Abstract: A secondary battery system according to one aspect of the present disclosure comprises: an electric load connected to a secondary battery; and a control device which, in an SOC-dV/dQ curve in which the dV/dQ that is the differential value of the change amount dV of the voltage V of the secondary battery with respect to the change amount dQ of the capacity Q of the secondary battery is plotted with respect to the charged state (SOC) expressed as a percentage of the capacity Q with respect to the fully charged capacity of the secondary battery, when at least one peak top SOC range including the peak top charge rate is set, and charging or discharging stops in the peak top SOC range, discharges the secondary battery by the electric load to avoid the peak top SOC range and ends charging or discharging of the secondary battery.