TM116140203141
ess because of the decomposition of the water compo-
nent of the electrolyte as hydrogen at the negative
a. Individual nickel-cadmium cell nominal voltage is
plates and oxygen at the positive plates. The electro-
1.2 volts while actual voltage is between 1.20 and 1.30
lyte conducts current between the plates of opposite
volts.
polarity and reacts to produce the electrochemical
b. Active materials of a nickel-cadmium cell are
changes without producing any significant change in
nickel oxide on the positive plate and metallic cad-
its own overall chemical composition. Thus, the mea-
mium on the negative plate.
surement of specific gravity of the electrolyte gives no
c. Nickel-cadmium cell plates are separated by a
indication of the state of charge in a nickel-cadmium
thin porous multilaminate of plastic materials.
cell. Because the charging of the nickel-oxide positive
d. Electrolyte used in the nickel-cadmium cell is a
electrode is not 100 percent efficient, an overcharge of
31 percent by weight solution of potassium hydroxide
20 to 40 percent is usually required to bring the bat-
in distilled water.
tery to full capacity.
e. Specific gravity of the electrolyte is the same
b. Discharge. During discharge, the chemical reac-
whether the nickel-cadmium cell is charged or dis-
tion is reversed. The positive plates gradually return to
charged. However, the electrolyte level rises during
a state of lower oxidation, while the negative plates
charge and lowers during discharge.
simultaneously regain lost oxygen. During the dis-
f. The vent valve on the nickel-cadmium cell must
charge process, the chemical energy is released as elec-
be capable of venting excessive gases.
trical current through the discharge load. The rate at
g. The value of charging voltage used depends on
which the chemical energy is converted is determined
the number of cells connected in series.
principally by the resistance of the load to current
h. The value of charging current depends on the
flow. A load with a very low resistance could cause
ampere-hour rating of the storage battery.
such a high current that the cell might be damaged or
destroyed.
Section Il. OPERATING CHARACTERISTICS
d. Plateau Voltage. Plateau voltage is the cell
voltage during the relatively flat portion of the dis-
a. Voltage. Regardless of size or shape, the voltage
charge curve. This voltage depends on the discharge
of a single nickel-cadmium cell is nominally 1.20 volts
rate and, in most applications, is about 1.2 volts.
based on the plateau voltage at a 5-hour discharge rate
but a number of voltage levels other than this maybe
26. Capacity
encountered as the voltage varies depending on the
Sintered-plate Ni-Cad cells are capable of delivering
rate of charge or discharge, state of a charge, and
almost full capacity over a wide range of discharge
temperature. Battery voltages higher than this are ob-
rates and temperatures. However, battery capacity can
tained by connecting integral multiples of a suitable
be influenced by the primary factors of discharge rate
number of cells in series. Accordingly, a 6-volt nickel-
and temperature and secondary factors such as stor-
cadmium battery consists of five cells connected in
age, age, and previous charge history of the battery.
series; a 12-volt battery contains 10 cells similarly con-
a. Rated Capacity. The capacity rating of a nickel
nected, a 24-volt battery contains either 19 or 20 cells
cadmium battery is that capacity measured in ampere-
serially connected. The quantity of cells a specific bat-
tery contains is listed in chapter 2 of TM
hours at a specified rate of discharge, discharge tem-
perature, and voltage end point.
11-6140-203-14-2 or TM 11-6140-203-14-3 for
(1) Aircraft batteries are normally rated at the 1-
aircraft and nonaircraft nickel-cadmium batteries re-
hour discharge rate; i.e., a 30-Ah battery is discharged
spectively.
at 30 amperes, at 20C (68F) to an end point voltage
b. Open Circuit Voltage. Open circuit voltage is the
of 0.9 volt/cell. However, in the past other rating sys-
voltage of a cell without a load. At room temperature,
tems have also been employed. Thus, a battery which
this voltage is 1.25 to 1.35 volts for a fully charged
battery, depending on how long the cell has been
is rated at 30 Ah at the 1-hour rate could also have a
rated capacity at 34 Ah at the 2-hour rate (17A) to an
standing after charge. Elevated temperatures will
lower the measure open circuit voltage.
end-point of 1.0 volt/cell.
(2) Nonairaft batteries for communications use
c. Closed Circuit Voltage. Closed circuit voltage is
are normally rated at the 5 hour-discharge rate, i.e., a
the voltage of a cell with a load applied. The value de-
14 Ah battery is discharged at 2.8 amperes at 20C
pends on the size of the load, the electrical capacity of
(68F) to an end point voltage of 1.0 volt.
the cell, the temperature, and the length of time on
b. Actual Capacity.
discharge.
