TM 116140203141
a maximum of 2 hours, the electrolyte level should be
b. Cell capacity decreases at temperatures lower
than 20F or higher than 100F.
-inch above the top of the plates.
c. Voltage level of nickel-cadmium batteries re-
f. The electrolyte level should be checked when the
mains essentially constant until approximately 90 per-
nickel-cadmium battery is fully charged and has been
cent of its capacity has been delivered.
at rest for a minimum of 30 minutes and a maximum
d. Nickel-cadmium batteries should be charged for
of 2 hours.
at least 125 percent of previously discharged ampere-
g. Refer to TM 11-6140-203-14-2 and TM
hours to be at approximately full charge state.
11-6140-203-14-3, respectively, for rest time limita-
e. Electrolyte is not visible from the top of the cells
tions for aircraft nickel-cadmium and nonaircraft
when the cell is at a low state of charge. When charged
nickel-cadmium batteries.
properly and rested for a minimum of 30 minutes and
Section Ill. FACTORS AFFECTING SERVICEABILITY
quired during an emergency. For example, during a
test being conducted on 19-cell, 34-ampere-hour, 24-
An important characteristic observed in nickel-cad-
volt nickel-cadmium batteries, 30 were given a capac-
mium batteries is temporary loss of capacity or "sleep-
ity test. The average capacity measured on 30 bat-
iness" (also referred to as the "memory effect"). When
teries was less than 15-ampere-hours, or less than half
this temporary loss occurs, the battery is unable to de-
of the rated capacity; two of the batteries delivered
liver the designed capacity. The loss of capacity is a re-
only 2-ampere-hours each. It is clear that the condition
sult of shallow discharge cycles. For instance, when a
of the batteries would not be ideal under emergency
battery is continually discharged, as for example, to a
conditions. To minimize the loss-of-capacity problem,
depth of only 20 percent of full charge, its voltage-
nickel-cadmium batteries should be serviced period-
time curve begins to drop abnormally near the 20-per-
ically and given a deep cycle discharge to 0 volt.
cent discharge point. The effect is that 80 percent of
the capacity has become temporarily inactive.
a. Causes. The loss-of-capacity effect is more com-
mon when recharging a battery across a constant po-
Thermal runaway is a condition in which the current
for a fully charged nickel-cadmium battery rises out of
tential bus, such as in aircraft, than when charging
all proportion to the impressed voltage.
with constant current. The loss of capacity is usually
an indication of an imbalance between the cells be-
a. Cause. Since runaway occurs in an overcharged
state after the battery is fully charged, the excess
cause of differences between individual cells in tem-
charging energy is dissipated as heat and electrolysis
perature, charge efficiency, and self-discharge rate.
Imbalance can be verified by a periodic check of indi-
of water. Continued overcharging under certain condi-
tions has the effect of reducing the internal battery re-
vidual cell voltages after the battery has shown a
sistance so that it draws a higher current from the im-
sharp rise in voltage while being charged by the con-
pressed voltage. As the temperature of the battery in-
stant current method, or after current has dropped
creases, the effective internal resistance continues to
and essentially stabilized during constant potential
decrease and the current becomes progressively
method. The individual voltage of each cell under full
greater. This process continues and eventually de-
charge should be 1.5 volts or more when the cell is at
stroys the battery unless it is properly protected by a
room temperature (about 70 F). A variation of more
cutout device.
than 0.10 volt between cells after completion of charge
b. Effect of Temperature and Voltage. Thermal
is an indication of imbalance and that the lower volt-
runaway primarily depends on temperature and charg-
age cells need to be additionally charged to reach
ing voltage; therefore, the higher the charging voltage,
equalization. Usually, a deep cycle discharge at the 10-
hour rate of 0 volt followed by a recharge of the bat-
the less the temperature required to effect the run-
away condition, or the higher the temperature, the less
tery is sufficient to equalize the cells. Figure 2-9
charging voltage required. Battery temperature
shows a battery discharge equalization fixture and a
should never exceed 120F.
list of available fixtures. Another cause of capacity
loss is a growth of the grain size of the active material
c. Effect of Battery Separator Material. Most ther-
mal runaways in the past have been caused by the
within the battery plates. This type of effect is nor-
breakdown of the cellophane, film separator, so that
mally associated with very low rates of discharge par-
oxygen generated on overcharge could reach the cad-
ticularly at high temperatures.
b. Consequences. The temporary loss-of-capacity ef-
mium electrode, thereby lowering its voltage and in-
fect should not be taken lightly. Even though a battery
creasing the current from a constant potential source.
may appear to be giving satisfactory performance, it
Plastic film separators such as "Permion" or "Celgard"
may deliver only a portion of its rated capacity if re-
used in many Army nickel-cadmium batteries since
