A battery system is provided that can prevent overcharging during charging, and that can demonstrate appropriate charging performance and output performance of a secondary battery even when the battery system is used in a low-temperature environment. The battery system includes a secondary battery and a protection circuit. The protection circuit includes a bypass electrical path that connects an upstream-side main electrical path that is connected to a positive terminal of the secondary battery to a downstream-side main electrical path that is connected to a negative terminal of the secondary battery. The battery system is configured so that, when the voltage of the secondary battery exceeds a predetermined voltage during charging, current flowing through the secondary battery decreases while current flowing through the bypass electrical path increases, and when the voltage of the secondary battery falls below the predetermined voltage during charging, the current flowing through the secondary battery increases while the current flowing through the bypass electrical path decreases. The bypass electrical path includes a heater that generates heat using the current flowing through the bypass electrical path, and the heater is arranged adjacent to or in close contact with the secondary battery so as to impart a thermal effect to the secondary battery.