Electrical Engineering ⇒ Topic : Methods of Charging Batteries
Methods of Charging Batteries
For charging batteries, direct current is essential and it may be obtained from a d.c. generator.In case the available supply is a.c., it is converted into d.c. by such means as motor-generator set, rotary convertor set or rectifier. In order to charge a new or a discharged battery, direct current is passed through the battery in a direction opposite to that in which it supplied current. The battery is considered fully charged when all the cells are gassing freely and specific gravity of the electrolyte and the terminal voltage of all the cells remain constant for three consecutive readings taken at hourly intervals. There are two main methods of battery charging viz .
(i) Constant current charging (ii) Constant voltage charging
(1) Constant current charging. In this method, the charging current is kept constant throughout the charging period by varying the supply voltage to overcome the increased back e.m.f. of the cells on being charged. The terminal voltage V of a battery on charge is given by
V = E + Ir
As the battery charges, its e.m.f. E increases progressively. Therefore, in order to maintain the charging current constant, the supply voltage must be increased progressively to compensate for the increased e.m.f. In case the d.c. shunt generator is used to charge the battrey, the necessary voltage change can be obtained by varying the field current of the generator. The drawback of this method is that it fails to take into account the state of charge of the battery because a high charging rate is required in the beginning when the battery is fully discharged and a low rate of charge is desirable as the battery approaches full state of charge. Further, this method requires longer time for complete charging.
(2) Constant voltage charging. In this method, the charging voltage is kept constant throughout the charging process. A variable resistance is connected in series with the circuit as shown in Fig.(a). As the battery charges, its e.m.f. increases progressively and series resistance is reduced progressively to maintain the correct charging current. This method is the most common method of charging lead-acid batteries. It is because this method reduces the charging time by about 50% and increases cell capacity by about 20%.
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