A battery system control device includes a load, a secondary battery connected to the load via a first power converter converting a voltage by a switching operation, a fuel cell, and a control unit. The battery discharges power supplied to the load. The fuel cell is connected to the load and to the battery and the first power converter, via a second power converter converting a voltage. The fuel cell generates low voltage power. The control unit charges the battery using the generated power from the fuel cell. The control unit includes fuel cell and secondary battery controllers. The fuel cell controller steps up the power generated by the fuel cell to a voltage chargeable by the battery to be supplied to the load by the second power converter. The secondary battery controller directly connects the load and the battery by stopping the switching operation of the first power converter.