The defence battery is a battery mounted on a station: a heavy frame turret on steel plate, turned by motors and aimed by circuits. It fires shells from the station's hold at the target the station's targeting picks, by default the ship with the least structure left. Under the default defensive stance it opens fire only once the station's side has been fired on. Like a ship's battery, it cannot aim at anything smaller than a corvette; a point defence array covers strike craft.
Unlike most facilities, it draws power all the time, so the station's generators burn fuel to keep it live. An unpowered defence battery does not fire. A Forward HQ or a combat station can hold one.