A battery charger for charging a plurality of secondary batteries is provided. The charger is configured to be connected to a power supply circuit and configured so that an output of the circuit is connected to the batteries. The battery charger includes a first switch for connecting the batteries in series, a second switch to selectively connect a positive polarity terminal of a first secondary battery having a highest electric potential, a DC-DC converter having the first and grounding terminals, an input terminal being to be connected to the positive polarity terminal via the second switch, a grounding terminal being to be connected to a negative polarity terminal of a second secondary battery having a lowest electric potential, an external power supply output terminal connected to an output terminal of the DC-DC converter, and a controller for controlling the first and second switches.