See Figure 1
The automobile charging system provides electrical power for operation of the vehicle's ignition and all the electrical accessories. The battery serves as an electrical surge or storage tank, storing (in chemical form) the energy originally produced by the engine-driven alternator. The system also provides a means of regulating alternator output to protect the battery from being overcharged and to avoid excessive voltage to the accessories.
The vehicle's alternator is driven mechanically, through belts, by the engine crankshaft. It consists of two coils of fine wire, one stationary (the stator), and one movable (the rotor). The rotor may also be known as the armature, and consists of fine wire wrapped around an iron core which is mounted on a shaft. The electricity which flows through the two coils of wire (provided initially be the battery in some cases) creates an intense magnetic field around both the rotor and stator, and the interaction between the two fields creates voltage, allowing the alternator to power the accessories and charge the battery.
Almost all vehicles today use alternators because they are more efficient, can be rotated at higher speeds, and have fewer brush problems. In an alternator, the field rotates while all the current produced passes only through the stator windings. The brushes bear against continuous slip rings rather than a commutator. This causes the current produced to periodically reverse the direction of its flow, very similar to the power supply to your house. Diodes (electrical one-way switches) block the flow of current from traveling in the wrong direction. A series of diodes is wired together to permit the alternating flow of the stator to be converted to a pulsating, but unidirectional flow at the alternator output. This inverter circuit switches the AC current unusable by the vehicle to the standard DC or direct current. The alternator's field is wired in series with the voltage regulator.
The voltage regulator is contained within the alternator. Simply described, it consists of solid-state components whose job it is to limit the output of the alternator to usable levels. Excess output can damage the battery and overvoltage can destroy electrical components.