A battery charger includes a supply voltage unit supplying a power supply voltage, a constant current generator receiving the power supply voltage and outputting a substantially constant current, and a timer whose output is coupled to the constant current generator. The timer outputs the timer output in response to a transition in the power supply voltage. The supply voltage unit changes a magnitude of the power supply voltage in response to the timer output so that the substantially constant current is changed from a first current level to a second current level.