Battery state sensing apparatus

A voltage-responsive optical sensing device such as a semiconductor device of a light emitting device and/or liquid crystal device of a sensing apparatus for sensing an excessive charge and/or excessive discharge state of a battery, such as a lithium ion secondary battery is incorporated into an inside of a cell of a cell group to form the battery. For example, with electrodes of the liquid crystal device connected in parallel to the cell and a light beam of an external light source introduced into the liquid crystal device, a photo sensor senses a change in a light-transmissive characteristic of the liquid crystal device so that the state of the cell constituting the battery can be sensed.