The charging system provides electrical power for operation of the vehicle's ignition, starting system and all of the electrical accessories. The battery serves as an electrical surge or storage tank, storing (in chemical form) the energy originally produced by the alternator. The system also provides a means of regulating the alternator output to protect the battery from being overcharged and the accessories from being destroyed.
The storage battery is a chemical device incorporating parallel lead plates in a tank containing a sulfuric acid-water solution. Adjacent plates are slightly dissimilar and the chemical reaction of the two dissimilar plates produces electrical energy when the battery is connected to a load such as the starter motor. The chemical reaction is reversible, so that when the alternator is producing a voltage (electrical pressure) greater than that produced by the battery, electricity is forced into the battery and it is returned to it's fully charged state.
Alternators are used on the modern vehicle for they are lighter, more efficient, rotate at higher speeds and have fewer brush problems. In an alternator, the field rotates while all of the current produced passes only through the stator windings. The brushes bear against the continuous slip rings; this causes the current produced to periodically reverse the direction of it's flow. Diodes (electrical one-way switches) block the flow of current from traveling in the wrong direction. A series of diodes are wired together to permit the alternating flow of the stator to be converted to a pulsating but unidirectional flow at the alternator output. The alternator's field is wired in series with the voltage regulator.