A charger power supply successively performs a constant-current charging process and constant-voltage charging process in accordance with a compensation signal. The process includes a plurality of first time periods during which a first output current is provided and a plurality of second time periods during which a second output current is provided. One of a current feedback loop and a voltage feedback loop is selected by comparing output signals of two loops. The current feedback loop has a first reference voltage compensated by a difference between a first voltage corresponding to a first output voltage during the plurality of first time periods and a second voltage corresponding to a second output voltage during the plurality of said time periods.