A battery system including: a plurality of battery modules; a plurality of switches configured to switch a connection state of the plurality of battery modules between a series state and a parallel state; and an electronic control unit configured: to control switching of the plurality of switches; and to fix ON-OFF states of the plurality of switches such that the connection state of the plurality of battery modules is the series state or the parallel state when a deterioration parameter indicating a degree of deterioration of a predetermined battery module out of the plurality of battery modules reaches a threshold value, the ON-OFF state representing a state in which a switch is open or closed.