A connector of a battery device includes a first terminal and a second terminal. The first terminal is coupleable to an opposed unit through a cable in a first state. The second terminal is coupleable to the opposed unit through the cable in the second state which is inverted upside down from the first state. A control device of the battery device is configured to charge the battery device with electric power from the opposed unit, when it is coupled to the opposed unit through the cable in the first state, and to supply the opposed unit with electric power from the battery device, when it is coupled to the opposed unit through the cable in the second state.