The present invention provides a battery charger and a method for preventing both an overshoot charging current and an overcharged battery voltage during a mode transition. The battery charger includes a charging regulation circuit having an input terminal, an output terminal, and a control terminal, in which the charging regulation circuit outputs a charging current whose amount is regulated based on a first regulation signal at the control terminal. The battery charger also includes an operational amplifier having a positive input terminal, a negative input terminal, and an output terminal for generating the first regulation signal; The battery charger contains a first switch unit and a second switch unit for controlling current flow in the battery charger.