An auxiliary battery which is a main power supply is used as power source electric power supplying means for supplying power source electric power to a control device, and an auxiliary power supply is further provided. When an abnormality occurs in the auxiliary battery, the power supply for supplying power source electric power to the control device is switched from the auxiliary battery to the auxiliary power supply by a power supply switching device, to operate the control device stably and perform discharging of a battery. Thus, the discharging can be stopped before the battery is overdischarged.