A battery system includes: a battery configured to be charged or discharged; a temperature sensor configured to detect a temperature of the battery; a heater configured to warm the battery; a charger configured to supply electric power from an external power supply to the battery and the heater; a relay provided in a passage through which the electric power from the charger is supplied to the battery; and a controller configured to control operations of the charger and the relay. The controller is configured to supply predetermined electric power from the charger to the heater and turn off the relay so as not to supply the electric power from the charger to the battery in a case where an SOC of the battery is equal to or more than a threshold and the temperature of the battery is lower than a predetermined temperature.