A battery is a device that converts chemical energy contained in
its activematerials directly into electrical energy by means of an
electrochemical oxidation-reduction reaction. This type of reaction involves
the transfer of electrons from one material to another through an external
electrical circuit and the motion of ions in an internal medium.
While the term "battery" is often used, the basic
electrochemical unit is the "cell." A battery contains one or more cells,
connected in series, parallel, or both, depending on the desired output
voltage and capacity.
A cell consists of three major components: electrodes,
electrolyte, and separator.
The electrodes are the heart of the cell. They determine
directly the capacity and, usually, the life to be achieved by the cell.
Electrodes generally consist of a structural member (the grid), a current
collector to conduct the current to or from the electrode, and an active
material for the oxidation-reduction reaction.
The negative electrode is where the oxidation reaction takes
place during discharge, giving up electrons to the external circuit. (In a
non- rechargeable cell, the negative electrode is also called the anode.)
The positive electrode is where the reduction reaction takes
place during discharge, accepting electrons from the external circuit. (In a
non- rechargeable cell, the positive electrode is also called the
The electrolyte is typically a liquid acting as the ionic
conductor which provides the medium for transfer of electrons, as ions, inside
the cell between the electrodes.
The separator is a porous membrane placed between electrodes of
opposite polarity, permeable to ionic flow but preventing electric contact of
Gassing, the evolution of gas from electrodes in a cell,
commonly results from local action (self-discharge) or from the electrolysis
of water in the electrolyte during charge in aqueous batteries. The design of
the battery must make provisions for gasses produced during the chemical
reactions. Gasses may be vented to relieve the internal pressure, but the lost
electrolyte must be replaced on a regular basis. If the gasses are not vented,
they must be recombined to maintain internal pressure below the burst pressure
of the container.
For a more complete set of terms refer to Appendix