Gassing

TM 116140203141

Figure 2-6. Typical Constant Potential Charging Curve.

tery during charge. This ratio is always less than 1;

ple, a 20-ampere-hour battery that is fully discharged

therefore, excess charge must always be returned to

should be recharged at constant current until at least

the battery after discharge to restore rated capacity.

1.25 times 20 ampere-hours are restored, or at least 25

The total charge necessary may be as low as 110 per-

ampere-hours are returned. In general, a 40 percent

cent, or much greater, depending on the temperature

overcharge is recommended, particularly when cell im-

and cell characteristics. It is generally recommended

balance has occurred, charge rates are at the 5-hour

that the nickel-cadmium battery be charged (constant

rate or lower, and the temperature is above 90°F.

current) for a time that will give at least 125 percent

210. Gassing

of the previously discharged ampere hours. For exam-

During charge, the tendency of nickel-cadmium cells

to gas increases with temperature rise and the charge

state. Cell gassing characteristics will also vary some-

what among manufacturers, cell types and for various

other reasons. A certain amount of gassing is neces-

sary for a battery to become fully charged. The danger

is in excessive or violent gassing, leading to possible

explosion, loss of electrolyte, or damage to the battery.

Monitor the charging procedure to insure that exces-

sive or violent gassing is controlled by reducing charg-

ing current or charging voltage.

211. Electrolyte Level

An inherent characteristic of nickel-cadmium battery

cells is that the electrolyte is absorbed within the

plates and separators to a point where it is not visible

from the top of the cells when at a low state of charge

or in a discharged condition. When the battery is re-

charged, the electrolyte level rises and reaches its max-

imum height at full charge. Ideally, the electrolyte

level should be checked on a fully charged battery that

has been at rest for at least 30 minutes and not exceed-

Figure 2-7. Constant Current Charging Curve.