TM 116140203141
cells is the range from 0F to 100F, with the opti-
age and tone is necessary to prevent excessive over-
mum temperature range being 50F to 90F. An in-
charge while still providing full capacity. The charac-
crease or decrease in temperature from the optimum
teristics of the various charging methods follow. In
range causes a corresponding reduction in cell capac-
general, the constant potential charge provided by the
ity. Vented nickel-cadmium batteries, however, may
aircraft bus does not provide a full charge.
be discharged at temperatures as low as 65F if dis-
a. Constant Potential. Constant-potential (CP)
charge rates are limited. Figure 2-5 indicates available
charging applies a fixed voltage to the cell. The result-
battery capacity as a function of temperature. The in-
ing charge current is determined by the applied volt-
formation given in figure 2-5 should be considered
age level, the state-of-charge of the cell and its temper-
only a guide because cell design and battery size will
ature. Figure 2-6 shows a typical constant potential
affect low temperature discharge performance consid-
charging curve of a partially discharged battery in an
erably. Discharging at high rates for sustained periods
aircraft. As shown by the figure, very large amounts of
will cause large increases in battery temperature. Bat-
current are input for a short period of time until the
teries should be allowed to cool to approximately room
back voltage of the battery rises as it nears full charge.
temperature after high rate discharge before starting
Adjustment of the constant potential setting of the
to recharge.
aircraft bus with temperature is required to match the
characteristics of the battery.
28. Charge Characteristics
b. Modified Constant Potential. Frequently, battery
A nickel-cadmium battery is capable of accepting
charging equipment used for charging nickel-cadmium
charge at rates many times its rated capacity for a
batteries cannot tolerate the very high currents which
would be drawn in the initial stages of charging at con-
short period. However, once the battery is fully
charged the rate of current input must be limited to
stant potential. In such cases, current-limiting devices
or electrical resistances are commonly connected be-
prevent excessive loss of water, spewage of electrolyte
and overheating. Control of charging current or volt-
tween the battery and its charging source. Because
these practices supply a fixed voltage when the battery
is fully charged, but reduced voltage in the initial
stage of charging, they are called modified constant
potential charges. The limitation of charging current
during initial stages of charging current lengthens the
time required to completely charge the battery. Cur-
rent can also be limited by manually reducing the volt-
age for the first few minutes of charge. For constant
potential charging rates for aircraft nickel-cadmium
batteries, refer to chapter 5, TM 11-6140-203-14-2.
For constant potential charging rates for nonaircraft
nickel-cadmium batteries, refer to chapter 5, TM
11-6140-203-14-3. A typical modified constant po-
tential charging curve is shown in figure 2-6.
c. Constant Current. The constant current method
of charge is not limited by the back voltage of the bat-
tery and as such will provide a full charge irrespective
of battery temperature. A typical constant current
charging curve is shown in figure 2-7. The figure indi-
cates that the battery voltage rises sharply as a 100
percent state-of-charge is reached. Charging current is
still applied, however, to achieve the desired 20 to 40
percent overcharge. When automatic equipment is em-
ployed, charging can be done at high rates; i.e., the 1-
hour rate, until the voltage rise takes place. At this
point the constant current value can be reduced to the
3- to 5-hour rate in order to provide the required over-
charge at a low enough rate to prevent spewage of elec-
trolyte. The length of the overcharge period is normal-
ly timed. If automatic equipment is not accessible, the
entire charge must be run at a rate which will be safe
for overcharge. These charging rates are provided with
19-Cell Nickel-Cadmium Battery at Normal and Low Temperature.
each specific battery.
