Batteries are maintained floating on the line in various installations and are connected in such a way that they are charged when load demands are low and automatically discharged when load demands are high or when the main power supply fails or is disconnected during peak hours. Other systems link the battery to the feeder circuit as needed, allowing it to deplete to a specific degree before being withdrawn and re-charged for future needs.
The following two main charging approaches (though there are various variants) are used for batteries other than the ‘floating’ and system-governed types.
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Constant-current charging System:
In this charging method To overcome the increased back e.m.f. of cells, the charging current is kept constant by adjusting the supply voltage. The current delivered by a charging booster (which is basically a shunt dynamo directly run by a motor) may be kept constant by changing its excitations. The current is regulated by altering the rheostat linked in the circuit if charged on a d.c. supply. The charging current should be adjusted so that no excessive gassing occurs during the last phases of charging and the cell temperature does not exceed 45°C. This procedure takes a lot longer than the others.
Constant-voltage charging System:
The voltage is kept constant in this method, but it requires a large charging current at the beginning when the cells’ back e.m.f. is low, and a low charging current is required as the cells’ back e.m.f. is high when they are charged. This method keeps the voltage constant, but it requires a high charging current at the beginning of the charging process when the cells’ back e.m.f. is low, and a low charging current is required when the cells’ back e.m.f. is high when the cells are almost charged. The time it takes to charge is nearly cut in half using this method. It boosts capacity by around 20% while lowering efficiency by about 10%.
When a secondary cell or a battery of such cells is charged, the emf of the cells works against the applied charging voltage. If V is the supply voltage that delivers a charging current of I against the back e.m.f. Eb, then the input power is VI, but the power required to overcome the opposition is EbI. This electric power, EbI, is transformed into chemical energy and stored in the cell.
The charging current may be calculated using the equation below.
R = total circuit resistance, which includes the battery’s internal resistance.
I = current of charging.
The charging current may be kept constant throughout by adjusting R.
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