A device to adjust the capacity of battery cells includes a controller that adjusts the cells to reduce the rate of heat generation in capacity adjustment. When capacity adjustment of cells that form battery set is performed, the capacity adjustment target cells are divided into multiple groups. When the capacity adjustment of the capacity adjustment target cells in one group comes to an end, the cells in the next group are adjusted. As a result, it is possible to reduce the rate of heat generation as compared with the case when capacity adjustment is performed for all of the capacity adjustment target cells at the same time.