A battery system includes a battery unit and a controller. The battery unit has a plurality of battery modules, a plurality of first relays and a plurality of second relays. Each battery module has a plurality of unit cells that are connected in series. The plurality of first relays are connected between the same polarity terminals between the plurality of battery modules and configured to allow parallel connection between the plurality of battery modules upon being closed. The plurality of second relays are connected between different-polarity terminals of the plurality of battery modules and configured to allow series connection between the plurality of battery modules upon being closed. The controller of the battery system is configured to control the closed/open state of the plurality of first relays and the plurality of second relays based on a voltage deviation between the plurality of battery modules.