In a system that shares a battery with an external device, the system includes a power storage device connected to the battery via a power supply line. The system includes a switch provided on the power supply line, and a control unit. The control unit controls on-off switching operations of the switch to selectively establish an electrical conduction between the battery and the power storage device or interrupt the electrical conduction therebetween. The battery has a battery voltage thereacross, and the power storage device has a power-storage voltage thereacross. The battery charges the power storage device while the electrical conduction is established so that the power-storage voltage follows the battery voltage. The control unit turns off the switch when the battery voltage is in a predetermined insufficient voltage state to prevent electrical power charged in the power storage device from being discharged to the battery.