A cooling fan of a secondary battery and a cooling fan of a DC/DC converter are arranged in parallel to share the same air discharge path. A temperature sensor on an air intake side and a temperature sensor on an air discharge side are attached to the secondary battery. When the cooling fan is in failure, a temperature deviation between the air intake side and the air discharge side in the secondary battery increases because a backflow component of a discharge air is generated through the discharge path as the cooling fan operates. Based on this phenomenon, when operation commands for both of the cooling fans are issued, the control circuit senses a failure in the cooling fan by monitoring the temperature difference between temperatures detected by the temperature sensors. Failure detection is thereby possible without providing a sensor at each of the cooling fans.