Patent ID: 7999511

Claim:
A battery charger for charging a plurality of secondary batteries, the battery charger being configured to be connected to a power supply circuit for converting an AC input to a DC output, and configured so that an output of the power supply circuit is connected to the plurality of secondary batteries, the battery charger comprising: a first switch for connecting the plurality of secondary batteries in series, the plurality of secondary batteries comprising a first secondary battery having a highest electric potential of the plurality of secondary batteries and a second secondary battery having a lowest electric potential of the plurality of secondary batteries; a second switch to selectively connect a first polarity terminal of the first secondary battery; a DC-DC converter having an input terminal connected to the first polarity terminal of the first secondary battery via the second switch, an external power supply input terminal connected to a second polarity terminal of the second secondary battery; an external power supply output terminal connected to an output terminal of the DC-DC converter to derive a stable power supply; and a controller for controlling the first and second switches; and further comprising a third switch for connecting the first polarity input terminal of the DC-DC converter to an output terminal of the power supply circuit, wherein: upon detection of the AC input, the controller controls the first and second switches to become OFF state and the third switch to becomes ON state, and a DC output of the power supply circuit is inputted to the first polarity input terminal of the DC-DC converter; and upon no detection of the AC input, the controller controls the first and second switches to become ON state and the third switch to becomes OFF state, and the plurality of secondary batteries is connected in series while a voltage of the highest electric potential of the secondary batteries is inputted to the first polarity input terminal of the DC-DC converter via the second switch.