A battery system according includes a secondary battery, a temperature sensor and a controller. The secondary battery includes a power generation element configured to perform charging and discharging, an electrolyte, and a battery case. The electrolyte is impregnated inside the power generation element. The power generation element and the electrolyte is housed in the battery case. The temperature sensor is configured to specify a temperature of the secondary battery. The controller is configured to calculate a deviation in salt concentration in the electrolyte. The controller is configured to calculate a first flow velocity and a second flow velocity at each position in the power generation element in the flow direction of the electrolyte using an equation defining a flow of the electrolyte. The first flow velocity is a flow velocity when the electrolyte flows from an inside of the power generation element toward an outside of the power generation element. The second flow velocity is a flow velocity when the electrolyte flows from the outside of the power generation element toward the inside of the power generation element. The equation includes, as a parameter, a density of the electrolyte. The density is specified from the temperature. The density changes according to the temperature of the secondary battery. The controller is configured to calculate a salt concentration distribution in the power generation element based on the first flow velocity and the second flow velocity.