Abstract: A battery device has a plurality of batteries connected in series. Each of the batteries includes a thermally responsive switch, a first conductive plate connected to a first terminal of the thermally responsive switch, a second conductive plate connected to a common terminal of the thermally responsive switch, and a third conductive plate which functions as a bypass when it is connected to a second terminal of the thermally responsive switch. In normal operation, the thermally responsive switch is in contact with the first fixed contact of the movable contact such that the first and second conductive plates are electrically connected. When the battery heats up abnormally, exceeding a maximum permissible temperature, the bimetal strip is inverted such that the movable contact comes into contact with the second fixed contact and thus the second and third conductive plates become electrically connected.