An assembled battery includes a plurality of secondary cells, a first arithmetic operator, a second arithmetic operator, and a discharger. The first arithmetic operator calculates a voltage change rate of each of the secondary cells during charging/discharging thereof. The second arithmetic operator determines a discharge condition for each of the secondary cells from the voltage change rate calculated in the first arithmetic operator and a state of charge of each of the secondary cells. The discharger performs discharging of each of the secondary cells on the basis of the discharge condition when the assembled battery is neither being charged nor being discharged. The second arithmetic operator determines the discharge condition of each of the secondary cells such that each of the secondary cells after the discharging has the same state of charge.