A battery system includes a battery pack including a plurality of cells, and an ECU. The ECU calculates a current distribution in each cell using a ladder circuit network model, the ladder circuit network model being obtained by geometrically modeling an interior of the cell using a plurality of resistance elements and a plurality of power storage elements. The ECU calculates the current distribution in the cell by applying, to the ladder circuit network model, a resistance distribution in the cell calculated based on a salt concentration distribution in the cell.