PURPOSE: To achieve the small size, the light weight, the low cost and the high efficiency of the title system by improving an auxiliary-battery charging device.

CONSTITUTION: The electric system of an electric vehicle is provided with a main battery for driving a vehicle, with an auxiliary battery for an auxiliary apparatus and with a charging device used to charge the auxiliary battery by the main battery. The charging device 7A is provided with a DC-AC converter composed of a switching element 703 converting a DC voltage into an AC voltage whose frequency and waveform are definite and of an ON-OFF control circuit 707, with an insulating step-down transformer 702 connected to the AC side of the converter and with a rectifier 705 connected across the output side of the transformer 702 and the auxiliary battery.