A battery system configured to be equipped in a vehicle, the battery system comprising: a battery configured to be charged and discharged, the battery being configured to be equipped in the vehicle, a charge level range of the battery including a hysteresis region and a non-hysteresis region, the hysteresis region being a charge level range where a significant hysteresis occurs, the significant hysteresis being hysteresis in which open-circuit voltage values with respect to a charge level of the battery after continuation of charging and after continuation of discharging are different from each other by a predetermined value or more, the non-hysteresis region being a charge level range where the significant hysteresis does not occur; a voltage detector configured to detect a voltage of the battery as a detected voltage value (Vb); a current detector configured to detect a current flowing through the battery as a detected current value; and an electronic control unit configured to control charge and discharge of the battery, the electronic control unit being configured to estimate an aging deterioration of the battery based on an open circuit voltage value (Vo; Vo1, Vo2) that is calculated from the detected voltage value and an integrated current value (ΔAh; ΔAh 12) that is calculated from the detected current value when the charge level of the battery is in the non-hysteresis region, and a charger configured to charge the battery while the vehicle is at a standstill, characterized in that the open circuit voltage value includes a first open circuit voltage value and a second open circuit voltage value that are acquired in the non-hysteresis region, the integrated current value (ΔAh 12) is a value resulting from integrating the detected current value (Ib) until the open circuit voltage value (Vo) changes to the second open circuit voltage value after the open circuit voltage value (Vo) becomes the first open circuit voltage value, and the electronic control unit is configured to estimate, as a characteristic indicating the aging deterioration, at least one of a full charge capacity of the battery at a current time and a change characteristic of the open circuit voltage value with respect to the charge level, based on the first open circuit voltage value, the second open circuit voltage value and the integrated current value (ΔAh 12), and the electronic control unit is configured to temporarily stop charge of the battery with the charger when the charge level of the battery reaches a first charge level or a second charge level in the non-hysteresis region in a middle of the charge of the battery with the charger, and acquire the detected voltage value that is obtained during a charge stop period, as one of the first open circuit voltage value and the second open circuit voltage value. a battery configured to be charged and discharged, the battery being configured to be equipped in the vehicle, a charge level range of the battery including a hysteresis region and a non-hysteresis region, the hysteresis region being a charge level range where a significant hysteresis occurs, the significant hysteresis being hysteresis in which open-circuit voltage values with respect to a charge level of the battery after continuation of charging and after continuation of discharging are different from each other by a predetermined value or more, the non-hysteresis region being a charge level range where the significant hysteresis does not occur; a voltage detector configured to detect a voltage of the battery as a detected voltage value (Vb); a current detector configured to detect a current flowing through the battery as a detected current value; and an electronic control unit configured to control charge and discharge of the battery, the electronic control unit being configured to estimate an aging deterioration of the battery based on an open circuit voltage value (Vo; Vo1, Vo2) that is calculated from the detected voltage value and an integrated current value (ΔAh; ΔAh 12) that is calculated from the detected current value when the charge level of the battery is in the non-hysteresis region, and the integrated current value (ΔAh 12) is a value resulting from integrating the detected current value (Ib) until the open circuit voltage value (Vo) changes to the second open circuit voltage value after the open circuit voltage value (Vo) becomes the first open circuit voltage value, and the electronic control unit is configured to estimate, as a characteristic indicating the aging deterioration, at least one of a full charge capacity of the battery at a current time and a change characteristic of the open circuit voltage value with respect to the charge level, based on the first open circuit voltage value, the second open circuit voltage value and the integrated current value (ΔAh 12), and the electronic control unit is configured to temporarily stop charge of the battery with the charger when the charge level of the battery reaches a first charge level or a second charge level in the non-hysteresis region in a middle of the charge of the battery with the charger, and acquire the detected voltage value that is obtained during a charge stop period, as one of the first open circuit voltage value and the second open circuit voltage value.