A battery system includes a battery panel which includes a battery pack including a plurality of secondary battery cells, and a battery management unit to monitor at least one of a voltage, a temperature and a degradation level of the secondary battery cell; and a DC/DC conversion device. The DC/DC conversion device includes an interactive DC/DC converter which converts power discharged from the battery pack into a prescribed voltage to output converted power, and converts power supplied from outside into a prescribed voltage to output converted power; a control unit which controls the interactive DC/DC converter; a communication unit which communicates with the battery management unit and the control unit; a control source unit; and a battery connected to a power supply line of the control source unit through a switch.