Functioning of the BB-5421U

TM 11-6140-224-13&P

If no flame is present allow battery to cool and ventilate battery box. If necessary a water spray may be used to

cool the battery box.

If flame is present, use a C02 fire extinguisher.

3-6. Functioning of the BB-5421U

The application of electrical energy to the storage battery (charging) causes the storage of a corresponding amount of

chemical energy within its plates; the storage of chemical energy is accomplished through changes in the composition of

the plate materials. When an electrical load is connected to the storage battery (discharging), the stored chemical energy is

converted into a flow of electrical current through the load. In a sealed nickel-cadmium storage battery the specific gravity

of the electrolyte cannot be measured to give an indication of the state of charge.

Charging. When the charging current (electrical energy) is applied to the storage battery, the cadmium-oxide

material of the negative plates gradually becomes metallic cadmium. The nickel oxide material of the positive plates is

brought to a higher state of oxidation. These changes (in both sets of plates) continue as long as the charging current is

applied. Toward the end of this process, the cell generates gas because of the evolution of oxygen at the positive plates. At

low rates of overcharge, i.e., 200mA, all the oxygen generated at the positive reacts with the negative electrode to maintain

the composition of the components of the cell unchanged. The steady state pressure reached within the cells is very low.

At higher rates of overcharge, i.e., 3A, enough pressure can be rapidly generated to cause cell venting if the charge is not

stopped soon enough after overcharge and oxygen generation begins. In the BB-542/U the amount of overcharge is

controlled by measuring the pressure in 2 of the cells of the battery and automatically terminating the charge once a

pressure of approximately 10 psig is reached in one of the two control cells. When the battery stands, its internal pressure

drops so that the pressure switches will cycle the charging current off and on If the battery is not removed from the

charging source.

Discharging. When a load is connected to the storage battery, a reverse chemical action is immediately initiated.

The positive plates gradually return to a state of lower oxidation, and the negative plates change from metallic cadmium to

cadmium oxide. During this process, the amount of chemical energy that is converted to current flow is determined

principally by the resistance of the load. In order to prevent reversal of any cells during deep discharge, the BB-542/U is

equipped with an internal discharge cutoff which will automatically disconnect the battery from the load when the battery

voltage falls to 20 volts. The battery will remain disconnected until a charging source of 28 volts, or higher, is connected to

the battery.

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