BATTERY SYSTEM

A battery system includes a battery made of a lithium-ion secondary battery, a sensor group configured to detect a voltage, a current, and a temperature of the battery, and a control device configured to control a charging current of the battery based on detection values of the sensor group. The control device is configured to acquire a deterioration state of the battery from the detection values of the sensor group, and set a threshold voltage at which lithium metal is deposited on an electrode of the lithium-ion secondary battery with respect to the voltage of the battery, in accordance with the acquired deterioration state of the battery. The control device is configured to reduce, when the voltage of the battery exceeds the threshold voltage during charging of the battery, the charging current to a limit current.