A battery charger for charging a battery through controlling a charging regulation circuit is provided. The battery charger includes a current sensing unit and an operational amplifier. The current sensing unit monitors a charging current applied to the battery when the battery charger operates under a constant current mode, thereby generating a first regulation signal to the charging regulation circuit. The operational amplifier compares a battery voltage of the battery with a first reference voltage to generate a comparison result. When the battery charger operates under the constant current mode, the comparison result controls a charging mode transition from the constant current mode to a constant voltage mode. When the battery charger operates under the constant voltage mode, the comparison result acts as a second regulation signal to control the charging regulation circuit.