A battery charger is a device used to put energy into a secondary cell or (rechargeable) battery by forcing an electric current through it.

An intelligent battery charger checks the charge condition of battery & stops charging the battery. Good charger can be specified as follows.

1. It should charge the battery with constant current till its final terminal voltage irrespective of changes in the line voltage.
2. It should not cross the predefined battery voltage and reduce charging current when demand from battery reduces.

In case the charger is not of good quality then it will keep on putting electrical energy into the battery which keeps on boiling the acid, reducing the water content. This do not adds any energy inside battery but if the fluid level drops below certain predefined level then it starts the damage of battery and it leads to permanent damage of battery.