Factors, which must be taken into account in this respect, are:
• Rated primary and secondary voltages
• Number of secondary windings
• Rated burden on each winding
• Accuracy class on each winding
• Rated frequency
• Rated voltage factor

The procedure is in principle as follows:
1. The number of turns are determined from
where

N = number of turns (primary or secondary)
U_n = rated voltage (primary or secondary)
f = rated frequency in Hz
A_j = core area in m2
B_n = flux density at rated voltage (Tesla)

The value of Bn depends on the rated voltage factor.
2. Determination of the short-circuit resistance R_k
The highest percentage resistive voltage drop ∆U_r permissible for the approximate accuracy class is estimated. R_k is determined from ∆U_r and the rated burden impedance Z_b
3. The cross-sectional areas of the primary and secondary winding conductors are chosen with respect to the calculated value of R_k.
4. The short-circuit reactance X_k is calculated when the dimensions of the windings are determined.
5. The errors are calculated. If the errors are too high the area of the winding conductors must be increased.

If a transformer is provided with two measuring windings it is often prescribed that each of these windings shall maintain the accuracy, when the other winding is simultaneously loaded. The load current from the other winding passes through the primary winding and gives rise to a primary voltage drop, which is introduced into the first winding. This influence must be taken into account when designing the windings.

1.3.6 Accuracy and burden capability
For a certain transformer design, the burden capability depends on the value of the short-circuit impedance. A low value for the short-circuit impedance (a high quantity of copper) means a high burden capability and vice versa. The burden capability must always be referred to a certain accuracy class.

If 200 VA, class 1 is performed with a certain quantity of copper, the class 0.5 capability is 100 VA with the same quantity of copper, on condition that the turns correction is given values adequate to the two classes. The ratio between accuracy class and burden capability is approximately constant. This constant may be called the “accuracy quality factor” K of the winding

K=100*A/P

where
A = accuracy class
P = rated burden in VA

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Basic Princicple of Battery

Basic Princicple of Battery

The basis for a battery operation is the exchange of electrons between two chemical reactions, an oxidation reaction and a reduction reaction. The key aspect of a battery which differentiates it from other oxidation/reduction reactions (such as rusting processes, etc) is that the oxidation and reduction reaction are physically separated. When the reactions are physically separated, a load can be inserted between the two reactions. The electrochemical potential difference between the two batteries corresponds to the voltage of the battery which drives the load, and the exchange of electrons between the two reactions corresponds to the current that passes through the load. The components of a battery, which are shown in the figure below, and consist of an electrode and electrolyte for both the reduction and oxidation reaction, a means to transfer electrons between the reduction and oxidation reaction (usually this is accomplished by a wire connected to each electrode) and a means to exchange charged ions between the two reactions.

Figure: Schematic of a battery in which (a) the electrolyte of the reduction and oxidation reaction are different and (b) the electrolyte is the same for both reactions.

The key components which determines many of the basic properties of the battery are the materials used for the electrode and electrolyte for both the oxidation and reduction reactions. The electrode is the physical location where the core of the redox reaction – the transfer of electrons – takes place. In many battery systems, including lead acid and alkaline batteries, the electrode is not only where the electron transfer takes places, but is also a component in the chemical reaction that either uses or produces the electron. However, in other battery systems (such as fuel cells) the electrode material is itself inert and is only the site for the electron transfer from one reactant to another. For a discharging battery, the electrode at which the oxidation reaction occurs is called the anode and by definition has a positive voltage, and the electrode at which the reduction reaction occurs is the cathode and is at a negative voltage. 

The electrode alone is not sufficient for a redox reaction to take place, since a redox reaction involves the interaction of more than a single component. The other chemical components of the reaction are contained in the electrolyte. For many practical battery systems, the electrolyte is an aqueous solution. One reasons for having an aqueous solution is the oxidized or reduced form of the electrode exists in an aqueous solution. Further, it is important that the chemical species in the electrolyte be mobile in order that they can move to the site on the electrode where the chemical reaction takes places, and also such that ion species can travel from one electrode to the other. 

The current in the battery arises from the transfer of electrons from one electrode to the other. During discharging, the oxidation reaction at the anode generates electrons and reduction reaction at the cathode uses these electrons, and therefore during discharging, electrons flow from the anode to the cathode. The electrons generated or used in the redox reaction can easily be transported between the electrodes via a conventional electrical connection, such as a wire attached to the anode and cathode. However, unlike a conventional electrical circuit, electrons are not the only charge carrier in the circuit. Electrons travel from the anode to the cathode, but do not return from the cathode to the anode. Instead, electrical neutrality is maintained by the movement of ions in the electrolyte. If each redox reaction has a different electrolyte, a salt bridge joins the two electrolyte solutions. The direction of the ion movement acts to prevent a charge build-up at either the anode or the cathode. In most practical battery systems, the same electrolyte is used for both the anode and the cathode, and ion transport can take place via the electrolyte itself, eliminating the need for a salt bridge. However, in this case a separator is also inserted between the anode and the cathode. The separator prevents the anode and cathode from physically touching each other since they are usually in very close physical proximity to one another, and if they were to touch it would short out the battery as the electrons can be transferred directly without flowing through the external circuit and load.

The redox reactions which comprise a particular battery system define many fundamental parameters about the battery system. Other key battery properties, including as battery capacity, charging/discharging performance and other practical considerations are also influenced by the physical configuration of the battery, for example the amount of material in the battery or the geometry of the electrodes. The following pages describe how battery characteristics – voltage behavior, battery efficiency, battery non-idealities (self-discharge, degradation of battery capacity, etc) – are dependent on the operation of the redox reactions and the battery configuration.

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Basic Concepts of Battery

Basic Concepts of Battery

Contents:

1. Components of Cells and Batteries
2. Classification of Cells and Batteries
3. Operation of a Cell
4. Theoretical Cell Voltage, Capacity, and Energy
5. Specific Energy and Energy Density of Practical Batteries
6. Upper Limits of Specific Energy and Energy Density

ABOUT THE EDITORS

David Linden has been active in battery research, development, and engineering for more than 50 years. He was Director of the Power Sources Division of the U.S. Army Electronics R&D Command. Many of the batteries and power sources currently in use, including lithium batteries and fuel cells, resulted from R&D programs at that Division. Mr. Linden is now a battery consultant working with Duracell, Inc. and other companies on the development and application of newer primary and rechargeable batteries. He is a member of national and international groups establishing standards for these new technologies.

1. COMPONENTS OF CELLS AND BATTERIES

A battery is a device that converts the chemical energy contained in its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. In the case of a rechargeable system, the battery is recharged by a reversal of the process. This type of reaction involves the transfer of electrons from one material to another through an electric circuit. In a nonelectrochemical redox reaction, such as rusting or burning, the transfer of electrons occurs directly and only heat is involved. As the battery electrochemically converts chemical energy into electric energy, it is not subject, as are combustion or heat engines, to the limitations of the Carnot cycle dictated by the second law of thermodynamics. Batteries, therefore, are capable of having higher energy conversion efficiencies.

While the term ‘‘battery’’ is often used, the basic electrochemical unit being referred tois the ‘‘cell.’’ A battery consists of one or more of these cells, connected in series or parallel, or both, depending on the desired output voltage and capacity.*

The cell consists of three major components:

1. The anode or negative electrode—the reducing or fuel electrode—which gives up electrons to the external circuit and is oxidized during the electrochemical reaction.
2. The cathode or positive electrode—the oxidizing electrode—which accepts electrons from the external circuit and is reduced during the electrochemical reaction.

*Cell vs. Battery: A cell is the basic electrochemical unit providing a source of electrical energy by direct conversion of chemical energy. The cell consists of an assembly of electrodes, separators, electrolyte, container and terminals. A battery consists of one or more electrochemical cells, electrically connected in an appropriate series / parallel arrangement to provide the required operating voltage and current levels, including, if any, monitors, controls and other ancillary components (e.g. fuses, diodes), case, terminals and markings. (Although much less popular, in some publications, the term ‘‘battery’’ is considered to contain two or more cells.)

Popular usage considers the ‘‘battery’’ and not the ‘‘cell’’ to be the product that is soldor provided to the ‘‘user.’’ In this 3rd Edition, the term ‘‘cell’’ will be used when describing the cell component of the battery and its chemistry. The term ‘‘battery’’ will be used when presenting performance characteristics, etc. of the product. Most often, the electrical data is presented on the basis of a single-cell battery. The performance of a multicell battery will usually be different than the performance of the individual cells or a single-cell battery (see Section 3.2.13).

3. The electrolyte—the ionic conductor—which provides the medium for transfer of charge, as ions, inside the cell between the anode and cathode. The electrolyte is typically a liquid, such as water or other solvents, with dissolved salts, acids, or alkalis to impart ionic conductivity. Some batteries use solid electrolytes, which are ionic conductors at the operating temperature of the cell.

The most advantageous combinations of anode and cathode materials are those that will be lightest and give a high cell voltage and capacity (see Sec. 1.4). Such combinations may not always be practical, however, due to reactivity with other cell components, polarization, difficulty in handling, high cost, and other deficiencies.

In a practical system, the anode is selected with the following properties in mind: efficiency as a reducing agent, high coulombic output (Ah/g), good conductivity, stability, easeof fabrication, and low cost. Hydrogen is attractive as an anode material, but obviously, must be contained by some means, which effectively reduces its electrochemical equivalence. Practically, metals are mainly used as the anode material. Zinc has been a predominant anode because it has these favorable properties. Lithium, the lightest metal, with a high value of electrochemical equivalence, has become a very attractive anode as suitable and compatible electrolytes and cell designs have been developed to control its activity.

The cathode must be an efficient oxidizing agent, be stable when in contact with theelectrolyte, and have a useful working voltage. Oxygen can be used directly from ambient air being drawn into the cell, as in the zinc/air battery. However, most of the common cathode materials are metallic oxides. Other cathode materials, such as the halogens and the oxyhalides, sulfur and its oxides, are used for special battery systems.

The electrolyte must have good ionic conductivity but not be electronically conductive, as this would cause internal short-circuiting. Other important characteristics are nonreactivity with the electrode materials, little change in properties with change in temperature, safety in handling, and low cost. Most electrolytes are aqueous solutions, but there are important exceptions as, for example, in thermal and lithium anode batteries, where molten salt and other nonaqueous electrolytes are used to avoid the reaction of the anode with the electrolyte.

Physically the anode and cathode electrodes are electronically isolated in the cell to prevent internal short-circuiting, but are surrounded by the electrolyte. In practical cell designs a separator material is used to separate the anode and cathode electrodes mechanically. The separator, however, is permeable to the electrolyte in order to maintain the desired ionic conductivity. In some cases the electrolyte is immobilized for a nonspill design. Electrically conducting grid structures or materials may also be added to the electrodes to reduce internal resistance.

The cell itself can be built in many shapes and configurations—cylindrical, button, flat, and prismatic—and the cell components are designed to accommodate the particular cell shape. The cells are sealed in a variety of ways to prevent leakage and dry-out. Some cells are provided with venting devices or other means to allow accumulated gases to escape. Suitable cases or containers, means for terminal connection and labeling are added to complete the fabrication of the cell and battery.

2. CLASSIFICATION OF CELLS AND BATTERIES

Electrochemical cells and batteries are identified as primary (nonrechargeable) or secondary (rechargeable), depending on their capability of being electrically recharged. Within this classification, other classifications are used to identify particular structures or designs. The classification used in this handbook for the different types of electrochemical cells and batteries is described in this section.

2.1 Primary Cells or Batteries

These batteries are not capable of being easily or effectively recharged electrically and, hence, are discharged once and discarded. Many primary cells in which the electrolyte is contained by an absorbent or separator material (there is no free or liquid electrolyte) are termed ‘‘dry cells.’’

The primary battery is a convenient, usually inexpensive, lightweight source of packaged power for portable electronic and electric devices, lighting, photographic equipment, toys, memory backup, and a host of other applications, giving freedom from utility power. Thegeneral advantages of primary batteries are good shelf life, high energy density at low to moderate discharge rates, little, if any, maintenance, and ease of use. Although large highcapacity primary batteries are used in military applications, signaling, standby power, and so on, the vast majority of primary batteries are the familiar single cell cylindrical and flat button batteries or multicell batteries using these component cells.

2.2 Secondary or Rechargeable Cells or Batteries

These batteries can be recharged electrically, after discharge, to their original condition by passing current through them in the opposite direction to that of the discharge current. They are storage devices for electric energy and are known also as ‘‘storage batteries’’ or ‘‘accumulators.’’

The applications of secondary batteries fall into two main categories:

1. Those applications in which the secondary battery is used as an energy-storage device, generally being electrically connected to and charged by a prime energy source and delivering its energy to the load on demand. Examples are automotive and aircraft systems, emergency no-fail and standby (UPS) power sources, hybrid electric vehicles and stationary energy storage (SES) systems for electric utility load leveling.
2. Those applications in which the secondary battery is used or discharged essentially as a primary battery, but recharged after use rather than being discarded. Secondary batteries are used in this manner as, for example, in portable consumer electronics, power tools, electric vehicles, etc., for cost savings (as they can be recharged rather than replaced), and in applications requiring power drains beyond the capability of primary batteries.

Secondary batteries are characterized (in addition to their ability to be recharged) by high power density, high discharge rate, flat discharge curves, and good low-temperature performance. Their energy densities are generally lower than those of primary batteries. Their charge retention also is poorer than that of most primary batteries, although the capacity of the secondary battery that is lost on standing can be restored by recharging.

Some batteries, known as ‘‘mechanically rechargeable types,’’ are ‘‘recharged’’ by replacement of the discharged or depleted electrode, usually the metal anode, with a fresh one. Some of the metal/ air batteries (Chap. 38) are representative of this type of battery.

2.3 Reserve Batteries

In these primary types, a key component is separated from the rest of the battery prior to activation. In this condition, chemical deterioration or self-discharge is essentially eliminated, and the battery is capable of long-term storage. Usually the electrolyte is the component that is isolated. In other systems, such as the thermal battery, the battery is inactive until it is heated, melting a solid electrolyte, which then becomes conductive.

The reserve battery design is used to meet extremely long or environmentally severe storage requirements that cannot be met with an ‘‘active’’ battery designed for the same performance characteristics. These batteries are used, for example, to deliver high power for relatively short periods of time, in missiles, torpedoes, and other weapon systems.

1.2.4 Fuel Cells

Fuel cells, like batteries, are electrochemical galvanic cells that convert chemical energy directly into electrical energy and are not subject to the Carnot cycle limitations of heat engines. Fuel cells are similar to batteries except that the active materials are not an integral part of the device (as in a battery), but are fed into the fuel cell from an external source when power is desired. The fuel cell differs from a battery in that it has the capability of producing electrical energy as long as the active materials are fed to the electrodes (assuming the electrodes do not fail). The battery will cease to produce electrical energy when the limiting reactant stored within the battery is consumed.

The electrode materials of the fuel cell are inert in that they are not consumed during thecell reaction, but have catalytic properties which enhance the electroreduction or electrooxidation of the reactants (the active materials).

The anode active materials used in fuel cells are generally gaseous or liquid (comparedwith the metal anodes generally used in most batteries) and are fed into the anode side of the fuel cell. As these materials are more like the conventional fuels used in heat engines, the term ‘‘fuel cell’’ has become popular to describe these devices. Oxygen or air is the predominant oxidant and is fed into the cathode side of the fuel cell.

Fuel cells have been of interest for over 150 years as a potentially more efficient and lesspolluting means for converting hydrogen and carbonaceous or fossil fuels to electricity compared to conventional engines. A well known application of the fuel cell has been the use of the hydrogen/oxygen fuel cell, using cryogenic fuels, in space vehicles for over 40 years. Use of the fuel cell in terrestrial applications has been developing slowly, but recent advances has revitalized interest in air-breathing systems for a variety of applications, including utility power, load leveling, dispersed or on-site electric generators and electric vehicles.

Fuel cell technology can be classified into two categories

1. Direct systems where fuels, such as hydrogen, methanol and hydrazine, can react directly in the fuel cell

2. Indirect systems in which the fuel, such as natural gas or other fossil fuel, is first converted by reforming to a hydrogen-rich gas which is then fed into the fuel cell

Fuel cell systems can take a number of configurations depending on the combinations offuel and oxidant, the type of electrolyte, the temperature of operation, and the application, etc.

More recently, fuel cell technology has moved towards portable applications, historicallythe domain of batteries, with power levels from less than 1 to about 100 watts, blurring the distinction between batteries and fuel cells. Metal/ air batteries (see Chap. 38), particularly those in which the metal is periodically replaced, can be considered a ‘‘fuel cell’’ with the metal being the fuel. Similarly, small fuel cells, now under development, which are ‘‘refueled’’ by replacing an ampule of fuel can be considered a ‘‘battery.’’

Fuel cells were not included in the 2nd Edition of this Handbook as the technical scopeand applications at that time differed from that of the battery. Now that small to medium size fuel cells may become competitive with batteries for portable electronic and other applications, these portable devices are covered in Chap. 42. Information on the larger fuel cells for electric vehicles, utility power, etc can be obtained from the references listed in Appendix F ‘‘

3. OPERATION OF A CELL

3.1 Discharge

The operation of a cell during discharge is also shown schematically in Fig. 1.1. When the cell is connected to an external load, electrons flow from the anode, which is oxidized, through the external load to the cathode, where the electrons are accepted and the cathode material is reduced. The electric circuit is completed in the electrolyte by the flow of anions (negative ions) and cations (positive ions) to the anode and cathode, respectively.

The discharge reaction can be written, assuming a metal as the anode material and a cathode material such as chlorine (Cl2), as follows:

Negative electrode: anodic reaction (oxidation, loss of electrons)

Zn → Zn²⁺ + 2e

Positive electrode: cathodic reaction (reduction, gain of electrons)

Cl₂+ 2e → 2Cl^-

Overall reaction (discharge):

Zn + Cl₂→ Zn²⁺+ 2Cl^- (ZnCl₂ )

3.2 Charge

During the recharge of a rechargeable or storage cell, the current flow is reversed and oxidation takes place at the positive electrode and reduction at the negative electrode, as shown in Fig. 1.2. As the anode is, by definition, the electrode at which oxidation occurs and the cathode the one where reduction takes place, the positive electrode is now the anode and the negative the cathode.

In the example of the Zn/Cl2 cell, the reaction on charge can be written as follows:

Negative electrode: cathodic reaction (reduction, gain of electrons)

Zn²⁺ +2e → Zn

Positive electrode: anodic reaction (oxidation, loss of electrons)

2Cl^-→ Cl₂ + 2e

Overall reaction (charge):

Zn²⁺ + 2Cl^-→ Zn + Cl₂

3.3 Specific Example: Nickel-Cadmium Cell

The processes that produce electricity in a cell are chemical reactions which either releaseor consume electrons as the electrode reaction proceeds to completion. This can be illustrated with the specific example of the reactions of the nickel-cadmium cell. At the anode (negative electrode), the discharge reaction is the oxidation of cadmium metal to cadmium hydroxide with the release of two electrons,

Cd + 2OH^-→ Cd(OH)₂+2e

At the cathode, nickel oxide (or more accurately nickel oxyhydroxide) is reduced to nickel hydroxide with the acceptance of an electron,

NiOOH + H₂O +e → OH^- + Ni(OH)₂

When these two ‘‘half-cell’’ reactions occur (by connection of the electrodes to an external discharge circuit), the overall cell reaction converts cadmium to cadmium hydroxide at the anode and nickel oxyhydroxide to nickel hydroxide at the cathode,

Cd + 2NiOOH + 2H₂O → Cd(OH)₂+2Ni(OH)₂

This is the discharge process. If this were a primary non-rechargeable cell, at the end ofdischarge, it would be exhausted and discarded. The nickel-cadmium battery system is, however, a secondary (rechargeable) system, and on recharge the reactions are reversed. At the negative electrode the reaction is:

Cd(OH)₂+ 2e → Cd + 2OH^-

At the positive electrode the reaction is:

Ni(OH)₂+ OH^-→ NiOOH +H₂O +e

After recharge, the secondary battery reverts to its original chemical state and is ready forfurther discharge. These are the fundamental principles involved in the charge–discharge mechanisms of a typical secondary battery.

3.4 Fuel Cell

A typical fuel cell reaction is illustrated by the hydrogen/oxygen fuel cell. In this device, hydrogen is oxidized at the anode, electrocatalyzed by platinum or platinum alloys, while at the cathode oxygen is reduced, again with platinum or platinum alloys as electrocatalysts. The simplified anodic reaction is

2H₂→ 4H⁺+ 4e

while the cathodic reaction is

O₂+ 4H⁺ + 4e → 2H₂O

The overall reaction is the oxidation of hydrogen by oxygen, with water as the reaction product.

2H₂+ O₂→ 2H₂O

4. THEORETICAL CELL VOLTAGE, CAPACITY, AND ENERGY

The theoretical voltage and capacity of a cell are a function of the anode and cathode materials. (See Chap. 2 for detailed electrochemical theory.)

4.1 Free Energy

Whenever a reaction occurs, there is a decrease in the free energy of the system, which is expressed as

Delta G⁰=-nFE⁰

where F = constant known as Faraday (~96,500 C or 26.8 Ah)

          n = number of electrons involved in stoichiometric reaction

          E⁰= standard potential, V

1.4.2 Theoretical Voltage

The standard potential of the cell is determined by the type of active materials contained inthe cell. It can be calculated from free-energy data or obtained experimentally. A listing of electrode potentials (reduction potentials) under standard conditions is given in Table 1.1A more complete list is presented in Appendix B.

The standard potential of a cell can be calculated from the standard electrode potentialsas follows (the oxidation potential is the negative value of the reduction potential):

Anode (oxidation potential) + cathode (reduction potential) + standard cell potential.

For example, in the reaction Zn + Cl₂→ ZnCl₂, the standard cell potential is:

Zn → Zn²⁺ + 2e

Cl₂ → 2Cl^-- 2e

                                                                                      E⁰=-(-0.76 V)/1.36V=2.12V

The cell voltage is also dependent on other factors, including concentration and temperature, as expressed by the Nernst equation.

4.3 Theoretical Capacity (Coulombic)

The theoretical capacity of a cell is determined by the amount of active materials in the cell.It is expressed as the total quantity of electricity involved in the electrochemical reaction and is defined in terms of coulombs or ampere-hours. The ‘‘ampere-hour capacity’’ of a battery is directly associated with the quantity of electricity obtained from the active materials. Theoretically 1 gram-equivalent weight of material will deliver 96,487 C or 26.8 Ah. (A gram-equivalent weight is the atomic or molecular weight of the active material in grams divided by the number of electrons involved in the reaction.)

The electrochemical equivalence of typical materials is listed in Table 1.1 and Appendix C.

The theoretical capacity of an electrochemical cell, based only on the active materials participating in the electrochemical reaction, is calculated from the equivalent weight of the reactants. Hence, the theoretical capacity of the Zn/Cl2 cell is 0.394 Ah/g, that is,

                                                     Zn +         Cl₂            →            ZnCl₂

                                             (0.82 Ah/g)  (0.76 Ah/g)

                                                  1.22 g/Ah + 1.32 g/Ah = 2.54 g/Ah or 0.394 Ah/g

Similarly, the ampere-hour capacity on a volume basis can be calculated using the appropriatedata for ampere-hours per cubic centimeter as listed in Table 1.1.

The theoretical voltages and capacities of a number of the major electrochemical systemsare given in Table 1.2. These theoretical values are based on the active anode and cathode materials only. Water, electrolyte, or any other materials that may be involved in the cell reaction are not included in the calculation.

4.4 Theoretical Energy*

The capacity of a cell can also considered on an energy (watthour) basis by taking both the voltage and the quantity of electricity into consideration. This theoretical energy value is the maximum value that can be delivered by a specific electrochemical system:

Watthour (Wh) = voltage (V) × ampere-hour (Ah)

In the Zn/Cl2 cell example, if the standard potential is taken as 2.12 V, the theoretical watthour capacity per gram of active material (theoretical gravimetric specific energy or theoretical gravimetric energy density) is:

Specific Energy (Watthours/gram) = 2.12 V × 0.394 Ah/g = 0.835 Wh/g or 835 Wh/kg

Table 1.2 also lists the theoretical specific energy of the various electrochemical systems.

5. SPECIFIC ENERGY AND ENERGY DENSITY OF PRACTICAL BATTERIES

The theoretical electrical properties of cells and batteries are discussed in Sec. 1.4. In summary, the maximum energy that can be delivered by an electrochemical system is based on the types of active materials that are used (this determines the voltage) and on the amount of the active materials that are used (this determines ampere-hour capacity). In practice, only a fraction of the theoretical energy of the battery is realized. This is due to the need for electrolyte and nonreactive components (containers, separators, electrodes) that add to the weight and volume of the battery, as illustrated in Fig. 1.3. Another contributing factor is that the battery does not discharge at the theoretical voltage (thus lowering the average

*The energy output of a cell or battery is often expressed as a ratio of its weight or size.The preferred terminology for this ratio on a weight basis, e.g. Watthours /kilogram (Wh/ kg), is ‘‘specific energy’’; on a volume basis, e.g. Watthours / liter (Wh/ L), it is ‘‘energy density.’’ Quite commonly, however, the term ‘‘energy density’’ is used to refer to either ratio.

voltage), nor is it discharged completely to zero volts (thus reducing the delivered amperehours) (also see Sec. 3.2.1). Further, the active materials in a practical battery are usually not stoichiometrically balanced. This reduces the specific energy because an excess amount of one of the active materials is used.

In Fig. 1.4, the following values for some major batteries are plotted:

1. The theoretical specific energy (based on the active anode and cathode materials only)
2. The theoretical specific energy of a practical battery (accounting for the electrolyte and non-reactive components)
3. The actual specific energy of these batteries when discharged at 20_C under optimal discharge conditions

These data show:

• That the weight of the materials of construction reduces the theoretical energy density or of the battery by almost 50 percent, and
• That the actual energy delivered by a practical battery, even when discharged under conditions close to optimum, may only be 50 to 75 percent of that lowered value

Thus, the actual energy that is available from a battery under practical, but close to optimum, discharge conditions is only about 25 to 35 percent of the theoretical energy of the active materials. Chapter 3 covers the performance of batteries when used under more stringent conditions.

These data are shown again in Table 1.2 which, in addition to the theoretical values, liststhe characteristics of each of these batteries based on the actual performance of a practical battery. Again, these values are based on discharge conditions close to optimum for that battery.

The specific energy (Wh/kg) and energy density (Wh/L) delivered by the major batterysystems are also plotted in Fig. 1.5(a) for primary batteries and 1.5(b) for rechargeable batteries. In these figures, the energy storage capability is shown as a field, rather than as a

single optimum value, to illustrate the spread in performance of that battery system under different conditions of use.

In practice, as discussed in detail in Chap. 3, the electrical output of a battery may bereduced even further when it is used under more stringent conditions.

6. UPPER LIMITS OF SPECIFIC ENERGY AND ENERGY DENSITY

Many advances have been made in battery technology in recent years as illustrated in Fig. 1.6, both through continued improvement of a specific electrochemical system and through the development and introduction of new battery chemistries. But batteries are not keeping pace with developments in electronics technology, where performance doubles every 18 months, a phenomenon known as Moore’s Law. Batteries, unlike electronic devices, consume materials when delivering electrical energy and, as discussed in Secs. 1.4 and 1.5, there are theoretical limits to the amount of electrical energy that can be delivered electrochemically by the available materials. The upper limit is now being reached as most of the materials that are practical for use as active materials in batteries have already been investigated and the list of unexplored materials is being depleted.

As shown in Table 1.2, and the other such tables in the Handbook, except for some ofthe ambient air-breathing systems and the hydrogen/oxygen fuel cell, where the weight of the cathode active material is not included in the calculation, the values for the theoretical energy density do not exceed 1500 Wh/ kg. Most of the values are, in fact, lower. Even the values for the hydrogen/ air and the liquid fuel cells have to be lowered to include, at least, the weight and volume of suitable containers for these fuels.

The data in Table 1.2 also show that the specific energy delivered by these batteries, basedon the actual performance when discharged under optimum conditions, does not exceed 450 Wh/ kg, even including the air-breathing systems. Similarly, the energy density values do not exceed 1000 Wh/L. It is also noteworthy that the values for the rechargeable systems are lower than those of the primary batteries due, in part, to a more limited selection of materials that can be recharged practically and the need for designs to facilitate recharging and cycle life.

Recognizing these limitations, while new battery systems will be explored, it will be more difficult to develop a new battery system which will have a significantly higher energy output and still meet the requirements of a successful commercial product, including availability of materials, acceptable cost, safety and environmental acceptability.

Battery research and development will focus on reducing the ratio of inactive to activecomponents to improve energy density, increasing conversion efficiency and rechargability, maximizing performance under the more stringent operating and enhancing safety and environment. The fuel cell is offering opportunities for powering electric vehicles, as a replacement for combustion engines, for use in utility power and possibly for the larger portable applications (see Chap. 42). However, the development of a fuel cell for a small portable applications that will be competitive with batteries presents a formidable challenge.

REFERENCES

1. Ralph J. Broad, ‘‘Recent Developments in Batteries for Portable Consumer Electronics Applications,’’ Interface 8:3, Fall 1999, Electrochemical Society, Pennington, NJ.

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BATTERIES MAINTENANCE

INDUSTRIAL BATTERIES/ACCU MAINTENANCE

(LEAD-ACID, NICKEL-CADMIUM, NICKEL-IRON)

    The installation and proper maintenance of three types of storage (secondary) batteries used in industrial applications are discussed: lead-acid, nickel-cadmium, and nickel-iron. Although a detailed discussion on the various considerations that enter into selecting the correct battery type for particular applications will not be presented, appropriate differences in the characteristics of these battery types will be outlined. The selection process is necessarily complex, involving a close examination of the proposed application, together with a thorough familiarity with the operating and other characteristics of the batteries themselves. In most cases it is advisable that the selection of a battery for an application (particularly if it is comparatively novel) be made with the assistance of a knowledgeable representative of one of the major battery manufacturers.

    All three of the battery types discussed in this chapter are common in one respect: they are vented to expel safely gases evolved during charge.

    A few special terms that the user will encounter in battery literature should be defined. Cell and battery sizes are specified in terms of a nominal or rated capacity. This is the amount of capacity, usually expressed in ampere-hours, that the cell or battery would be expected to deliver under normal conditions through most of its life. It is important to know the discharge rate under which the nominal capacity was established, since the capacity delivered does depend on discharge rate. Hourly rates are convenient means for expressing the charge and discharge rates at which a cell is operated. Hourly rates are established by the manufacturer for each cell type and are reported in his technical literature. The hourly rate is that discharge amperage which will exhaust the cell in the stated number of hours. For example, when a cell is discharged at the amperage given at the 3-hr rate, the cell would be approaching exhaustion and the voltage would start to decline rapidly, at the end of the third hour of discharge. When reading hourly rates, it is important to note the cutoff voltage at which the hourly rate was established; the higher the required voltage at the end of discharge, the shorter the discharge period will be at that given rate. The cutoff voltage is that discharge voltage at which a discharge should be stopped; repeated discharging beyond this point may damage the cells. Efficiency is that percentage of charge input which can be withdrawn from the cell on the following discharge. Energy density is the power delivered per unit weight or per unit volume of the battery; it is expressed as watt-hours per pound or watt-hours per cubic inch.

I. LEAD-ACID BATTERIES

    Two types of lead-acid batteries are manufactured for industrial applications. These are generally referred to as motive-power and stationary batteries. Typical applications for motive-power batteries include material-handling trucks, mine locomotives, mine tractors, mine shuttle cars, floor sweepers and scrubbers, heavy-duty personnel carriers, transport vehicles, golf carts, and lawnmowers. Typical applications for stationary batteries are switchgear and emergency power for electric-utility substations, switchgear and emergency power for generating plants, computer and other no-fail systems, telephone-company equipment for a variety of operations, emergency lighting, and railway signal service. Depending upon the application, three types of stationary batteries are available through most manufacturers, those made with lead-calcium-alloy grids and those made with lead-antimony grids.

    The active material of the positive electrode of a lead-acid battery is lead dioxide, and the negative is highly reactive spongy lead. The electrodes are electrically insulated from each other by separators. Many different types of separators are used, such as resin-impregnated cellulose materials, microporous rubber, microporous plastic, and fiberglass-mat separators with micro-porous backing. Most separators are fabricated with vertical ridges or ribs on the surface that face the positive electrode, and the other surface is flat with no ribs. The electrolyte in fully charged batteries is a solution of sulfuric acid with a specific gravity ranging from 1.215 to 1.300, depending on intended service. The positive and negative active materials are supported on lead-grid structures in all types except the plante. (The positive electrode in a plante battery is a solid piece of pure lead that is scored with evenly spaced ridges to create a large surface area, then electrochemically converted to lead dioxide by an electrolytic forming process.) For most applications, however, the grid is cast from an alloy of lead with 4.5 to 7 percent antimony and small amounts of arsenic and tin. The function of the antimony is to harden the lead and facilitate casting. For telephone standby power where low self-discharge rates and float currents are required, a small amount of calcium (less than 0.10 percent), instead of antimony, is used in the grid alloy. Generally no arsenic or tin is used in the calcium alloy.

    In the fully charged state, the negative active material exists as lead, the positive as lead dioxide, and the concentration of sulfuric acid is at its maximum level. As the cell is discharged, the positive electrode is converted to lead sulfate as follows:

   The overall reaction results in the consumption of sulfuric acid and the equivalent production of water. The consumption of sulfuric acid during the discharge of a lead-acid battery provides a convenient method by which the state of charge can be measured.

Installation and Operation.

    Upon receiving a new battery, it is extremely important to examine the exterior packing case. An examination should be made for wet spots on the sides and bottom of thecase. Wet spots may indicate leaking jars, broken in shipment because of rough handling by the carrier. If any damage has occurred, take immediate and proper claim measures. If any jars are damaged and the electrolyte has leaked out, make immediate repairs and replace broken jars at once. If replacement jars are not immediately available, withdraw the elements from the damaged jar (see under “Repairs” below) and place the elements in a glass, porcelain, rubber, or other nonmetallic vessel containing water suitable for battery use. Sufficient water must be added to cover the plates and separators completely. Damage or complete destruction of the cell may result if these procedures are not followed. Note: Use distilled water or tapwater that has been analyzed and approved for battery use.

    Special attention should be given to cells that have been put into new jars. The cells should be filled with electrolyte of the same specific gravity as the balance of the cells at the time of initial filling, and a charge should be applied at a low finishing rate until the specific gravity of the electrolyte ceases to rise. If the specific gravity after charging is lower than that of a normal, fully charged cell, a small amount of electrolyte should be withdrawn and replaced with electrolyte of 1.400 specific gravity. The battery should then be given an additional charge for 1/2 to 1 hr to mix the liquid thoroughly. Another reading for specific gravity should be taken which should indicate full charge. If not, repeat the latter process until the normal specific gravity is obtained.

    All specific-gravity readings of electrolyte must be corrected to 80°F (27°C) to compensate for different densities at different temperatures and obtain a constant basis for comparison.

Placing the Battery in Service.

    Upon receipt of the battery, a useful practice is to give the battery a freshening charge of from 3 to 6 hr or until the specific gravity indicates no further rise. The charge should be given only with a direct-current charger and with the terminals properly connected.

    Cell temperature during the charge should not exceed 115°F (46°C). All points of contact between the charger and the battery should be clean to ensure good conductivity to terminal connections. If terminal connections are copper, apply a coat of petroleum jelly or no-oxide grease to prevent corrosion.

     If the battery is installed in a vehicle, properly fasten it in place by holddown lugs on the battery or jar, or bars of the vehicle, to reduce vibration and jarring. If the battery is being installed in a metal compartment, make sure the compartment is thoroughly dry and free of moisture prior to installation.

     If the battery is to be installed in a locomotive, block the battery into position, allowing a 1/8-in space between the block and battery tray. Do not wedge the battery into position. All connections between the battery and the vehicle must be flexible. All vent caps must be in place while the battery is in service. Failure to keep caps in position will result in loss of electrolyte (and therefore loss of capacity) and will cause corrosion outside the battery.

Charging the Battery.

    The batteries used in most industrial situations (other than standby emergency use) are used in what is called cyclic operation. That is, the battery is either being charged or being used (discharged). In most such applications batteries are charged about 1500 to 2000 times during their lives. Incorrect charging for a few cycles will do little harm, but incorrect charging day after day will shorten the life of the battery.

    Correct charging means charging the battery sufficiently, without overcharging, overheating, or excessive gassing. To accomplish this, the charging of batteries is usually started at a high rate of amperage known as the starting rate. Later in the charge, this rate of current flow is reduced to the finishing rate. Manufacturers generally suggest as a rule of thumb that the finishing rate should not exceed 5 A per 100 A-hr of rated battery capacity. The starting rate may be four or four and one-half times the finishing rate. Lead-acid batteries should be charged for a sufficient length of time at a rate which will introduce into the battery the same number of ampere-hours removed on discharge, plus a 5 to 15 percent overcharge. The specific value of the overcharge depends almost entirely upon the charging temperature, and the age and history of the battery. In general, it is more harmful to overcharge excessively an older battery at a high rate or a battery operating at high temperature than a freshly manufactured unit or one being charged at room temperature or lower. Any charge rate is permissible which does not produce excessive gassing or cell temperature greater than 115°F (46°C).

    Four methods of charging are discussed below; they are:

1. Modified constant-voltage
2. Taper
3. Two-rate
4. Constant-current

    The selection of the appropriate method will be governed by considerations such as the type of battery, service conditions, time available for charging, and the number of batteries to be charged at one time. It should be noted that in charging motive-power batteries, the end-of-charge rate (finishing rate) is extremely important and should not be exceeded. Normally, batteries can be charged in 8 hr, assuming a normal-duty discharge; however, if time permits, a longer period can be used.

    Figure 3.1 shows that a discharged battery can absorb high currents at relatively low battery voltages. For example, after the introduction of about 20 percent capacity (20 A-hr at 40 A), a 100-A-hr battery is at a voltage of about 2.22 V per cell. The curves also show that as the charge progresses at a given rate, the voltage increases, the higher charge rates yielding higher voltages. For example, at 110 percent charge (10 percent overcharge), the voltage at 5 A is 2.55 V, and at 20 A, the voltage is 2.74 V.

    The generally used finishing rate for lead-acid batteries is the 20-hr rate. With most charging schemes, the normal start-of-charge rate is about the 5-hr rate, or 20 A per 100 A-hr of rated capacity.

     Modified Constant-Voltage Method. In the modified constant-voltage method, a fixed resistor is in series with the charger and battery. A 2.63-V-per-cell bus is used for an 8-hr charge. Table 3.1 shows the relationship between volts per cell and time available for charge. In order to use a single fixed resistor and achieve proper start and finish rate, the voltages indicated in the table are required. For an 8-hr charge, the initial current is 22.5 A per 100 A-hr, and for a 16-hr charge, 8.5 A. It should be noted that the charging resistor should be of sufficient current-carrying capacity. The normal “tap” value of the resistor determines the finishing rate. For example, at the 8-hr rate, the “tap” value is 0.022 Ω. This number is calculated as follows: the terminal voltage Et of the battery at the end of charge, at a finishing rate of 5 A, is 2.52 V (see Fig. 3.2). Therefore, with a bus voltage EB = 2.63 V, the tap resistance must be

     When charging several batteries at once, from either a constant-voltage source derived from a motor generator, or from a rectifier, the modified constant-voltage method of charge is preferred because the current tapers during charge, reducing the possibility of excessive charge currents.

The following formula should be used to calculate the kilowatt requirements when using motor generators to charge several batteries from a fixed-voltage bus in 8 hr:

Taper Method. This method can be used with either a generator or rectifier equipment, and can be considered a variation of the modified constant-voltage charge method. It is employed when only one size of battery is to be charged. Shunt-wound motor generators, and rectifier chargers can be designed so that their voltage versus current characteristics correspond closely to the modified constant-voltagetype charger. No ballast resistor is required. The circuitry of the charger is such that the initial and finish charge rates are matched to the battery. As was previously mentioned, the finish rate is generally the 20- hr rate, and the initial rate about the 5-hr rate. Figure 3.3 shows the typical voltage, current, and specificgravity

profile of a cell being charged by the taper method. The charge characteristics are nearly the same as those shown in Fig. 3.2 for the modified constant-voltage method. In order to meet the requirements for charging a single battery from a motor generator, the following design parameters must be met.

• The nominal voltage of the generator must be 2.25 V per cell.
• The initial load voltage of the generator should be about 2.135 V per cell.
• At the end of charge, the charging current should be less than 5 A per 100-A-hr battery capacity, and the corresponding voltage 2.52 V per cell.

    To calculate the kilowatt requirements for a single motor-generator set, the following formula should be used:

    Two-Rate Method. The principle of this method is to begin charging at the recommended start-ofcharge rate, then switch to a lower rate when gassing occurs (at about 2.37 V per cell), the proper finishing rate is produced toward the end of the 8-hr period. Figure 3.4 illustrates the charge curve for the two-rate method. When the second resistor is brought into the circuit, a sharp drop in current occurs.

    Constant-Current Method. Constant-current charging is seldom used for 8-hr or shift charging of motive power batteries, because this would require manual control during and at the end of charge. If, however, the charge time available were about 12 to 16 hr, constant-current charging could be used. Strictly speaking, the charge period is longer, the initial current is lower than for the 8-hr charge rate, and the taper much shallower. As shown in Fig. 3.5, the initial rate for the 16-hr charge is about 8.5 A, and the finish rate 5 A, yielding a taper ratio (the ratio of the initial to finish current) of 1.7 to 1, and for the 8-hr charge, the ratio is approximately 4.0 to 1.

Maintenance. Inspect the battery once every week to make certain all connections are tight.

Remove dust or dirt accumulations from the battery top and then wash the battery clean with water and dry with compressed air. At least once each month, neutralize the acid on the battery covers and terminals with either 1 lb of ammonia or sodium bicarbonate solution per 1 gal of water prior to water rinse. Keep terminals and metal parts free of corrosion.

    Check the electrolyte level daily and replace any water lost by evaporation. And never allow the electrolyte level to drop below the top of the battery plates. Caution: Never overfill the cells. When replacing water that has evaporated, fill the cells only to the underside of the vent well. Overfilling causes loss of acid, thus reducing battery capacity.

     To ensure that water is thoroughly mixed with the electrolyte and to prevent overfilling, additions should only be made while the battery is on charge and gassing at its finish rate. The only exception to this is when the electrolyte level is below the separator protector and not discernible. In this case, add just enough water to bring the level up even with the separator protector prior to charging. Then, make the final adjustment toward the end of the charge period. It is advisable to keep accurate records of the amount of water used and the date of each filling, since the water requirements are an indication of battery overcharging. Battery water should be stored in a covered glass, plastic, earthenware, or other nonmetallic container. Only suitable water should be used for batteries, as certain impurities are harmful and will reduce battery life. Water sources in certain geographic areas are not suitable at any time and in other areas are only satisfactory during certain seasons of the year. If the quality of the local water supply is unknown, arrangements can be made with your battery manufacturer to have an analysis made on a sample at a nominal cost.

   Make sure that vent plugs are always kept tightly in place and see that the small gas escape holes do not become clogged. If plugs need cleaning, let them stand in clear water for 30 min or so.

How to Prevent Overdischarging. This is one of the most common causes of battery problems.

   Past Experience. This is an obvious but common method. Batteries should be suited to the job for which they are being used. A well-suited battery is a fully charged battery capable of doing the desired amount of work or lasting the desired length of time in a specific service. As long as the job is reasonably standardized (i.e., the equipment powered by the battery is not called on to do extra work during the cycle time), a schedule can be made for battery recharging with very few production failures.

   Operator’s Experience. An experienced operator can tell from the action of the equipment when the batteries are reaching a point at which they should be charged.

   Discharge Indicator. State-of-charge meters which are permanently mounted on materialhandling equipment are commercially available. They monitor voltage and rate of discharge and, if properly calibrated and adjusted, will light a warning light on the fuel gage dial just prior to the battery being 80 percent discharged. At 80 percent, a relay is activated which cuts off power to the lifting devices, but allows power to the drive motors.

   Ampere-Hour Meter. With this type of meter, the number of ampere-hours removed from the battery is recorded. (Some scales are calibrated in ampere-hours remaining.) Thus, the operator knows how much power is left in the battery.

How to Determine Battery Condition

   Records. The purpose of records is to provide a day-to-day case history so that any variations from normal can be detected quickly and acted upon. Daily records should show battery number, identification of the truck the battery was taken from, specific gravity of battery when put on charge (pilot-cell reading), temperature of pilot cell, time put on charge, time taken off charge, and specific gravity when taken off charge. These are enough facts to keep a good case history on the battery. If specific gravity (corrected for temperature) and time-on-charge data are compared with the previous day’s reading, any abnormal battery use or abuse would be indicated and can be acted upon. As a long-run check, most battery manufacturers recommend that special specific-gravity and voltage readings be taken of each cell of the battery every 6 months, after an equalizing charge has been applied. Comparisons of these readings with the readings of the last such test will show any longterm changes in battery condition as well as differences between cells.

   Test Discharge. Such a test should be made at any time there is a question as to whether or not the battery is delivering its rated capacity. The procedure is as follows.

   The battery is given an equalizing charge and the fully charged specific gravity of each cell is adjusted to normal. Starting time is noted, and the battery is discharged at the standard 6-hr rate given in the operating data supplied by the battery manufacturer. Individual cell voltages and the overall battery voltage should be recorded 15 min after the test is started, and then hourly until the voltage of any cell reaches 1.8 V; thereafter, voltage measurements should be made at 15-min intervals. Record the time when each cell voltage reaches 1.75 V. When the majority of cells reach 1.75 V, record the time and terminate the test. Measure the specific gravity of each cell immediately.

   Record all cell voltages and stop the test discharge when the battery voltage reaches the termination voltage of 1.70 times the number of cells in the battery. Record the specific gravity of each cell immediately after terminating the test discharge. The readings will help determine whether the battery is uniform or if any one or more cells are low in capacity. If the battery is uniform and delivers 80 percent or more of its rated capacity, it can be returned to service.

   Internal Inspection. If the test discharge indicates that the battery is not capable of delivering at least 80 percent of rated capacity and all cells are uniform, an internal inspection of one of the cells is indicated. Failure to meet capacity ratings may be caused by an internal shunt which can be repaired. The positive plates, which wear out first, should be examined. If they are falling apart or the grids have many frame fractures, a new battery is needed. If the positive plates are in good condition and the cells contain little sediment, the battery may be sulfated. The negative plates of a sulfated battery will have a slatelike feeling, being hard and gritty and having a sandy feeling when rubbed between the fingers. (A good negative plate, when fully charged, is spongy to the touch and gives a metallic sheen when stroked with the fingernail.) Sulfation is such a common condition that a special discussion on its causes and treatment follows.

   A sulfated battery is one in which abnormal lead sulfate is formed in the plates. This affects the normal chemical reactions within the battery, causing loss of capacity. The most common causes of sulfation are undercharging, repeated partial charges, neglect of equalizing charge, standing in a partially or completely discharged condition, low electrolyte, specific gravity more than 0.015 above normal, and high temperature. The following steps will usually restore a sulfated battery:

1. Clean battery.
2. If no electrolyte is visible, add water to bring level up to the separator protector.
3. Put battery on charge at the prescribed finishing rate until full ampere-hour capacity has been supplied the battery. If during the charge the temperature of the battery exceeds 115°F (46°C), reduce the charge rate. If any cells give test-voltage readings 0.20 V below the average cell voltage, pull and repair the cell before continuing the charge.
4. Continue the charge at the finishing rate until the specific gravity shows no change for a 4-hr period.
5. Give the battery a test discharge.
6. (a) If the battery gives rated capacity, no further special treatment is needed except that the battery should be immediately recharged before being returned to service. (b) If the battery does not deliver at least 80 percent of rated capacity, continue the discharge until one or more cells reach 1.0 V. Repeat steps 3, 4, 5, 6a, and then go to step 7 if step 6a is not met.
7. If the battery does not deliver at least 80 percent of capacity, repeat steps 2, 3, 4, and 5 again. If the battery does not now deliver 80 percent of capacity, assume that it should be replaced.

Causes and Remedies of Common Battery Troubles. In the listing that follows it is impossible to consider all sources of battery trouble. The ones listed are common troubles and will serve as a starting point for investigating the cause of unsatisfactory performance. Eight symptoms are listed; after each are listed possible causes. Where the remedy for the cause is perfectly obvious, it is omitted. Where, however, there might be some doubt as to the correct remedy, it is indicated along with the cause, is marked with the symbol R, and is enclosed in parentheses.

   Symptom: Battery Will Not Take a Charge. Possible causes: (1) Direct-current charging-circuit fuse blown or missing. (2) Circuit in charging receptacle or plug open, or connection of cable to stud loose. (3) Alternating-current line fuses blown or missing. (4) Alternating-current line switch open. (5) Circuit in control lead or circuit open, preventing contactor from pulling in. (6) Charging plug not pushed all the way into receptacle. (7) Charging rate too low. (R: Check ammeter for accuracy. See below, under “Symptom: Battery Takes Too Long to Charge.”) (8) No voltage output from generator. (R: Check field circuit; if open, correct. Check brush contact to armature; correct by replacing brushes or adjusting so they don’t stick.) (9) Bus voltage too low, caused by incorrect tap setting in rectifier or too low voltage from generator. (10) With initial equipment, connections to charging receptacle reversed.

   Symptom: Battery Takes Too Long to Charge. Possible causes: (1) Connection poor in charging circuit. (R: Check lugs, bolted connections, charging leads, plugs, and receptacles for high-resistance joints, and correct.) (2) Battery overdischarged. (3) With two-rate charging, charging equipment does not provide high starting rate. (R: Check for open in control circuit to provide high rate. Determine cause and correct.) (4) With two-rate rectifier charging equipment: (a) voltage relay connected for smaller number of cells than in battery; (b) applied ac voltage too low under load conditions (R: Install greater-capacity line to rectifier to reduce voltage drop, or relocate rectifier nearer to incoming ac source); (c) primary transformer taps not set for voltage applied; (d) voltage relay operating below standard voltage (i.e., 2.37 V per cell); (e) start-of-charging rate too low; (f) end-of-charging rate too low; (g) start-of-charging rate too high. (5) Where voltage relay is used in control circuit for two-rate charge, temperature of charging control equipment may be materially higher than battery operating temperature. (R: Provide better ventilation for charging equipment, or relocate it to an area where atmospheric temperature is the same as temperature in area where battery operates.) (6) With modified constant-voltage charging: (a) bus voltage too low; (b) bus voltage decreases as load decreases (R: Adjust generator for flat characteristic); (c) ballast resistance too great. (7) Charging leads reversed, or charging-equipment polarity reversed. (8) Battery not placed on proper charging circuit when installation has various battery sizes. (9) Charge not terminated when battery is fully charged.

   Symptom: Battery Will Not Work Full Shift. Possible causes: (1) Cell voltages and specific gravity uneven. (R: Give an equalizing charge.) (2) Electrolyte level low. (3) Battery not charged before going into service. (4) Two or more cell leakers in steel tray. (R: Replace broken jars.) (5) One or more cells cut out of battery. (6) Battery with incorrect number of cells assigned to equipment. (7) Specific gravity below normal. (8) Impurities in electrolyte. (9) Operator riding brakes. (10) Operator using reverse instead of brakes. (11) Load too great. (12) Wheels, axles, and bearings need grease. (13) Tires underinflated. (14) Brakes dragging. (15) Wheels deeply grooved. (16) Ruts in roadbed deep. (17) Series field in motor shorted or grounded. (R: Clear grounds and insulate wiring.) (18) Armature needs repairs. (19) Grounds on equipment. (20) Excessive grades along route traveled. (21) Service required exceeds capacity of equipment. (22) When batteries are in two halves, discharged half has been paired with a charged half. (23) Uneven number of cells in two halves, where split batteries are used in parallel-start, series-run control circuits.

   Symptom: Battery Overheats on Charge. Possible causes: (1) Finish rate too high. (2) High-charge rate on too long. (R: Reduce voltage-operating point of voltage relay.) (3) Timer not set correctly. (4) Ampere-hour meter not set correctly. (5) Percent overcharge setting of ampere-hour meter set above correct level of 12 percent. (6) Timer set for too many hours. (7) Two-rate charge did not change over to low rate. (R: check operation of voltage relay. Check for open voltage-relay circuit. Check for open in charge-rate control lead. See if voltage relay is connected for same number of cells as in battery.) (8) Bus voltage too high. (9) Charge rate too high. (10) Charge not stopped—automatic mechanism does not terminate charge. (R: See that voltage relay is connected for same number of cells as in battery. Check timing mechanism. Check for open in control leads. Check operation of voltage relay. Check amperehour meter for accuracy and operation at low rates; clean and calibrate it.) (11) Ventilation poor. (12) Separators worn through. (13) Sediment space filled. (14) Internal shunt. (15) Fully charged specific gravity is below normal, and attendant continues charge to increase specific gravity. (R: Adjust specific gravity with acid.)

   Symptom: Battery Overheats on Discharge. Possible causes: (1) Overdischarge (beyond allowable limit of 1.130). (2) Battery too small. (3) Ventilation poor. (4) Burn of connectors to cell terminals poor. (5) Load excessive. (6) Battery worn out. (7) Separators worn through. (8) Internal shunt. (9) Battery capacity temporarily reduced because of low fully charged specific gravity. (10) Battery not fully charged before being put in service, resulting in overdischarging. (11) Electrolyte level low. (12) Battery not heat-insulated from resistor in charging equipment. (13) Atmospheric temperature too high.

   Symptom: Electrolyte Level Low. Possible causes: (1) Jar broken or cracked. (2) Water additions neglected. (3) Cell overlooked when adding water. (4) Too much overcharging. (R: If automatically controlled, check voltage relay, timer, and charge-rate relay. If manually controlled, terminate charge when specific gravity is 10 points below last equalizing charge value. Change from high rate to low rate when specific gravity reaches 1.200.)

   Symptom: Cell Voltages Unequal. Possible causes: (1) Overdischarge. (R: Give an equalizing charge.) (2) Equalizing charges lacking. (3) Internal shunt. (4) Top of battery very dirty. (5) Cells operated with low electrolyte level. (6) Fully charged specific gravity of cell low. (7) Sediment space filled. (R: Replace battery.) (8) Positive plates worn out. (R: Replace battery.) (9) Half tap on cells for lower voltage circuit. (R: Remove tap, and connect load to battery terminals through resistance.) (10) External source (such as charging resistance on locomotive) heating certain cells. (11) Contact poor in controller on split-circuit batteries (parallel and series on discharge, all series on charge). (12) Impurities in cell. (13) Charging rate varies. (14) See also symptom below.

   Symptom: Unequal Specific Gravity between Cells. Possible causes: (1) All items under Cell Voltages Unequal above. (2) Overfilled with water. (3) Cell operated with cracked jar. (4) Acid not adjusted properly when jar was changed. (5) Battery operated with vent caps out of place. (6) Sealing compound leaks. (7) Battery operated with broken cover. (8) Neutralizing material in cell.

Repairs. Most of the repairs to storage batteries consist of removing a part of the battery and replacing it. In this section, we shall therefore outline the procedure for disassembling and reassembling a typical battery. The manufacturer’s instructions that were received with the battery will undoubtedly outline any specific procedures in handling their units.

   Drilling Intercell Connectors. In most batteries, the lead insert of the cover, the cell post, and the intercell connectors are all welded together. To remove a cell from the circuit or an element from a jar, it is therefore necessary to remove the connector or cut it in two.

   There are two methods of removing a connector. One is to use a special drill that allows the cell post to remain but cuts the bond to the lead insert of the cover. The other method is to drill through the center of each post, using a 15/16-in drill, to a depth of 3/8 in. After the intercell connectors are drilled, they can be lifted off. On some batteries it is possible to saw a connector. It should be cut above the space where the two jars meet. Then the cell can be pulled out of the tray.

   Removing Cell from Tray. After the connector is removed, use a warm compound knife and cut the compound from between the jar of the cell and the adjacent cells or tray. Penetrating oil or kerosene mixed with regular oil should be run into the space between cells to act as a lubricant. Work the cell back and forth to see that it is loose; then lift straight up. Small cells can be lifted manually. To lift heavy cells, attach a cell puller (a self-tapping nut with loop attached). Always attach the puller on the negative post. If the cell has two negative posts, use two pullers with a piece of wood through the loops. Lift slowly and carefully, vibrating the lifting rope after a strain is put on to loosen the cell.

   Replacing a Jar. Have the new jar ready. Remove the jar to be replaced from the tray (as outlined above). If the cell is to remain out of the battery for a day or two, the space in the tray should be blocked to prevent jars in the tray from bowing out into the space from which the cell was removed.

    Cut the compound from around the top of the jar with a warm compound knife, keeping it very close to the inside of the jar cell. Heat the outside of the jar on all four sides with a blowtorch. Place the jar holddown clips and chains on the jar, and use the cell puller to lift the element halfway out of the jar. Allow the element to remain in this position a minute or so to drain. Then remove the element and lay it down on a wood board surface with the flat side of the negative plate down. The element should not be exposed to air any longer than necessary. (If the element starts to heat, sprinkle it with water and place in a jar.)

   After warming the clean jar so that it is pliable, slide it over the bottom of the element carefully, using the compound knife as a guide. Then lift up the jar and lower the element into the jar slowly and carefully so that the separators are not broken or damaged. (If the jar is square, be sure that the element is placed in the jar so that the ribs of the jar are in the correct direction at right angles to the plate.)

   Clean, neutralize, and dry the surface of the jar, and cover. Reseal between the repaired cell and its adjacent cells with compound. (When pouring a seal, use a compound knife in one hand and hold the saucepan of compound in the other. The knife is used to cut off the pour and catch excess compound.) Remove any excess compound that may have run down the outside of the jar. Fill the cell with correct electrolyte (the same as in adjacent cells or higher) to the top of the splash plate. The cell should then receive an equalizing charge, and the acid should be adjusted. Place the cell in the tray, being sure polarity is correct. The final step is to reburn the intercell connector. This is covered separately below. But first it is important to note that the post and connectors have been cleaned in preparation for burning.

   Reconnecting Sawed Connector. As mentioned above, sometimes connectors are sawed through when a cell is removed. The connector can be burned together by using a connector mold, which is simply a shallow trough that fits under the break. It is blocked into place with small wedges. Place the tip of the carbon on the piece of the connector that is in the electric circuit for the carbon burner, and hold the carbon there until it is white hot. Add new lead, and move the tip of the carbon through the molten lead to ensure that the new lead is fused with the lead of each half of the connector.

II.VENTED NICKEL-CADMIUM BATTERIES

   Nickel-cadmium batteries are alkaline batteries, having a solution of potassium hydroxide as the electrolyte. These batteries are very rugged physically and will sometimes withstand more shock and vibration than the equipment they are powering. They are also capable of sustaining considerable electrical abuse (overcharging, standing in the discharged state, and occasional overdischarging). Characteristically they have low internal resistance, and consequently have good charge acceptance and perform well at high rates. Compared with nickel-iron storage batteries, they have a relatively low self-discharge rate. Their performance at low temperatures is excellent; many designs will deliver 80 percent of their rated capacity at temperatures as low as 240°F (115°C). As a rule they are not intended for cyclic applications, being used rather in engine starting, railroad signaling, emergency lighting, communications, alarm, switchgear, marine, and standby applications.

   Specific points of comparison with the lead-acid storage battery that may be mentioned are as follows: Nickel-cadmium batteries can be left standing for long periods of time in the discharged condition without fear of deterioration. During charge, no corrosive fumes are released. The nickel-cadmium battery with sintered plates (described below) can, if required, be recharged quite rapidly—in 1 or 2 hr. They can be overcharged with little damage, provided that the temperature is controlled. Further, nickel-cadmium batteries are not damaged by freezing.

   A few of the disadvantages of nickel-cadmium cells are as follows: they have a considerably lower voltage than the lead-acid type (both operating and open-circuit); the average discharge voltage is between 1.2 and 1.25 V at ordinary discharge rates. They are not capable of extended deepcycle service. With the pocket plates (discussed below), the ratio of energy per unit volume and unit weight is no greater, in some cases less, than that of lead-acid storage batteries. The state of charge cannot be readily determined, as can be done with a hydrometer with lead-acid batteries. And, because of the considerably greater cost of the prime metals, nickel and cadmium, on an energy basis their cost is substantially greater than that of lead-acid batteries.

   The electrochemical characteristics of the nickel-cadmium system are similar to those of other alkaline batteries such as the silver-cadmium and the nickel-iron but differ greatly from the electrochemical reactions of the lead-acid battery. As was mentioned under “Lead-Acid Batteries,” sulfuric acid electrolyte is consumed in reacting with the positive and negative; consequently the state of charge can be determined by measuring the specific gravity of the electrolyte. However, in all alkaline batteries the electrolyte serves only as a carrier of charge. The potassium hydroxide is not consumed but serves only to shuttle electrons back and forth between the positive and negative plates as the battery is being charged or discharged. Consequently, the electrolyte remains relatively constant during both charge and discharge. The discharge-voltage curve of alkaline batteries is relatively flat and the batteries are not as vulnerable to freezing as other storage batteries. To emphasize again: The state of charge of an alkaline battery cannot be measured by the specific gravity of its electrolyte.

Plate Processing and Battery Construction. Two basic types of vented nickel-cadmium cells are

available, those with pocket plates and those with sintered plates. Pocket plates are most generally used in vented nickel-cadmium cells and batteries. These plates are extremely rugged and are used in applications requiring maximum life, great resistance to shock or vibration, and maximum cell size. Nickelcadmium batteries using sintered plates have much lower cell internal resistance and are therefore used in applications requiring very high discharge rates, such as engine-starting and switchgear applications.

Pocket Plates. Production of pocket plates begins with strips of thin steel ribbons, perforated with roughly 2000 holes per square inch, and then nickel-plated. The edges of this ribbon are turned up into a troughlike configuration. The active materials—nickel hydroxide plus graphite for the positive, and cadmium hydroxide (or cadmium oxide) plus iron powder for the negative—are pressed into this trough. A second piece of perforated steel ribbon is applied over this, and the edges of the two ribbons are crimped together, forming a very long flat pocket of perforated steel containing the active material. The material can be cut to pieces of any length to form plates of any desired width. These pockets are then laid horizontally into plate frames stamped from steel sheet; these frames have the length and width of the desired plate and are open in the center to receive the pockets. The pockets are crimped into this frame in such a way that the joints formed along the sides of the plate frame serve also to seal off the cut end of the individual pockets. The plates are then assembled into positive and negative groups, bolted together to the proper terminal post by means of a threaded connector rod passing through the base of the post, or in some cases to comblike teeth extending from the terminal post. The positive and negative groups are then interleaved. The separators—either plastic rods placed vertically between the plates of corrugated or perforated plastic sheet—are inserted. The assembled element is then placed in the cell case, and the cover, with its insulating and sealing washers and nuts, is placed over the terminals and welded (or cemented) in place.

   Smaller pocket-plate cells are available either in plastic or in steel cases. Plastic cases have many advantages: They are transparent or translucent. In applications involving large numbers of cells, this means a considerable saving of maintenance time because electrolyte levels can easily be checked visually and it is easy to fill to the proper level when watering. Since the cases are nonconductive, they can be touching, thus saving installation space. There is less likelihood of accidental grounding with these cases, and it is a little safer to work around them with metal tools. Plastic cases are resistant to electrolyte corrosion. For some applications requiring great physical ruggedness, they are assembled into steel battery trays.

   Steel cell cases are formed of welded steel sheets, are nickel-plated, and can be produced in a variety of sizes without a large tooling expense. Most importantly, they offer the advantage of great strength. It is for this reason that large cells are built only in steel cases.

Sintered Plates. Sintered plates involve first a sintered nickel plaque, which serves as the plate grid. This plaque is made by sintering fine nickel powder (made by the carbonyl process) to a piece of nickel screen or perforated nickel sheets. The resulting plaque material is a very porous, tough, flexible sheet of pure nickel, usually between 0.025 and 0.08 in thick. Though this material appears solid to the eye, roughly 80 percent of the volume is open space. The positive and negative active materials are then deposited into these pores by any of several different methods of impregnation. The resulting cells have very low internal resistance and consequently perform well at very high discharge rates; this is the prime advantage of the sintered nickel-cadmium cell.

Voltage. The open-circuit voltage of nickel-cadmium cells is about 1.35 V. The average discharge voltage, which can be used for calculating the number of cells used for particular applications, is generally stated as 1.2 V. At lower discharge rates (5 to 8 hr and lower), the average voltage would be about 1.25 V; at these rates, the voltage would drop 0.15 to 0.2 V from the beginning to the end of the discharge. Pocket cells can be discharged at the 1-hr rate and sintered cells at two or three times the 1-hr rate before the average discharge voltage falls below 1.2 V. Sintered cells will deliver 12 to 16 times the 1-hr-rate discharge at voltages no less than 1.0 V. At low-temperature operation, the voltage naturally is reduced to a lower level, but not significantly until the temperature gets down to the range of 220°F (104°C). For lower-voltage applications (i.e., 6 V, 12 V), the number of cells used is the exact (or the nearest) equivalent to the quotient obtained by dividing the application voltage by 1.2. For example, five cells are used for a 6-V application and 10 cells for 12 V. With higher-voltage applications this factor may not be strictly adhered to. For example, 18 to 20 cells may be used for 24-V circuit-breaker application, and 92 to 95 cells for a 125-V control application. The exact number of cells selected depends on the discharge rate, line loss to be counteracted, float voltage available, and other factors. As a general practice, it is recommended that the manufacturer’s service engineer be consulted in selecting the proper number of cells and electrical operating characteristics to ensure a proper interface between the battery system and the proposed application.

Performance Characteristics. Most pocket-plate designs will deliver their normal capacity, though to a lower end voltage, when discharging at rates as high as the 1-hr rate. Some high-rate sintered plate cells will deliver nominal capacity at discharge rates several times the 1-hr rate. When discharging into loads which offer very low resistance (switchgear, engine-starting applications), sintered cells will deliver some 15 to 18 times the 1-hr rate for about 10 sec. Some manufacturers, for sintered-type plates, publish the 5- and 1-sec capacity ratings. Most nickel-cadmium cells will deliver about 80 percent of nominal capacity at 0°F (−17.7°C) at normal discharge rates. It is suggested that when discharging nickel-cadmium batteries at the 3- to 4-hr rates or lower, care should be exercised to avoid repeatedly discharging below 1.0 V. At very high rates, such as are involved in engine-starting applications, the discharge may, however, be carried down to 0.65 V. Repeated discharging below these limits will lead to a declining capacity. Sintered plates, however, can tolerate overcharging somewhat better than pocket plates.

Life. The life expectancy provided by most pocket plates in float operation is about 15 years, depending on the severity of service conditions. Occasional standby applications have been reported in which a 25-year life was attained. Sintered-plate batteries have a much shorter life; in severe service, such as vehicle-starting applications, lives of 5 to 7 years have been reported. Batteries in emergency lighting, alarms, and communications can be expected to survive for 8 to 15 years. Some manufacturers provide types that are satisfactory for cycling under carefully controlled conditions and will deliver up to 10,000 cycles, depending upon the depth of discharge.

Selection. Having determined the general battery design that should be considered for your application, the approximate number of cells that will be needed, and the ampere-hours to be supplied between chargings, contact the firm manufacturing that cell type and supply them with the detailed information needed to recommend the proper battery configuration for your application. This information should include the following.

   Voltage Required. The allowable maximum and minimum values, the degree of voltage regulation preferred. Capacity and Rate Capability Required. The currents the battery will be expected to deliver and the length of time over which these stated currents will flow. If the current is unknown, state as explicitly as possible the work that is to be done by the equipment which the battery will power, in terms of torque or horsepower delivered, transmitting and receiving power, and so on.

   Charging Conditions. Type of charge routine to be used—constant current, constant voltage, float, trickle, type of charge equipment you plan to use, rates and voltages it can deliver, degree of control it exercises, length of time allowable for each charge, and frequency at which charging can be done.

   Shock and Vibration Resistance Required.

   Angular Inclination. Slant or tilt to which the battery will be subjected.

   Installation Conditions. Available space, ambient temperature, ventilation available, proximity of lead-acid batteries, or other contaminating conditions.

   Any Special Maintenance Conditions. Desire to minimize frequency of watering, need to avoid special tools, and type of personnel who will care for the battery.

Installation. In the discussion that follows, the assumption is made that the purchaser of a new battery system has received detailed instructions from the manufacturer on installation. In general, nickel-cadmium batteries are shipped charged and filled with electrolyte (except for thoseexported, which are shipped discharged and dry). Throughout the unpacking and installation procedure, nickelcadmium batteries should be handled with caution, since they are charged. For example, chains or metal hooks must not be used to hoist cells from the packing crates; rope slings, passed under the intercell connectors, may be used with caution. After having checked for shipping damage, remove whatever shipping plugs have been put into the cell vents and replace with the vent plugs provided. Then check the electrolyte level in each cell. With most medium and large cell sizes, the electrolyte should be at least 1/2 in above the plates. The maximum level should be obtained from the manufacturer’s literature; it will often be about half the distance from the top of the plates to the underside of the cell cover. If electrolyte has been spilled during shipment and the level is below the top of the plates, add refill electrolyte to bring the level to its stipulated level. In general the battery manufacturer is the best source for details on electrolyte replacement, and his recommended procedure should be followed.

   Cells in steel cases and battery trays may be permanently installed. Nickel-cadmium batteries do not release corrosive fumes and may be installed next to machinery or instruments; however, this equipment should not be subject to direct spray from the cells. Appropriate battery racks may be purchased from the manufacturer or, if convenient, may be built by the user to the manufacturer’s specifications. A small amount of space should be left between trays for circulation of air. Smaller batteries should be placed on shelves. Batteries in trays with special extended sides may be set directly on the floor. Plastic-case cells in plastic or steel battery trays require no special installation. Vehicular or marine batteries must be securely held down. In all cases, if the batteries are to be serviced from the side of the compartment, a minimum of 8 in, preferably 12 in, should be provided between the cell tops and the compartment roof. Batteries should not be installed in areas where the temperature will rise frequently to above 100°F (38°C).

   After positioning the batteries and installing the intercell connectors, check the polarity of all cells, following the connectors from one battery terminal throughout the battery to the other terminal to make sure that the cells are correctly connected in series. Any cells connected into the circuit with reverse polarity will be damaged. Look for a “plus” sign marked on the cell terminal or on the cover next to the terminal, a red mark on the side of the vent wall toward the positive terminal, or (in some larger cell types) a red-rubber insulating band on the positive terminal. Then make sure that all terminalpost nuts are tight. Check that the main battery cables to the battery are heavy enough to carry the maximum current that will be required without excessive voltage drop. Battery cables should be fitted with nickel-plated lugs; bare copper lugs are likely to corrode. All cables should be kept off the cell tops. Wipe off any electrolyte that may have splashed out onto the cell tops during installation. When the cell tops of steel-cased cells are perfectly clean and dry, completely cover them with a thin coating of petroleum jelly. This will prevent electrolyte spray from gradually building up into a hard-to-remove crust on the cell tops. If the battery is not to be placed in service within 90 days, it should either be put on continuous trickle charge or given a charge at the 5-hr rate for 3 hr when it is put back into service.

Charging. It is important to establish a reliable regular charge procedure with nickel-cadmium batteries and to adhere to it, since it is not possible to check the state of charge quickly as can be done with lead-acid batteries. However, if there is a strong need to determine the state of charge, it is possible in many applications to design a method of simultaneously reading voltage and current during a brief high-rate discharge, or reading the amount of current drawn when the battery is placed on a brief constant-voltage charge. The manufacturer’s service engineer should be contacted for establishing the most satisfactory method for the particular application.

   Nickel-cadmium batteries tolerate overcharging fairly well. In any questionable case, it is always better to overcharge than undercharge. The cautions against excessive overcharge are stated, not because of any direct effect on the plates, but rather so that maximum permissible temperatures are not exceeded and that the electrolyte loss and buildup of conductive film on the cell tops is minimized. On any charge routine the battery temperature should not exceed 115°F (46°C). Occasional temperatures of 125°F (52°C) can be tolerated, but repeated charging at these high temperatures is likely to result in reduced capacity and shortened battery life. When checking temperature, always take it from the cells in the middle of the battery, as these are likely to be the warmest cells.

   Nickel-cadmium batteries can be recharged with either of two basic types of charge routine: constant voltage (constant potential) and constant current. They are also maintained in the fully charged state by trickle- or float-charge routines. The equalizing charge discussed below is a variation of the constant-current routine. Constant-voltage charging involves supplying charge current at a fixed regulated voltage. The voltage level is selected so that the current, high at first, tapers off to a very low level as the battery nears full charge, and the countervoltage of the battery rises. This is one of the two most commonly used methods. It readily lends itself to automatic control and can be performed rapidly. The constant-current method offers the advantage of easy calculation of the ampere-hours of charge put into the battery. However, if it is to be performed manually, it calls for frequent adjustment of the rate. With the trickle-charge method, the battery is left permanently connected to a source delivering very small amounts of charge current; an example is the charger/battery combinations included in many emergency-lighting or alarm units. Trickle charging can be done with either constant current or constant voltage.

   Float charging, the second very common charge method used for nickel-cadmium batteries, differs from trickle charging in that the battery is permanently connected in parallel across the line between the power source and the equipment to be powered. The power source normally supplies both the equipment and the charge current to the battery. The battery is discharged in the event of failure or inactivity of the power source. This is the typical standby application.

   Constant Voltage. In most cases a modified rather than a true constant-voltage method is used so as to limit the high initial surge of current that would otherwise be absorbed by a discharged battery. With this scheme the voltage is automatically reduced below the preselected value until the current taken by the battery at that voltage drops to a value that can be supplied by the charger. This reduces the size and hence the cost of the charging equipment. This method has been found particularly suitable for sinteredplate batteries. For information on the voltage values that should be used for either constant-potential or modified constant-potential charging, the manufacturer’s engineers should be consulted.

   Constant Current. If it is necessary to charge a battery fully within 7 hr using the constant-current method, the charger must be capable of delivering current at the 5-hr rate, at a voltage of approximately 1.8 to 1.85 V per cell. In many installations smaller chargers are used, delivering a lower rate (8- to 10-hr rate), thus requiring only 1.55 to 1.65 V per cell. Longer charge times are therefore necessary. Water consumption will be lower with these lower charge rates. A variable resistance must be placed in series with any battery to be charged at constant current; make sure that this variable resistor and the ammeter and shunts used in the charging equipment are capable of handling the currents involved. The resistance should be adjusted at least every 1/2 hr to hold the charge rate steady. Batteries to be seriesconnected and charged on one charger must be of similar design type and in a similar state of charge. If the state of charge is unknown, charge each battery on a separate charger or at a separate time.

   A charge-back factor of about 140 percent is recommended for all nickel-cadmium batteries; that is, the battery is charged until 140 percent of the amperes taken out on the previous charge is returned. If the amount of capacity previously withdrawn is unknown, simply start the charge at a convenient rate, preferably at or near the 5-hr rate. Observe the on-charge battery voltage. Using approximately the 5-hr rate, the initial on-charge voltage of a fully discharged battery will be about 1.35 V per cell. During the charge the voltage will gradually increase to about 1.45 V per cell. At this point a major portion of the normal capacity will have been returned and gassing will begin.

   As the cell approaches, and reaches, full charge, the cell voltage will rise quite rapidly to about 1.5 to 1.8 V per cell (depending on the cell design and actual charge rate); charging should be continued until the on-charge voltage has remained steady at this level for 60 min (as indicated by three identical readings taken 30 min apart). When using this method, it is particularly important to watch the end-of-charge voltage point. If the charge is not terminated, the battery will continue to accept current, which will go entirely into the formation of hydrogen and oxygen; water loss will therefore be very rapid, and the battery temperature may rise above the maximum permissible level.

   Because of the sharp voltage rise at the end of charge, charging can conveniently be terminated by a voltage-sensing relay or other similar device. In almost all cases nickel-cadmium batteries can be automatically charged on modified constant-current charge equipment designed for use with leadacid batteries of comparable size. However, note that the end-of-charge voltage of a nickel-cadmium battery differs substantially from that of a lead-acid battery.

Trickle Charge. Trickle charging should be used only to keep a charged battery in the fully charged condition. It is not an acceptable method to charge a completely discharged battery. Pocket-plate batteries may be maintained on trickle charge at voltages between 1.40 and 1.45 V per cell; for sintered-plate batteries an acceptable voltage level is 1.36 to 1.38 V per cell. However, the manufacturer’s instruction should be followed as to the exact values. Self-discharge losses will be replaced when operating at the lower end of the voltage range. (Operating at the higher voltage will ensure return of capacity taken out in a partial discharge.) At all events, stay below the gassing potential of approximately 1.45 to 1.47 V. If water consumption is observed to be excessive, decrease the on-charge voltage. If the battery is cold (32°F [0°C]or colder), raise the voltage by about 0.05 V per cell. These voltages are critical. If charge voltage fluctuates because of changes in line voltage, it may be necessary to monitor the voltage for the initial period of operation and then choose the average value for routine operation.

   Trickle charging can also be done by the constant-current method. Set the charger to supply a few milliamperes of current for each ampere-hour rated capacity of the battery. The exact value that will provide a balance between minimizing water consumption and maintaining full charge can be determined through trial and error or by the technical data of the battery manufacturer.

   Float Charge. Pocket-plate batteries are maintained on float at between 1.40 and 1.45 V per cell, and sintered-plate types at 1.36 to 1.38 V. As for trickle charging, the lower values cited are adequate to replace self-discharge losses and will ensure minimum water loss but will not replace any significant amount of discharge current withdrawn from the battery. The voltage must be held below 1.45 V to avoid gassing and excessive water loss. Operating at these voltages, the battery will draw current at approximately the 35- to 50-hr rate.

   Equalizing Charge. Batteries operating on float- or trickle-charge routines should occasionally be given an equalizing charge to keep the cells in balance. Cells are said to be out of balance when, because of small unavoidable differences in chemical or physical condition, they begin to differ in their state of charge. When this happens, some of the cells in the battery will reach full charge before the others and will exhibit an early increase in cell voltage. In float operation, where the charge voltage is not too far above that voltage at which the cells will accept no charge current, this early rise in the voltage of some cells will result in decreased current delivered to the battery as a whole, before the other cells have reached full charge.

   Some commercially available chargers have two charge positions, one for normal charging and one for equalizing charge. In the equalizing position these chargers usually deliver current equivalent to the 15- to 20-hr rate for the battery. This is barely adequate. To ensure complete equalization, charging should be done at the 5- to 10-hr rate if possible. On float or trickle applications, once a year when the battery is observed to have lost capacity or to have gone out of balance, charge it at the equalizing rate until the voltage of each cell, measured individually, has reached a plateau (at about 1.65 V per cell) and has ceased to rise.

Maintenance. Once nickel-cadmium batteries are properly installed and are being operated correctly, the major maintenance effort involved is maintaining the electrolyte level and the specific gravity and keeping the battery exterior clean. Typical instruments and materials needed for maintaining and overhauling nickel-cadmium batteries are as follows:

• Refill electrolyte (potassium hydroxide of 1.220 specific gravity), or as specified
• Renewal electrolyte (1.240 specific gravity), or as specified
• Adjustment electrolyte (1.300 specific gravity), or as specified
• Petroleum jelly
• Pure mineral oil, acid-free, nonsaponifying
• Asphalt-base paint, caustic- and corrosion-resistant
• Hydrometer (reading 1.150 to 1.300 specific gravity)
• Spirit thermometer (reading 0 to 160°F); special types with scale indicating gravity-correction factors are available
• Electrolyte-level test tube
• Filling squeeze bottle or bulb
• Equalizing bottle or bulb (see below)
• Special post nut and vent tools (as recommended by the manufacturer)

   The principles to be observed in maintaining nickel-cadmium batteries are as follows.
   Follow carefully the prescribed charge procedures as described previously.

   Maintain proper electrolyte level, gravity, and purity. Having ensured that the electrolyte level of new cells as received is correct, set up a schedule for the regular checking of level, to be followed as long as the cells are in operation. Cells in standby applications, which may have to be recharged fairly frequently, should be checked once a month for the first 6 months; by this time the user will see the pattern of electrolyte loss and may find it possible to reduce the frequency of checking. In float- and trickle-charge applications, the electrolyte level should be checked every 3 to 6 months. In the infrequent cases where nickel-cadmium batteries may be used in cycle applications, the level should be checked every other cycle until the pattern of electrolyte loss becomes clear.

   Water should be added to maintain solution levels in accordance with the manufacturer’s instructions. Where these are not available, the following instructions will give satisfactory results: electrolyte must never be allowed to fall below the plate tops; if the tops of the plates are exposed to the air, serious damage will usually result. The maximum level in most cells is one-half to two-thirds of the distance between the tops of the plates and the underside of the cell cover. If this maximum is exceeded, there is danger that during a heavy-charge routine, electrolyte will overflow, causing leakage currents and loss of potassium hydroxide.

   Many manufacturers of cells with transparent plastic cases put two marks on the side of the cell case, the lower one corresponding to the plate tops (the minimum level) and the upper mark indicating the maximum level. In small sintered-plate cells there may be only one mark, indicating maximum level; this mark will be slightly above the tops of the plates. In cells with opaque plastic or metal cases, the electrolyte level may be checked with a level test tube. These tubes may be obtained from the battery manufacturer, but any clean, uncontaminated, clear plastic tube of convenient length (8 to 12 in), having a bore of roughly 3/16 in, will do. The tube is held vertically and placed into the cell until it comes to rest on the plate tops. The forefinger is then placed tightly over the end, and the tube is withdrawn, permitting one to view the height of the electrolyte above the plate tops. Of course, the electrolyte contained in the tube must be returned to the cell. Wash out the tube in water after each use.

   Normal charging procedures do not cause any significant loss of potassium hydroxide. Only water is lost, through the formation of hydrogen and oxygen that is characteristic of any storage cell being charged. Water alone should be added to correct the level drop due to charging and evaporation; potassium hydroxide electrolyte is added to a cell only in the case of spillage. As a general rule, use only distilled or deionized water. In some parts of the country tapwater has the necessary purity, but this can be decided only by chemical analysis; some manufacturers will perform this service if requested.

   If it seems that the frequency at which water must be added is excessive, or that spray is building up on the cell tops at an extraordinary rate, check the charging operation. It may be necessary to decrease the charge voltage, or to decrease current and use longer charge periods. If constant-current charging is being done, make sure also that the charge is being terminated at the proper point.

   Water may conveniently be added with a squeeze bottle or bulb, sometimes furnished with the battery. When watering cells of larger batteries, establish a regular orderly pattern of working through the cells, and use this pattern consistently. This will decrease the likelihood of missing a cell. Most plastic-cased cells have removable, screw-type vent plugs. When watering these batteries, it is good practice to remove the plugs and soak them in warm water for several minutes to remove crystallized deposits from the vent passages. When replacing these plugs, screw them in with only moderate force; otherwise undue pressure will be exerted on the O ring or washer generally used to provide a seal between the plug and the cover.

   If there are many cells to maintain, it may be convenient to provide a second “equalizing” bottle or bulb to withdraw excess electrolyte. This may be acquired from the manufacturer, or it may be prepared by the user. To prepare it, first determine the exact distance above the plate tops at which the maximum electrolyte level falls. Measuring this same distance from the end of the spout of a squeeze bottle or bulb, drill a small hole through the side of the spout. The end of this spout is then seated on the plate tops, and excess electrolyte is drawn into the bulb; the electrolyte level will fall until it reaches the proper height, at which point the hole in the spout will draw air. If there are a great number of cells and watering is fairly frequent, an automatic filler may be justified (check with manufacturer).

   The concentration (specific gravity) of the electrolyte is important. Most pocket-plate cells as manufactured are filled with electrolyte of 1.190 to 1.210 specific gravity; the exact value will be specified by the manufacturer for the particular cell-design type. More concentrated electrolyte (1.280) may be used for cells intended for low-temperature operation; this concentration would damage cells operated at room temperature, however. Refill electrolyte also usually has a concentration of 1.190 to 1.210, and is used to replace electrolyte lost by spillage. Renewal electrolyte is generally about 1.240 specific gravity, and is used to replace electrolyte in cells in which plates have been covered by distilled water after shipping or installation accidents, or to replace electrolyte which has become excessively contaminated or diluted through use.

   When ordering or preparing refill or renewal electrolyte, gage the quantity needed by the rule of thumb that in most pocket-type cells 1 qt of electrolyte will be needed for each 70 to 90 A-hr of rated capacity. Considerably less electrolyte is needed for sintered cells. For more accurate values check with the battery manufacturer.

   When filling cells in which plates have been covered with distilled water following accidental spillage, it may happen that the renewal electrolyte will be diluted by the water in the plates, so that the resulting concentration is below the recommended value. In this case it will be necessary to adjust the gravity, using 1.300 specific gravity electrolyte. This adjustment must be done while the cell is being overcharged, so that the gassing will mix the electrolyte as readings are taken. When the battery has been charging at a steady voltage of 1.6 to 1.7 V per cell for 30 min, check the specific gravity. Then estimate the total amount of electrolyte contained in the cell. For each quart of electrolyte, a difference of 20 gravity points (0.020) below the necessary value calls for the addition of roughly 60 mL (or 2 fl oz) of 1.300 electrolyte. Add this amount of 1.300 electrolyte, let the cell charge for another 30 min, and check the gravity again. Repeat this procedure until the gravity is correct. If gravity is too high, it may be corrected by withdrawing a portion of the electrolyte from the cell and replacing it with distilled water.

   All gravity readings, taken in the course of any maintenance procedure, must be corrected for temperature. This is particularly important when adjusting electrolyte gravity, since the battery is on charge and is likely to be warm. For each 4°F that electrolyte temperature is about 72°F, add 0.001 to the observed gravity reading; for each 4°F below 72°F, subtract 0.001. Similarly, electrolyte must be at the proper level over the plates whenever gravity readings are taken. Always place the hydrometer all the way into the cell, so that its tip rests on the tops of the plates. This will prevent mineral oil from being drawn up into the hydrometer.

   With pocket batteries operating on uninterrupted float routines, check the gravity once a year. When operating on any routine that involves recharging, gravity should be checked once every 6 months. The concentration of the electrolyte will decline slowly as small quantities of potassium hydroxide are thrown out along with the gases and spray is released during the charging. When the gravity has dropped to the minimum value specified by the manufacturer (usually in the neighborhood of 1.160), the electrolyte must be renewed. Continued operation beyond this point will result bin a fairly rapid decrease in cell life.

   The procedure for renewing electrolyte is as follows: first prepare or acquire the necessary amount of renewal electrolyte. Then discharge the battery at the 7-hr rate to a voltage of 0.5 to 0.8 V per cell. This will minimize the danger of shocks or damage through shorting. With cells maintained in wooden trays, disconnect the intercell connectors, incline the tray to one side, and remove the slats from the side. Take out and invert each cell individually, emptying out all electrolyte. Do not allow the cell to touch any conductive material, causing short circuiting. Batteries assembled in steel or plastic trays are simply inverted so that all cells are emptied simultaneously. The electrolyte is injurious to aluminum, copper, zinc, or tin. Do not rinse cells with water or electrolyte. Do not allow any cell to stand empty for more than 30 min, or the plates will be damaged through exposure to air. Fill each cell as it is emptied with renewal electrolyte to the maximum permissible level (halfway between plate tops and cell cover in most cell designs). Wash out the vent cap and replace it immediately.

   Clean each cell, preferably by a blast of low-pressure steam followed by compressed-air drying. It is good practice at this time to repaint the cell cases with corrosion-resistant paint. Reassemble the cells into the trays, coat the covers with petroleum jelly, make sure the intercell connectors are tightened securely, and charge the battery at the 7-hr rate for 14 hr. The battery is now ready to be returned to service.

   With sintered cells, there is usually not enough free electrolyte in the cell to obtain a gravity reading. Judgment as to when to renew electrolyte is therefore based on electrical performance. If the cell has been cycled considerably or overcharged, or has been used for a period of a few years and is beginning to decline in capacity in spite of good maintenance and proper charging, the electrolyte probably needs renewing. Dump the electrolyte, following the same general procedure as described above. Replace the electrolyte with 1.300 specific gravity solution. The cell will be discharged at this point. Therefore, fill it only to the tops of the plates, and then charge it. Following the charge, the electrolyte level may be brought up the rest of the way to the maximum mark.

   Potassium hydroxide reacts with carbon dioxide in the air to form potassium carbonate, which will decrease capacity when its concentration in the electrolyte exceeds a few percent. Formation of carbonate can be minimized by several means: (1) Open cell vents no more frequently, and for no longer, than is absolutely necessary. (2) Make sure that vent components and the glands or washers around the terminals make a good seal against the cell cover. (3) Maintain a layer of oil on the surface of the electrolyte. (4) Minimize overcharging, particularly overcharging at high rates. This condition causes agitation of the electrolyte and formation of crusts of carbonate on the underside of the cell cover, which then fall back into the electrolyte. (5) Control electrical operation and scheduling of maintenance so that frequency of adjustment of electrolyte level is minimized. (6) Store the electrolyte stock in tightly sealed containers only.

   Carbonate concentration can be determined by chemical analysis. This is a service that most manufacturers will provide. This service is also available from many commercial testing laboratories. The decision as to when to have an analysis performed can be based on the performance of the battery. It should not be necessary to have this analysis done more frequently than every 2 years. If the battery is not yet approaching end of life but exhibits marginal performance in spite of proper charging and correct electrolyte level and concentration, carbonate contamination should be suspected, and an analysis should be done. On the other hand, if, at the end of a 2-year period, performance is good, an analysis can be deferred. When carbonate concentration in the electrolyte reaches 10 percent by weight, the electrolyte should be renewed.

   Electrolyte can be procured from most manufacturers either as dry crystals, to be mixed to the proper concentration by the user, or as solution mixed to a specified concentration. Using dry crystals can save considerable shipping cost and avoids having to order several different concentrations but does involve handling and mixing. Solutions should be mixed in a large glass, porcelain, or plastic vessel that is perfectly clean and free from contaminants. Electrolyte crystals should be ordered from the battery manufacturer; the container will usually include mixing instructions. As a general rule, preparing 1.240 specific gravity solution calls for about 2.56 lb of pure potassium hydroxide per gallon of water—2.33 lb per gal will produce 1.220 solution. Some users may prefer to mix just one solution strength, the strongest needed, for storing, and then dilute this to the other strengths required as they are needed. Starting with 1.300 solution and mixing 7 parts of this with 2 parts water will produce 1.240 specific gravity electrolyte; 1/2 parts of 1.300 and 4 parts water will yield 1.220 specific gravity. Also, 10 parts of 1.240 solution and 1 part water can be mixed to yield 1.200 specific gravity.

   When handling potassium hydroxide in any of these procedures, it should be remembered that it is a corrosive chemical, injurious to skin and eyes. The standard goggles, face mask, and rubber garments should be considered for use. If electrolyte is spilled or splashed on skin or clothes, wash immediately with liberal quantities of water. It is wise to have on hand a stock of boric acid solution to neutralize spilled electrolyte. Diluted pharmaceutical-grade boric acid can be used to rinse the eyes.

    Guard against stray currents and shorts by the following means:

   Under no circumstances allow metal cell cases to touch each other. Even though both terminals are insulated from the steel cases and covers by rubber glands, current will be conducted by the electrolyte from the plates to the cases, and thence via touching cell cases to plates of the opposite polarity, thus shorting out the battery.

   Keep the cases and the covers clean. Films and paths of dirt, moisture, and electrolyte spray not only will conduct current between points of opposite polarity and self-discharge the battery, but also will lead to electrolytic corrosion of the steel cases and covers. Wipe off moisture and carbonate that build up on the cover; keep the cover coated with petroleum jelly. Prevent debris from building up between the cell cases, or under the cells so as to bridge to ground. It is good practice to go over the entire battery periodically with a low-pressure blast of steam, followed by an air blast to dry the cells thoroughly.

   Never stack cells or trays on top of one another.

   Do not overfill cells with electrolyte and risk overflow during charging.

   Dress all cables up and away from cell tops. Never allow cables to lie on cell tops or on intercell connectors.

   After installing or doing maintenance on the battery, make sure that no tools, screws, or other metal parts are left in the battery compartment.

   Use only spirit thermometers. Mercury is an electrical conductor. If a mercury thermometer should break, allowing mercury to run down into the cell interior between the plates, serious shorting would be likely.

   When taking battery voltage readings, check also for possible voltages between each of the battery terminals and ground. Such a voltage is an indication of a ground somewhere in the system.

   Instruments or devices which would cause a constant drain of current must not be left connected across the battery permanently. As an example, if the user wants to have a voltmeter connected in readiness, it should be wired through a normally open push-button switch so that it is connected to the battery only when the switch is depressed.

   Make sure connections are tight and making good electrical contact and that post and vent seals are maintained.

   Good electrical contact at terminal connections will prevent wasteful voltage drop. This can be checked by putting the battery on a high-rate discharge for 15 to 20 min. Defective connections will have resistance and will feel warm to the touch. Take these connections apart and clean the contact areas of the terminal posts, connectors, and nuts with solvent or cleanser and fine emery cloth or steel wool.

   If the seal around posts and vents does not remain tight, air and impurities may be admitted to the cell interior, and there will be excessive buildup of carbonate and electrolyte film on the covers. Leaks will be indicated by encrusted carbonate developing at the seal area. In these cases, tighten the lower terminal-post nut or the vent plug. If the rubber sealing glands on the terminal posts or the seal components on the vents have become brittle or deformed, they should be replaced. If special tools are necessary to turn terminal or vent components, they will be available from the manufacturer.

III. NICKEL-IRON BATTERIES

Nickel-iron alkaline storage batteries are traditionally used in cyclic service, although they have also been used successfully in standby and emergency-power applications.

   The number of cells needed in a battery is determined by the voltage requirements of the equipment it is to operate in relation to the average operating voltage per cell. When discharging at their normal rates, all types and sizes of nickel-iron cells have an average discharge voltage of 1.2 V per cell. In most cases the number of cells required for a particular application can be calculated on the basis of 1.2 V per cell. For example, an electrical industrial truck having a 36-V motor should have a 30-cell battery.

   The ampere-hour capacity of the cells required in the battery is determined by the rate of current consumed by the equipment and the length of time it is to be operated on a single charge of the battery. This time period, in the principal application in which the battery is the normal power supply for the equipment (cycle service), is usually the regular daily working period (in the majority of cases, one 8-hr shift). In standby applications, it is usually the maximum expected outage of the normal power supply.

   After the required ampere-hour capacity has been determined, a cell with 20 percent additional capacity should be selected. This safety factor should be considered adequate for contingencies, and assures that the battery will have ample capacity up to the end of its normal service life.

   In standby installations, it is important not only that the battery be of suitable voltage and capacity to carry the load satisfactorily during outages in the primary power supply but also that the power available for charging be ample to recharge the battery without undue delay following the intervals of discharge, and to maintain it in a satisfactory charged condition. How much power is required depends mainly upon how often, how long, and at what rates the battery is on discharge.

   If the discharge is infrequent, short, and at low rates, power sufficient only for continuous trickle charging would be sufficient. Emergency power-supply systems for call-bell signals and other equipment having small and infrequent current amounts are examples.

   On the other hand, when the discharge is frequent or prolonged, especially at relatively high rates, sufficient power may be needed to charge the battery at an average of its full normal rate if it is to be maintained at a satisfactorily high state of charge.

Operation. The required charging voltage varies according to the method of charging employed and ranges from approximately 1.50 to 1.55 V per cell for trickle charging to 1.84 V per cell or more for charging at normal to high rates. The number of ampere-hours required to charge the battery fully is equal to the number of ampere-hours previously discharged, plus an overcharge factor which averages approximately 25 percent. Charging at an average of the normal rate of the battery usually gives the best overall results and is generally recommended.

   For rating purposes, a discharged battery is defined as one that has been discharged to the equivalent of 1 V per cell at the normal rate. This usually represents the lower limit of the range in voltage needed for fully satisfactory operation of the equipment for which the battery supplies power. It is not necessary, however, that the discharge be stopped at this or any other prescribed limit if further output at a low voltage can be utilized. This will not harm the battery. The temperature rise is the principal limitation on charge rates. Any rate is safe as long as it does not result in raising the electrolyte temperature above 115°F (46°C).

   Boost charging or supplementary charging at high rates during brief periods of idleness is sometimes useful as an emergency measure only in order to obtain more than the usual amount of work from a battery that is regularly cycled. Regular or frequent boosting is an indication the battery is of inadequate capacity for the work and is not recommended. It is not a substitute for a correctly applied battery. The following information is useful as a guide to determine the amount of current that should be employed in a boosting operation:

• Five times the normal rate for 5 min
• Four times the normal rate for 15 min
• Three times the normal rate for 30 min
• Two times the normal rate for 60 min

When a battery is being boosted, it is useful to take temperature readings of the electrolyte in the cells nearest the center, or warmest part, of the battery and to stop the charge if the temperature rises to 115°F (46°C). Any frothing at the filler openings is also an indication that boosting has gone too far and should be discontinued immediately. A battery that has been discharged need not be immediately recharged. No injurious reactions will take place if charging is delayed.

Charging Batteries That Are Cycled. Sources of dc power for charging batteries that are cycled

may be:

• Direct-current power lines
• Motor generators which accept either dc or ac primary power
• Rectifiers which accept ac primary power

To ensure maximum cooling, be sure that the battery is exposed to free-air circulation while it is on charge. If it is charged in an enclosure of any kind, such as a battery box of an industrial truck or locomotive, open the cover.

Charging Standby-Power Batteries. At normal temperatures, trickle charging voltage is usually between 1.50 and 1.55 V per cell, and 1.70 to 1.72 V per cell is usual for constant-potential charging at an average of approximately the normal rate.

   But these values are not exact; they vary with the age of the battery, the specific gravity of the electrolyte, the temperature, and other conditions. With this in mind it is necessary, therefore, to adjust the voltage on the basis of ammeter readings, not voltmeter readings. Voltmeter readings are useful, however, in determining when a battery is fully charged. Stabilization of the voltage at the battery terminals for about 1/2 hr while current is flowing through the battery at a constant rate is a trustworthy indication that the battery is fully charged.

   For any given charge rate the voltage necessary at the battery terminals varies with the electrolyte temperature. Therefore, for batteries exposed to seasonal changes, a higher setting would be needed in the winter than in the summer.

   It is important that the rates employed for trickle charging result in overcharging rather than undercharging. In practice it is virtually impossible to arrive at a charge rate which will result in precisely the amount of input required, especially since the output usually tends to vary from day to day. In the interest of consistently high dependability of operation, the best practice is to use rates ample for the maximum rather than the average or minimum requirements, especially since any overcharging that may result at low trickle rates is not harmful. If a case should arise in which a battery on trickle charge should undergo a prolonged discharge, set the voltage for a higher rate until voltage stabilization occurs, indicating that the battery is again fully charged.

   For batteries furnishing large amounts of power each day and requiring correspondingly more input, the best voltage setting is one that results in the highest average rate during the charging interval consistent with rates that are not excessive after the batteries are fully charged, that is, rates that will not result in raising the electrolyte temperature above 115°F (46°C). As long as these rates are not excessive, it is desirable to adopt settings which on an average will tend to result in a slight amount of overcharging rather than undercharging.

Watering. During the operation of the battery, water is dissipated from the electrolyte chiefly as the result of gassing during charge. This loss must be made up by adding distilled or approved water, using as a guide the recommended and minimum levels suggested by the manufacturer. (Caution: Do not add electrolyte, as this will raise the specific gravity of the solution; if the specific gravity is allowed to exceed 1.230 in standard cells or 1.215 in high, wide cells, the battery may be damaged.)

   The best time to add water in batteries that are cycled is just before charging; then the gassing during charge will mix the solution. Never add water during or immediately after charging. This avoids getting a false solution-level reading, caused by gassing during and immediately after charging, which makes it virtually impossible to add the correct amount of water.

Maintenance

   Putting New Batteries into Service. Always unpack and inspect batteries immediately on arrival so that in case of damage, a claim may be filed promptly with the transportation company. Test the height of the electrolyte in a few cells to see if any has been spilled. If the electrolyte is below the recommended level but is above the plate tops or can be seen with a flashlight, raise it to the recommended level with distilled water. If it is so low that it cannot be seen, raise to the recommended level by adding refill solution.

   Batteries are shipped in a charged condition unless otherwise ordered; so they may be put into service immediately on arrival. In case a charged battery stands idle for a period from a week to a month, charge it at an average of its normal rate for 2 or 3 hr before putting it into service. If you expect to hold a battery idle for more than a month, order it shipped discharged and store it in that condition. Then when you are ready to put it into service, give it a 15-hr charge at its normal rate. A charged or partially charged battery left standing idle for more than a month is likely to become sluggish. Before placing such a battery in service, charge it 15 hr at normal rate; then discharge it at normal rate to an average of 1 V per cell. If it does not deliver normal rate for at least 5 hr before 1 V per cell is reached, it may need further cycling.

   Batteries assembled in cradles or demountable boxes have their cell-to-cell connectors in place so that all that is necessary to complete the assembly is to apply the tray-to-tray jumpers. If a battery is assembled in trays only and consists of more than one tray, first arrange the trays so as to ensure correct polarity. Then apply the jumpers. The necessary jumpers and tools (pole-nut wrench and lugdisconnecting jack) are usually included with each shipment.

   The lugs on the ends of the jumpers are provided with an inside taper that corresponds to the taper on the poles of the cells. Be sure both these contact surfaces are clean. Remove any oil, grease, or dirt that may stick to them, using a clean cloth. If an abrasive is necessary, use 00 sandpaper or 00 emery cloth; never use a file or other cutting tool that might score or abrade the contact surfaces. Then slip the jumpers into place. If the lugs do not fit exactly on the poles, bend the jumpers until they do; never hammer or force them on. After the lugs are in place, grease the pole threads slightly. Then apply the hexagonal pole nuts.

   After completing the connections, you can check their tightness by putting the battery on charge or discharge at its normal rate for 15 or 20 min. Any loose or dirty connections will cause excessive heating of jumper lugs, which will be readily perceptible to the touch. (Caution: Disconnect battery from charging circuit before touching jumper or connector lugs.) Remove any such jumpers, clean the contact surfaces of the lugs and poles, and reapply. Check the tightness of the connector lugs in the same manner. Connectors are removed and applied in the same manner as jumpers. By having all connections clean and tight, you will avoid unnecessary voltage drop in the battery circuit.

   Cleaning. Keeping a battery clean is not merely a matter of good housekeeping but is also an assurance of good performance and life. By keeping the cell tops and connectors clean, you lessen the risk of getting impurities into the cells when you open the filler caps to add water. By keeping dirt from accumulating below or between the cells, you reduce the possibility of ground, especially if the battery is exposed to dampness.

   Batteries assembled in cradles or demountable boxes are best cleaned when supported so that dirt can be blown out through the bottom. Use a wet steam jet followed by an air blast to blow off any accumulated moisture. Clean the cell tops and connectors first; then blow out any dirt that may become lodged between cells. Be sure all filler caps are closed so that no dirt can get into the cells. Wear goggles when using the steam jet and air blast.

   Batteries assembled in trays only can be cleaned by wiping cell tops, connectors, and jumpers occasionally with a wet cloth. In this way you can avoid letting dirt fall down into the spaces between the cells, but if you see dirt beginning to accumulate there, remove the trays to a floor drain or other suitable place and clean them by wet steam or warm water followed by an air blast as already described. Be sure cells and trays are dry before reassembling, also that the contact surfaces of the cell terminals and jumper lugs are clean and that all connectors are tight and of correct polarity.

   Inspect the cells for any necessary attention. Make sure the filler caps, hinge bands, and lid springs are in proper alignment to ensure free operation and correct seating of the valves. To prevent contamination of the electrolyte, it is just as important to maintain the valves so that they seat properly as it is to keep the filler caps normally closed. Screw down the gland caps of any cells showing evidence of leakage around the stuffing-box assembly. Use the special wrench available for the purpose, and be careful not to damage the gland caps.

Cycling. A battery that is not kept in regular use or is used only intermittently may become sluggish and deliver less than the capacity of which it is capable. This can be corrected by cycling the battery as follows:

1.Charge the battery if it is not already charged.

2.Discharge through a resistance that can be varied to keep the rate at normal until the potential of the battery falls to the equivalent of 0.5 V per cell (15 V for a 30-cell battery, etc.).

3.Short-circuit each tray, and let stand until the resulting heat is dissipated and the electrolyte cools to not more than 5°F above room temperature.

4.Water as necessary to bring the electrolyte to the recommended level, and charge at normal rate

for 15 hr.

5.Discharge at normal rate, and keep a record of the time until the potential of the battery falls to the equivalent of 1 V per cell.

   Except while the battery is short-circuited, keep the electrolyte temperature below 115°F (46°C). Take the voltage readings only while current at normal rate is flowing. Usually one such cycle is sufficient, although if the battery still appears sluggish, another cycle or two may bring further improvement.

   A discharge at normal rate for 5 hr before the equivalent of 1.00 V per cell is reached indicates full rated capacity. If less capacity is indicated, continue the discharge as in step 1 and repeat steps 2, 3, and 4.

   Laying Up. In case a battery is to be laid up for a month or more, discharge and short-circuit as described in operations 1 and 2 under Cycling. Check height of electrolyte solution, and add water if necessary to raise to correct level. Then store in a clean, dry place. Batteries may be left standing idle in this condition indefinitely without injury. When the battery is to be returned to use, charge it at normal rate for 15 hr. If it was laid up for a year or more, follow this charge by a discharge at normal rate to an average rate of 1 V per cell; then follow with operations 1, 2, 3, and 4 under Cycling. Also inspect the cells for any necessary attention as described under “Cleaning.”

   Renewing Electrolyte. When a battery is new and fully charged, the electrolyte has a specific gravity of approximately 1.200 at 60°F (15.5°C) if thoroughly mixed and at the recommended level. During use of the battery, the electrolyte tends to gradually weaken and must be renewed if its specific gravity falls to between 1.160 and 1.170. Do not operate a battery with an electrolyte of a gravity below 1.160. (Caution: Do not attempt to raise the specific gravity of weakened electrolyte by adding solution.)

   To test the electrolyte for specific gravity, use a hydrometer. Take readings only when the electrolyte has been thoroughly mixed by charging, and wait 1/2 hr or more after the charge has been completed to allow for dissipation of gas. Using a thermometer and a test tube, check the temperature and the height, and correct for any variation from 60°F (15.5°C) and the recommended level.

   To renew the solution, proceed as follows:

1.Discharge, short-circuit, and cool the battery as described in operations 2 and 3 under “Cycling.”

2.Pour out the old solution.

3.Fill immediately with standard renewal solution.

4.Charge at normal rate for 15 hr.

   For ease in pouring out the old solution, disconnect the jumpers so you can do it one tray at a time. Avoid splashing. Do not shake or rinse; just tip the trays so that the old solution will run out. The electrolyte is injurious to wood, brass, copper, lead, aluminum, and zinc. Short lengths of scrap 2 by 4s or similar timbers can be used to support the trays while they are tipped over.

   Always keep in mind that the solution is injurious to the skin and clothing. Wear rubber gloves, goggles, and preferably also a rubber apron. If, in spite of these precautions, any solution should be splashed or spilled on the skin or clothing, wash it away immediately with plenty of water. As a further precaution, it may be well to keep available a supply of 4 percent sterile boric acid solution and an eyecup for additional treatment of the skin and eyes. Meanwhile, arrange the containers of standard renewal solution so that you can refill immediately. Do not let the cells stand without solution. The containers may be elevated, and the solution poured into the cells through a hose, or for small cells and small containers, the solution may be poured in directly from the container through a funnel.

   Replacing Spilled Electrolyte. An accident which overturns a battery rarely causes damage because of the steel-cell construction but may spill electrolyte solution from the cells. To replace spillage use standard refill solution. Standard renewal solution may also be used in an emergency if diluted with pure distilled water to a gravity of 1.215 at 60°F (15.5°C); an easy way to do this is to mix 1 part of water by volume with 5 parts of renewal solution by volume.

   If you have no electrolyte solution on hand, the best thing to do depends on how much solution was spilled. If the solution left in the cells is still above the plate tops or can be seen by a flashlight after the battery has been turned right side up, merely add water and continue the battery in service. If so little solution remains in the cells that it cannot be seen with a flashlight, take the battery out of service, make sure all filler caps are closed in order to keep out impurities, and wait until you can obtain a supply of refill solution.

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