PURPOSE: To maintain two storage batteries the same discharging state by providing output terminals of the two batteries connected in series and a low voltage output terminal from one battery, and supplementing a low voltage output current from a high voltage terminal by voltage converting means.

CONSTITUTION: Storage batteries 3, 2 are charged in series by the DC output of a generator 1, a high voltage load is connected to a high voltage output terminal 6, and a low voltage load is connected to the connecting point 5 of the batteries 2, 3. The output voltages of the terminals 6, 5 are divided by resistors 15, 16 and 19, 20, and input to an operational amplifier 13. When the output of the terminal 5 is increased and the voltage difference of the terminals 5, 6 exceeds a predetermined value, the amplifier 13 produces an output, which is compared by a comparator 11 with the output voltage of a triangular wave oscillator 12, and a transistor(Tr)10 is opened or closed in response to the output of the amplifier 13. When the Tr10 is closed, a current flows from the battery 3 to the Tr10, a coil 8, while the Tr10 is opened, energy stored in the coil 8 is rectified by a diode 9, thereby charging the battery 2. Thus, the batteries 2, 3 are uniformly discharged.