A battery system includes a discharge amount obtaining portion that obtains a discharge amount (determination zone discharge amount) from a determination start voltage until a predetermined determination end voltage of a battery; an internal resistance obtaining portion that obtains an internal resistance value of the battery; a correlation map in which a combination of internal resistance value information and determination zone discharge amount information for a battery that is the same kind as a target of a determination and in which lithium deposition has not occurred is stored for each degree of age-related deterioration of the battery; and a deposition determining portion that determines a degree of lithium deposition in the target battery by comparing the determination zone discharge amount during the determination of the target battery with the internal resistance value during the determination of the target battery on the same scale by converting at least one via the correlation map.