Linden D., Reddy T.B. (eds.) Handbook of batteries

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PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.17

28.4.10 Cycle Life

The cycle life is usually measured to the point when the battery will not deliver more than

a given percentage (usually 60 to 80%) of its rated capacity. Sealed nickel-cadmium batteries

have long cycle lives. Under controlled conditions over 500 cycles can be expected on a full

discharge, as illustrated in Fig. 28.21. On shallow discharges considerably higher cycle life

can be obtained, as shown in Fig. 28.22. Cycle life is also very dependent on the many

conditions to which the battery has been exposed, including charge, overcharge, and dis-

charge rates, frequency of cycling, the temperatures to which the battery has been exposed,

and battery age, as well as battery design and battery components. Specially designed bat-

teries, such as those using alkali-resistant materials, are also manufactured, which have longer

life, particularly at the higher temperatures (see Sec. 28.6).

FIGURE 28.21 Cycle life of sealed nickel-cadmium batteries at 20⬚C. Cycle conditions:

charge—0.1C ⫻ 11 h: discharge—0.7C ⫻ 1 h. Capacity-measuring conditions: charge—

0.1C ⫻ 16 h; discharge—0.2C, end voltage—1 V.

FIGURE 28.22 Cycle life of sealed nickel-cadmium batter-

ies at shallow discharge.

28.18 CHAPTER TWENTY-EIGHT

28.4.11 Life Expectancy and Failure Mechanisms

The useful life of a nickel-cadmium battery can be measured either in terms of the number

of cycles before failure or in units of time. It is virtually impossible to know all the detailed

information necessary to make any kind of accurate prediction of battery life in a given

application. The best that can be provided is an estimate based on laboratory test data and

ﬁeld experience or extrapolation of accelerated test data.

Basically, failure of a battery occurs when it ceases to operate the device, for whatever

reason, at the prescribed performance level, despite the possibility that the battery may still

be useful in another application with less demanding requirements.

Failure of a nickel-cadmium battery can be classiﬁed into two general categories: revers-

ible and irreversible failure. When a battery fails to meet the speciﬁed performance require-

ments but can, by appropriate reconditioning, be brought back to an acceptable condition, it

is considered to have suffered a reversible failure. Permanent or irreversible failure occurs

when the battery cannot be returned to an acceptable performance level by reconditioning

or any other means.

Reversible Failures

Voltage Depression (Memory Effect). A sealed nickel-cadmium battery may suffer a

reversible loss of capacity when it is cycled repetitively on shallow discharges (discharge

terminated before its full capacity is delivered) and recharged. For example, as shown in

Fig. 28.23, if a battery is cycled repetitively, but only partially discharged and then recharged,

the voltage and delivered capacity will gradually decrease with cycling (curves 2 representing

repetitive cycling). If the battery is then fully discharged (curve 3), the discharge voltage is

depressed compared to the original full discharge (curve 1). The discharge proﬁle may show

two steps and the battery may not deliver the full capacity to the original cutoff voltage.

This phenomenon is known as ‘‘voltage depression.’’ It also is referred to as ‘‘memory

effect,’’ as the battery appears to ‘‘remember’’ the lower capacity of the shallow discharge.

Operation at higher temperatures accelerates this type of loss.

The battery can be restored to full capacity with a few full discharge-charge reconditioning

cycles. The discharge characteristics on the reconditioning cycles are illustrated in Fig. 28.23

(curves 4 and 5).

FIGURE 28.23 Voltage depression and subsequent recovery.

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.19

The voltage drop occurs because only a portion of the active materials is discharged and

recharged during shallow or partial discharging. The active materials that have not been

cycled, particularly the cadmium electrode, may undergo a change in physical characteristics

and an increase in resistance. The effect has also been ascribed to structural changes in the

nickel electrode.

Subsequent cycling restores the active materials to their original state.

The extent of voltage depression depends on the depth of discharge and can be avoided

or minimized by the selection of an appropriate end voltage. Too high an end voltage, such

as 1.16 V per cell, terminates the discharge prematurely. (A high end voltage should be used

only if an extended cycle life is desired and the lower capacity can be tolerated, as in some

satellite applications.) A small voltage depression may be observed if the discharge is ter-

minated between 1.16 and 1.10 V per cell. The extent of the depression is dependent on the

depth of discharge, which is also rate-dependent. Discharging to an end voltage below 1.1 V

per cell should not result in a subsequent voltage depression. Discharging to too low an end

voltage, however, should be avoided, as discussed in Sec. 28.4.6.

This phenomenon varies with the design and formulation of the electrode and may not

be evident with all sealed nickel-cadmium batteries. Modern nickel-cadmium batteries use

electrode structures and formation processes that reduce the susceptibility to voltage depres-

sion, and most users may never experience low performance due to memory effect. However,

the use of the term ‘‘memory effect’’ persists, since it is often used to explain low battery

capacity that is attributable to other problems, such as ineffective charging, overcharge,

battery aging, or exposure to high temperatures.

Overcharging. A similar reversible failure can occur with long-term overcharging, par-

ticularly at elevated temperatures. Figure 28.24 shows the voltage ‘‘step’’ near the end of

discharge that can be induced by long-term overcharging. The capacity is still available, but

at a lower voltage than when it was freshly cycled. Again, this is a reversible failure; a few

charge and discharge cycles will restore normal voltage and expected capacity.

FIGURE 28.24 Typical discharge voltage proﬁle of sealed nickel-cadmium

batteries after long-term overcharge (dotted line) vs. 16-h charge, both at C

/ 10 rate.

28.20 CHAPTER TWENTY-EIGHT

Irreversible Failures. Permanent failure in nickel-cadmium batteries results from essen-

tially two causes: short-circuiting and loss of electrolyte. An internal short circuit may be of

relatively high resistance and will be evidenced by an abnormally low on-charge voltage and

by a drop of voltage as the battery’s energy is dissipated through the internal short circuit.

A short circuit may also be of such a low resistance that virtually all the charge current

travels through it or the battery electrodes are totally shorted internally.

Any loss of electrolyte will cause degradation in capacity. Charging at high rates, repeated

voltage reversal, and direct short-circuiting are ways that can cause loss of electrolyte through

the pressure relief device. Electrolyte can also be lost over a long period of time through

the battery seals, and capacity is lost in proportion to the reduction in electrolyte volume.

Capacity degradation caused by electrolyte losses is more pronounced at high discharge rates.

High temperature degrades battery performance and life. A nickel-cadmium battery gives

optimum performance and life at temperatures between 18 and 30

⬚C. Higher temperatures

reduce life by promoting separator deterioration and increasing the probability of short-

circuiting. Higher temperatures also cause more rapid evaporation of moisture through the

seals. These effects are all long-term; but the higher the temperature, the more rapid the

deterioration. Table 28.2 lists the recommended temperature limits for sealed nickel-cadmium

batteries.

TABLE 28.2 Operating and Storage Limits for

Sealed Nickel-Cadmium Batteries

Type

Temperature,

⬚C

Storage Operating

Button ⫺40 to 50 ⫺20 to 50

Standard cylindrical

⫺40 to 50 ⫺40 to 70

Premium cylindrical

⫺40 to 70 ⫺40 to 70

28.5 CHARGING CHARACTERISTICS

28.5.1 General Characteristics

Sealed nickel-cadmium batteries are usually charged by means of the constant-current

method. The 0.1C rate can be used and the battery is charged for 12 to 16 h (140%). At

this rate the battery can withstand overcharging without harm, although most sealed nickel-

cadmium batteries can be safely charged at the C /100 to C/ 3 rate. At higher charge rates

care must be taken not to overcharge the battery excessively or develop high battery tem-

peratures.

The voltage proﬁle of a sealed nickel-cadmium battery during charge at the C /10 and

C/ 3 rates is shown in Fig. 28.25. A sharp rise in voltage to a peak near the end of the charge

is evident.

The voltage proﬁle of a sealed nickel-cadmium batteries is different from the one for a

vented one, as illustrated in Fig. 28.26 The end-of-charge voltage for the sealed battery is

lower. The negative plate does not reach as high a state of charge as it does in the vented

construction because of the oxygen recombination reaction.

Constant-potential charging is not recommended for sealed nickel-cadmium batteries as

it can lead to thermal runaway. It can, however, be used if precautions are taken to limit the

current toward the end of charge.

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.21

FIGURE 28.25 Typical pressure, temperature, and voltage relationships of sealed

nickel-cadmium battery during charge.

FIGURE 28.26 Charge vs. voltage for sealed and vented nickel-cadmium

batteries at 25⬚C; 0.1C charge rate.

28.5.2 The Charge Process

The charge process is summarized in Fig. 28.27. Figure 28.27a plots the charge efﬁciency,

discharge energy (on subsequent discharge)

Charge efficiency ⫽

charge input energy

against total input energy. At the start of the discharge (area 1), the charge energy is con-

sumed by the conversion of the active materials into a chargeable form, and the charge

efﬁciency is low. In area 2 charging is most efﬁcient and almost all of the input energy is

used to convert the discharged active material into the charged state. As the battery ap-

proaches the charge state (area 3), most of the energy goes to the generation of oxygen, and

the charge efﬁciency is low.

Figure 28.27b presents the relationship of charge efﬁciency to charge rate. It shows that

the charge efﬁciency, as well as the output capacity, is lower at a lower charge rate.

The charge efﬁciency also depends on the ambient temperature during charge. This re-

lationship is shown in Fig. 28.27c. There is a decrease in capacity in the high-temperature

range due to a fall in potential for oxygen gas generation at the positive electrode.

The principle of the sealed battery is based on the ability of the negative electrode to

recombine this oxygen gas and prevent the buildup of internal gas pressure. The capacity

for this recombination is limited. Hence the maximum charge rate that can be tolerated is

the one that keeps the rate of oxygen generation below the gas recombination rate so that

the internal gas pressure does not build up excessively.

28.22 CHAPTER TWENTY-EIGHT

Overcharging, at rates beyond the ability of oxygen recombination or heat dissipation,

can result in failure. ‘‘Fast’’ charging methods can be used successfully, but a means must

be provided for monitoring and terminating the charge before excessive overcharging occurs.

Temperature rise, voltage, or pressure can be monitored and used effectively as a cutoff.

FIGURE 28.27 Charge process of sealed nickel-cadmium bat-

teries. (a) Charge efﬁciency at 20⬚C. Charge—0.1C ⫻ 16 h;

discharge—0.2⬚C; end voltage—1 V. (b) Charge efﬁciency vs.

charge rate at 20⬚C.

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.23

FIGURE 28.27 (c) Charge efﬁciency vs. ambient temperature

during charge at 0.1C rate (Continued ).

28.5.3 Voltage, Temperature, and Pressure Relationships

Figure 28.25 also shows the relationship of voltage, temperature, and pressure of a typical

sealed battery during charging at the C /10 and C /3 rates. The voltage increases gradually

during the charge until the battery is about 75 to 80% charged. The voltage then rises more

sharply due to the generation of oxygen at the positive electrode. The temperature remains

relatively constant during the early part of the charge as the charge reaction is endothermic.

It then rises as the battery reaches the overcharge state due to the heat generated by the

oxygen recombination reaction. Similarly the internal pressure remains low until the battery

goes into the overcharge condition, when most of the current is used to produce oxygen and

the pressure rises. Finally, as the battery reaches full charge, the voltage drops because of a

decrease in the battery’s internal resistance due to the increase in the battery temperature.

This drop in voltage can be used effectively in a control circuit to terminate the charge.

As shown, when the battery is overcharged at acceptable rates, the pressure and temper-

ature tend to stabilize. These steady-state conditions are governed by such factors as ambient

temperature, overcharge rate, heat transfer characteristics of the cell and battery, cell design

and components such as the separator, recombination capability of the negative electrode,

and the resistance of the cell and battery. Charging at higher rates, such as the C /3 rate

compared to the C/ 10 rate, results in higher temperatures and internal pressures. At higher

charge rates, temperature and pressure will rise more signiﬁcantly, particularly if the oxygen

recombination rate is exceeded. Because of the possibility of venting and other deleterious

effects on battery performance due to these high temperatures and pressures, it is necessary

to terminate the charge before these conditions are reached.

28.5.4 Voltage Characteristics during Charge

The voltage proﬁle of a sealed nickel-cadmium battery during charge at various charge rates

at 20

⬚C is shown in Fig. 28.28. The charge voltage also depends on temperature, as shown

in Fig. 28.29. The voltage and voltage peak decrease with a rise in temperature. Charging

at temperatures between 0 and 30

⬚C is best for sealed batteries. At lower temperatures the

voltage increases, recombination of oxygen is slower, and the internal gas pressure tends to

increase. Charging rates must be reduced. Above 40

⬚C the charging efﬁciency is low, and

higher temperatures cause battery deterioration.

28.24 CHAPTER TWENTY-EIGHT

FIGURE 28.28 Voltage proﬁle during charge at various charge

rates at 20⬚C.

FIGURE 28.29 Voltage proﬁle during charge at 0.1C rate at

various temperatures.

28.5.5 Charge Methods

There are a number of different methods for charging sealed nickel-cadmium batteries. The

standard method is a quasi-constant current charge at a relatively low rate. ‘‘Fast’’ charge

methods are becoming more popular in order to reduce the time required for charging. A

control circuit is needed when charging at these high rates to cut off the charge or reduce

the charge current at the completion of charge. Figure 28.30 shows the voltage and current

proﬁles during charge for each of the charge control methods. (Also see Sec. 5.5.)

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.25

FIGURE 28.30 Methods for charging sealed nickel-cadmium batteries and charge control. (a) Sem-

iconstant current (V

—battery voltage; I

—charge current; t—time). (b) Timer control ( t

—timer start;

—timer count up). (c) Temperature detection ( T—battery temperature). (d ) ⫺⌬V detection. (e) Trickle

charge. (Courtesy of Panasonic Industrial Co., Division of Matsushita Electric Corp. of America.)

Standard Method. (Fig. 28.30a). This is the simplest method, which uses a relatively in-

expensive circuit, controlling the charge current by inserting a resistance between the DC

power supply and the battery. The battery is charged at a constant current at a low (C/10)

rate so that the generation of oxygen is below the recombination rate. This rate also limits

the temperature rise. Excessive overcharge should be avoided. The battery should be charged

to about 140 to 150% charge input.

28.26 CHAPTER TWENTY-EIGHT

Timer Control. (Fig. 28.30b). For moderate charge rates a timer can be used to cut off the

charge or reduce the charge current to the trickle charge level. This is a relatively inexpensive

control device and suitable for applications where the battery is usually fully discharged

before charging. It is not suitable for applications where the battery is frequently charged

without prior deep discharging as this could result in excessive overcharge. A thermal cutoff

control should be used when charging at rates higher than the C /5 rate or without deep

discharging to prevent the battery from reaching high temperatures.

Temperature Detection. (Fig. 28.30c). This control system uses a sensor to detect the

temperature rise of the battery and terminate the charge. A thermostat or thermistor is used

as the detection device, and the detecting temperature is usually set at 45

⬚C. It is important

that the sensor be located so that it can accurately determine the battery’s temperature.

Charging in high ambient temperatures can result in an insufﬁcient charge while low ambient

temperatures may result in overcharge. The cycle life with this method may be shorter than

with the

⫺⌬V method or peak voltage methods as the battery could be subjected to more

overcharge.

Negative Delta V (⫺⌬V ). (Fig. 28.30d ). This is one of the preferred charge control systems

for sealed nickel-cadmium batteries. The drop in voltage of the battery is detected after the

battery voltage has reached its peak during charge. The signal can be used to terminate the

charge or reduce the charge current to a trickle charge. The method provides a complete

charge regardless of ambient temperature or residual capacity from the previous charge. A

value of 10 to 20 mV per cell is usually used for the control. The method is not suitable

for charging below the 0.5C as the

⫺⌬V value is too low to be detectable.

Trickle and Float Charge. (Fig. 28.30e). Trickle charge systems are used in two different

situations (1) in a standby power application where the battery is on continual charge to

maintain it in a state of full charge (compensating for self-discharge) until it is connected to

the load when the prime power fails, and (2) as a supplementary charge after the termination

of rapid charging. Charging is at the 0.02 to 0.05C rate, depending on the frequency and

depth of discharge. A periodic discharge every six months followed by a charge is advisable

to ensure optimum performance.

28.6 SPECIAL-PURPOSE BATTERIES

Special-purpose sealed nickel-cadmium batteries are manufactured with speciﬁcally designed

characteristics, overcoming some of the limitations of standard batteries, to meet the require-

ments for certain applications. Manufacturers’ recommendations should be followed because

of the speciﬁc performance characteristics of these batteries.

28.6.1 High-Capacity Batteries

These batteries incorporate design features, such as nickel foam substrate positive plates,

pasted negative electrodes, thin-walled battery containers, and increased amounts of active

material. These changes result in a 20 to 40% increase in capacity. These batteries are also

designed with improved oxygen recombination capability and are capable of being charged

at the 0.2C rate or less without control. They are capable of fast 1-h charging using

⫺⌬V

charge control. Figure 28.31 compares the discharge characteristics of the high-capacity

battery with those of a standard battery. Figure 28.32 shows the relationship of battery

capacity and discharge current for the two designs.