A vehicle has a low-voltage power generator driven by an engine, a standard low-voltage battery and a standard low-voltage electric load. Additional equipment has an additional low-voltage battery for supplying electric power to an additional low-voltage electric load. The additional low-voltage battery is charged with electric power from the low-voltage power generator. The additional equipment further has a high-voltage battery for supplying electric power to a high-voltage electric load, and a converter for converting low-voltage electric power from the low-voltage power generator to high-voltage electric power. When relays are opened during a period in which engine operation is stopped, the electric power is supplied from the additional low-voltage battery to the converter, so that the high-voltage electric power is supplied to the high-voltage electric load, such as a motor-driven compressor.