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

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PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.7

FIGURE 28.6 Voltage proﬁle of nickel-cadmium battery in a typ-

ical charge/ discharge cycle.

28.4.2 Discharge Characteristics

Typical discharge curves for the cylindrical battery at 20⬚C at various discharge loads are

shown in Fig. 28.7. The ﬂat voltage proﬁle, after the initial voltage drop, is characteristic.

The capacity that can be obtained from a battery is dependent on the rate of discharge,

the voltage at which discharge is terminated, the discharge temperature, and the previous

history of the battery. Figure 28.8 shows the percentage of rated capacity delivered during

discharges at various rates and temperatures. The midpoint voltage during discharge de-

creases as the discharge rate increases (see Fig. 28.11). If the battery were allowed to dis-

charge to a lower cutoff voltage, a greater percentage of the C/ 5 rate capacity will be

obtained. However, batteries or batteries should not be discharged to too low a cutoff voltage

as the battery may be damaged (see Sec. 28.4.6).

FIGURE 28.7 Constant-current discharge curves for sealed nickel-cadmium

batteries at 20⬚C, charge 0.1C,16h.(Courtesy of Sanyo Energy Corp.)

28.8 CHAPTER TWENTY-EIGHT

FIGURE 28.8 Percent of C /5-rate capacity vs. discharge

rate to 1.0-V cutoff for typical sealed nickel-cadmium battery.

28.4.3 Effect of Temperature

The sealed nickel-cadmium battery is capable of good performance over a wide temperature

range. Best operation is between

⫺20 and ⫹30⬚C, although usable performance can be

obtained beyond this range. The low-temperature performance, particularly at high rates, is

generally better than that of the lead-acid battery but usually inferior to that of the vented

sintered-plate battery. The reduction in performance at low temperatures is due to an increase

in the internal resistance. At high temperatures, the loss can be due to a depressed operating

voltage or to self-discharge.

Figure 28.9 shows some typical discharge curves of the sealed battery at various tem-

peratures at the 0.2C and 8C rates; Fig. 28.10 shows typical discharge curves at

⫺20⬚C. A

ﬂat discharge proﬁle is still characteristic, but at a lower operating voltage than at room

temperature. Figure 28.11 shows the effect of temperature on the midpoint voltage. Ambient

temperatures signiﬁcantly above or below 20 to 25

⬚C have a depressing effect on the average

discharge voltage.

The effect of temperature and discharge rate on the capacity of a sealed nickel-cadmium

battery is shown in Fig. 28.12. These data are typical of standard batteries. Manufacturers

should be contacted to obtain performance characteristics of speciﬁc batteries.

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.9

FIGURE 28.9 Constant-current discharge curves of sealed nickel-cadmium batteries at var-

ious temperatures. (a) 0.2C discharge rate. (b)8C discharge rate.

28.10 CHAPTER TWENTY-EIGHT

FIGURE 28.10 Constant-current discharge curves of sealed nickel-

cadmium batteries at ⫺20⬚C.

FIGURE 28.11 Midpoint discharge voltage vs. rate at various tem-

peratures for sealed nickel-cadmium batteries, 1-V cutoff.

FIGURE 28.12 Percentage of rated capacity vs. temperature at different discharge

rates for typical sealed nickel-cadmium batteries, 1.0-V cutoff.

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.11

28.4.4 Internal Impedance

The internal impedance of a battery is dependent on several factors, including ohmic resis-

tance (due to conductivity, the structure of the current collector, the electrode plates, sepa-

rator, electrolyte, or other features of the battery design), resistance due to activation and

concentration polarization, and capacitive reactance. In most cases the effects of capacitive

reactance can be ignored. Polarization effects are dependent in a complicated way on current,

temperature, and time; they decrease with increasing temperature (see Chap. 2). The effect

may be negligible for pulses of short duration, that is, less than a few milliseconds.

The nickel-cadmium battery is noted for its low internal resistance due to the use of thin

and large-surface-area plates with good electrical conductivity, a thin separator with good

electrolyte retention, and an electrolyte having a high ionic conductivity. During discharge,

the activation and concentration polarization effects are negligible, at least at low and mod-

erate rates, and the internal resistance of the battery, and the discharge voltage, remains

relatively constant from the state of full charge to the point where almost 90% of the capacity

has been discharged. At that point the resistance increases due to the conversion of active

materials in the electrode plates, which tends to lower electrical conductivity. Figure 28.13

shows the change in internal resistance with the depth of discharge for two batteries of

different size and capacity. Figure 28.14 illustrates the effect of temperature. The internal

resistance increases as the temperature drops because the conductivity of the electrolyte and

other components is lower at the lower temperatures.

With use over time, a nickel-cadmium battery gradually loses capacity, resulting in a

gradual increase in internal resistance. This is caused by gradual deterioration of the separator

and electrodes and by loss of liquid through the seals, which changes the electrolyte con-

centration and level. The net effect is an increase in internal impedance.

FIGURE 28.13 Resistance vs. state of charge at

20⬚C, discharged at 0.2C rate, for sealed nickel-

cadmium batteries. a—AA-size battery, b—sub-C-

size battery. (Typical for sintered-plate electrode type

batteries.)

FIGURE 28.14 Resistance vs. temperature for fully

charged sealed nickel-cadmium batteries. a—AA-size

battery, b—sub-C-size battery. (Typical for sintered-

plate electrode type batteries.)

28.12 CHAPTER TWENTY-EIGHT

28.4.5 Service Life

The service life of a sealed nickel-cadmium battery, normalized to unit weight (kilograms)

and size (liters), at various discharge rates and temperatures is summarized in Fig. 28.15.

The curves are based on a capacity, at the C/ 5 discharge rate at 20

⬚C, of 30 Ah /kg and 85

Ah/ L, reﬂecting the performance of standard-type sealed cylindrical batteries. Manufacturers

should be contacted for performance characteristics of speciﬁc batteries.

FIGURE 28.15 Service life of sealed nickel-cadmium battery at various

constant-current discharge rates and temperatures; end voltage at 1.0 V.

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.13

FIGURE 28.16 Discharge of sealed nickel-cadmium battery

showing polarity reversal.

28.4.6 Reversal of Voltage Polarity

When three or more batteries are series-connected, the lowest-capacity battery can be driven

into voltage reversal by the others. The larger the number of batteries in series, the greater

the possibility of this occurring. During reversal, hydrogen may evolve from the positive

electrode and oxygen from the negative. Figure 28.16 shows the complete discharge curve

of a battery, including polarity reversal. Section 1 is the normal period of discharge with

active materials remaining on both electrodes. Section 2 represents the period when the

discharge has been extended and all of the active material on the positive electrode has been

discharged and hydrogen gas is generated at this electrode. Active material still remains on

the negative electrode and its normal discharge reaction continues. The battery voltage varies

with the discharge current, but stays at about

⫺0.2 to ⫺0.4 V. In Section 3 the negative

active material has been discharged and oxygen gas is generated at this electrode.

Continued discharging during polarity reversal will lead to high battery pressure and

opening of the safety vent. This then results in a loss of gas and electrolyte and breakdown

of the capacity balance of the positive and negative electrodes.

Some battery designs provide a limited amount of built-in protection against deep reversal

by adding a small amount of cadmium hydroxide to the positive electrode. The term used

for the material added to the positive electrode for reversal protection is ‘‘antipolar mass’’

(APM). When the positive electrode is completely discharged, the cadmium hydroxide in

that electrode is converted to cadmium, which, combining with the oxygen generated from

the negative electrode, depolarizes the positive, preventing hydrogen generation for a time.

This reaction occurs at about

⫺0.2 V. This reaction can sustain for only a limited time, after

which hydrogen is evolved from the positive electrode. Because hydrogen combines with

the battery materials to only a limited extent, repetitive battery reversal will gradually in-

crease a battery’s internal pressure, ultimately causing the battery to vent.

Discharging to the point of reversal should be avoided. In order to prevent voltage reversal

in any of the cells of a multicell battery, particularly with a series string of more than four

cells, the battery should not be discharged to a voltage below 0.8 V per cell. In applications

of multicell batteries where it is likely that the battery will be frequently discharged below

1.0 V per cell, a voltage-limiting device is recommended to avoid cell reversal.

28.14 CHAPTER TWENTY-EIGHT

28.4.7 Types of Discharge

As discussed in Sec. 3.2.3, a battery may be discharged under different modes (such as

constant resistance, constant current, or constant power), depending on the characteristics of

the equipment load. The type of discharge mode selected has a signiﬁcant impact on the

service life delivered by a battery in a speciﬁed application. The voltage proﬁles of a nickel-

cadmium battery discharged under the three different modes are plotted in Fig. 28.17. The

data are based on a discharge of a 650-mAh battery so that, at the end of the discharge (1.0

V per cell), the power output is the same for all modes of discharge. In this example the

power output is 130 mW. To discharge at 130 mW at 1.0 V, the constant-current discharge

is 130 mA (C/ 5 rate) and the constant-resistance discharge is 7.7

⍀. As shown, the longest

service life is obtained under the constant-power mode as the average current is the lowest

under this mode of discharge.

FIGURE 28.17 Discharge curves of AA-size (650-mAh) nickel-cadmium battery—constant power (⽧) vs.

constant current (䉱) vs. constant resistance (䡲).

PORTABLE SEALED NICKEL-CADMIUM BATTERIES 28.15

28.4.8 Constant-Power Discharge

The discharge characteristics of the nickel-cadmium battery under the constant-power mode,

at several different power levels, are shown in Fig. 28.18. These are similar to the data

presented in Fig. 28.7 for constant-current discharges, except that the performance is pre-

sented in hours of service instead of percent discharge capacity. The power levels are shown

based on the E-rate. The E-rate is calculated in a manner similar to calculating the C-rate,

but based on the rated watthour capacity. For example, for the E /5 power level, the power

for a battery rated at 780 mWh is 156 mW.

FIGURE 28.18 Constant-power discharge curves at various E rates for sealed nickel-cadmium bat-

teries at 20⬚C.

28.16 CHAPTER TWENTY-EIGHT

28.4.9 Shelf Life (Capacity or Charge Retention)

Nickel-cadmium batteries lose capacity during storage. The rate of this self-discharge is a

function of storage temperature and battery design. Figure 28.19 can serve as a guide for

the shelf life (capacity or charge retention) at several temperatures for typical standard type

nickel-cadmium sealed batteries. Speciﬁcally designed batteries, as discussed in Sec. 28.6,

may have considerably different charge retention characteristics. For example, the button

batteries designed for memory-backup application have signiﬁcantly better charge retention

characteristics than the standard lower-resistance higher-discharge-rate cylindrical batteries.

Sealed nickel-cadmium batteries can be stored in a charged or a discharged condition.

Except for extended storage at high temperatures, they can be restored to full capacity after

storage by recharging (two or three charge-discharge cycles). Figure 28.20 illustrates the

capacity recovery after prolonged storage at several temperatures. The recovery time may

be longer after high-temperature storage.

FIGURE 28.19 Capacity retention (shelf life) of standard type

sealed nickel-cadmium batteries.

FIGURE 28.20 Capacity recovery of standard type sealed nickel-

cadmium batteries, discharge after 2-year storage at 0.2C rate. Storage

temperatures: at 20⬚C(䡩); 35⬚C(●); and 45⬚C(⫻).