Provided is a battery system for a vehicle and a control method thereof. In a case where high-rate deterioration of a secondary battery (100) progresses before execution of external charging by a charging device (210), an electronic control unit (ECU)(300) executes an SOC adjustment process of adjusting an SOC of the secondary battery (100) such that the SOC falls within a predetermined range. Thepredetermined range is a range where an expansion-contraction change of a negative electrode of the secondary battery (100) along with charge and discharge of the secondary battery (100) is small ascompared with a case where the SOC is out of the predetermined range. After the execution of the SOC adjustment process, the ECU (300) executes a process of moderating the high-rate deterioration so as to moderate the high-rate deterioration, and then executes the external charging.