According to one embodiment, an AC blocking filter (12) is provided between charging/discharging terminals of a storage battery (11) and an inverter (20) that converts a DC output of the storage battery to an AC output. Further, a DC blocking filter (13) is connected to the charging/discharging terminals. An alternating current generation device supplies a warm-up alternating current to the storage battery (11) via the DC blocking filter (13).