A battery system is configured to estimate a presence or absence of deterioration of a battery. This deterioration is a phenomenon in which an internal resistance of the secondary battery increases due to uneven ion concentration in a non-aqueous electrolyte. The battery system includes a voltage sensor and an ECU. The voltage sensor detects a voltage value of the battery. The ECU is configured to perform a calculation process and an estimation process. The calculation process includes a process of calculating, in accordance with the voltage value, an evaluation value for evaluating a degree of polarization mitigation after charging or discharging of the battery. The estimation process includes a process of estimating the presence or absence of the deterioration in accordance with the evaluation value.