BATTERY SYSTEM

There is fear that some battery cells among battery cells which are serially connected may consume electric power all the time, thereby causing expansion of unbalance in voltage of the battery cells and hindering electric discharge of a battery system. When a second battery has a sufficient voltage, an electric current control board supplies operating power to a battery control unit and a relay via an external minus line and an external plus line. On the other hand, when the voltage of the second battery has decreased, the electric current control board supplies the operating power from a first battery to the battery control unit and the relay via an internal minus line and an internal plus line. A first electric current control unit and a second electric current control unit control the supply of the operating power according to, for example, the decrease in voltage of the second battery.