While a drive battery (11) is charged, the output voltage of a DC-DC converter (20) is set to a first predetermined voltage (e.g. 14.4 V) at which thermal runaway does not occur even if the temperature of an auxiliary battery (19) is high and at which the auxiliary battery (19) can be charged even if the temperature thereof is low. During warm-up operation of the auxiliary battery (19), the output voltage of the DC-DC converter (20) is set to a second predetermined voltage (e.g. 14.3 V) lower than the first predetermined voltage. While the vehicle (10) is traveling, the output voltage of the DC-DC converter (20) is set to a third predetermined voltage (e.g. 14.2 V) lower than the second predetermined voltage. If lamps (17) are lit, the output voltage of the DC-DC converter (20) is set to a fourth predetermined voltage (e.g. 13.9 V).