A battery system is provided with a plurality of battery units each including a battery module consisting of a plurality of battery cells, a unit control unit, and a connector, and controls the overall operation of the respective battery units in a main control unit. To the plurality of connectors of the battery units, an external unit can be selectively connected so that voltage adjustment for inhibiting voltage variations among the plurality of battery cells or the plurality of battery modules can be realized by using the external unit. The external unit is connected to the battery unit for which voltage adjustment is necessary on the basis of a voltage adjustment necessity determination.