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A Quick Look
The alternator is designed to keep the car battery fully charged, it is not designed to charge dead  batteries. They are, on the whole, very reliable and will run for years without failing. They work by producing alternating current (hence alternator). The current is produced through a series of windings, which sit in a steel (normally) housing around the one moving part of the alternator. This winding is called the stator, and the part that spins; the rotor.

Alternating current is fine but it has a problem Ė you canít store it. All batteries hold DC voltage and  therefore need to convert the output. The device for this is called the rectifier. The rectifier is a series of diodes. Diodes are electrical one way valves. Each winding from the starter (on average 3) has its own pair of diodes one only allows positive (+) through and the other negative (-).

Voltage Regulation
Left alone, an alternator has no method of controlling its output, so it will over-charge the  battery. To prevent this, a voltage regulator is introduced. All regulators work in the same manner; by cutting off the supply to the rotor. The voltage regulator is responsible for switching this off and on. Typically a regulator switches the magnetic field off at about 14.5 volts and turns it back on at 13.5 this can happen numerous times each second giving an illusion of a steady voltage of 14v.

Although that is basically how  every alternator works,  there are two distinct variations, one called self excited and the other battery excited. The most common type is the self excited alternator. These have an extra diode added to the rectifier (known as the excitation diode). When  the ignition is turned on, the car the battery light comes on this is because it is connected to the rotor. With the light on there is a small flow of electricity around the rotor and at the same time a small electro- magnetic field is generated.

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