Techniques effectively prevent an overcurrent from occurring in a system comprising assembled batteries connected in parallel to a power supply line, when a given assembled battery is connected to the power supply line. Multiple battery units are each configured such that they can be connected to a power supply line. Each battery unit includes a battery cell group configured including multiple battery cells, a switching unit arranged between the power supply line and the battery cell group so as to control a current that flows between the power supply line and the battery cell group, and a control unit that controls the switching unit.