A power line interface is electrically connectable to a direct-current power system including a power line. A bidirectional DC-DC converter is electrically connected to the power line interface and a battery. A controller receives a first current value measured by a first current sensor and a current command value and performs constant current control processing that controls the bidirectional DC-DC converter so that the first current value equals the current command value. The controller receives a first voltage value measured by a voltage sensor and a voltage command value and performs constant voltage control processing that controls the bidirectional DC-DC converter so that the first voltage value equals the voltage command value. The controller switches operation from the constant current control processing to the constant voltage control processing.