A quick battery charger according to the present invention comprises a circuit for detecting the voltage of the battery (3), a constant-current charging circuit, a constant-voltage charging circuit, and a timer circuit responsive to the voltage detecting circuit. At the beginning in charging operation, the battery is charged at a constant current, and when the voltage of the battery exceeds a predetermined value, charging mode is changed to constant-voltage charging which last for a predetermined period of time determined by the timer circuit. The charger may also comprise a trickle charge circuit so that trickle charge takes place after the constant-voltage charge. One or more heat-sensitive elements (5, 6, 69) may be used for providing temperature compensation or fail-safe in response to the heat from the battery and heat from parts of the battery charger. As a timer may be used a capacitor (50) which may be forcibly charged to insure a given period of time of the constant-voltage charging. A differential amplifier (42) may be used for detecting the voltage of the battery so that a power transistor (13) used as an element of the constant-current charging circuit is controlled in response to an output signal from the differential amplifier (42) to perform constant-voltage charging.