NOVELTY - The photovoltaic-charged battery charging device (11) has a battery pack (12) that is included with multiple battery cells (13) that are connected in series. A control unit (15) is configured to change the series connection of the battery cells in response to a variation in output voltage of a solar battery (16). A current backflow prevention circuit is arranged in an input section of the battery charging device and connected between a solar battery and the control unit.    USE - Device for charging secondary battery such as lithium ion secondary battery, magnesium ion secondary battery, aluminum ion secondary battery used in electronic device (claimed) such as personal computer, television receiving set, mobile phone, personal digital assistant, electronic book, electronic paper e.g. electronic newspaper, digital still camera, video camcoder, music player, lighting fixture e.g. interior lamp and medical instrument, and electric vehicle (claimed) such as electrical motorcycle, electrical assist bicycle, golf cart, electrical cart, Segway (RTM: two wheeled self-balancing personal transport), air craft and ship.    ADVANTAGE - The increase or decrease of the series connection of the battery cells in response to the variation in output voltage of solar battery can be possible. Since the multiple battery cells are provided, the charge state of the battery cells can be allowed to be optimized, and lowered charge efficiency due to building in a maximum power point tracker (MPPT) circuit can be prevented. Since the current backflow prevention circuit is arranged in input section of the battery charging device, the solar battery can be prevented from damage. The battery charging device can be manufactured at low cost.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an electronic device;    (2) an electric vehicle;    (3) a method of charging battery charging device;    (4) a method of operating electronic vehicle;    (5) a photovoltaic generator; and    (6) a method of generating power in photovoltaic generator.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of battery charging device.    Battery charging device (11)    Battery pack (12)    Battery cells (13)    Control unit (15)    Solar battery (16)