During charging, the chemical action takes place which is exactly opposite to that of discharging. Thus current in opposite direction to that at the time of discharge, is passed through the battery. For this the voltage applied is in excess of the voltage of the battery or cell. The battery voltage acts in opposite direction to that of the applied voltage and hence called back e.m.f. the charging current can be obtained as,
Where Ea = Applied voltage
Eb = Back e.m.f. i.e. battery voltage
R = External resistance in the circuit
r = Internal resistance of the battery
Simple battery charging circuit used to charge the battery from d.c. supply is shown in the Fig.1.
The ammeter measured the charging current which is called charging rate, which can be adjusted using the external resistance R. the voltmeter measures the battery voltage. It is necessary that the positive terminal of the battery must be connected to the positive of the D.C. supply.
The charging current must be adjusted such that the temperature of the electrolyte will not increase beyond 100 to 110 °F.
1. Indications of Fully Charged Battery
The various indications of the fully charged cells are,
1. Specific gravity : The specific gravity of the fully charged cell increase upto 1.28 from about 1.18.
2. Gassing : When the cell is fully charged, it starts liberating the gas freely. In lead acid battery the hydrogen is liberated at cathode while oxygen at the anode. Gassing is a good indication of fully charged battery. Some acid particles may go out with the gases hence the charging room must be kept well ventilated.
3. Voltage : The voltage of the fully charged cell is about 2.7 V.
4. Colour : The colour of the plates changes for fully charged cell. Colour of the positive plate changes to dark chocolate brown while that of negative plate changes to grey colour. But as plates are immersed in the electrolyte, this indication is not clearly visible.