Electro-chemical cells transform chemical energy into electrical energy.
There are two types of cell, primary and secondary. In a primary cell, this transformation is not reversible, and the cell is discarded at the end of its life. In the secondary cell, the transformation is reversible, and it can be re-charged.
There are two types of secondary cell, wet and dry. In automotive use, the usual main-storage device is the wet cell of a lead acid battery. It has two plates of dissimilar materials immersed in an electrolyte – a solution that conducts electricity by using ions.
The accepted, or nominal, voltage of a cell does not depend on the size of the cell, however, its current capacity does. The surface area of the plates in a cell determines its current capacity.
In a lead acid battery, the plates are assembled so there is always one extra negative plate. The plates are close to each other but do not touch, which would cause a short circuit.
The nominal voltage of a cell is 2 volts. Cells connected in series make a battery, and the number of cells determines its nominal voltage. The cells are sealed from each other and filled with dilute sulfuric acid. The battery case is usually plastic or hard rubber.
One set of plates is connected to the negative side of a DC source, the other to the positive side. Direct current is applied to the plates, changing them chemically, until the battery is ready for service.