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The alternator is an electrical generating device. It has a belt driven pulley that is turned or rotated by the engine. It is like the main pumping station in a city water supply system in that it regulates how much energy is allowed into the electrical system. During peak electrical demand, such as when the lights, heater, seat warmers, windshield wipers and rear windshield defroster are all being used on a cold, dark, rainy morning, the alternator is unable to supply all the electricity needed. When this happens, the electrical system will draw energy from the battery in order to maintain this peak in systemwide electrical demand. When the electrical loads are reduced, the alternator recharges the battery back to its ideal voltage of 12.6 volts.
Note: There are time limiting circuits in the window defroster and seat warmer circuits in order to protect the battery from overload and/or becoming too discharged.
Mechanics Corner: More Technical Detail
The alternator is an electrical-generating device with a belt-driven pulley, which is turned or driven by the engine. Once the engine is started, the alternator supplies the electrical energy the vehicle needs.
The alternator sends electrical energy back to the battery in order to keep it fully charged. When the vehicle is running, the battery stores a large reserve of electrical energy to be used during moments of peak electrical demand (like on a cold, dark, and rainy morning when the lights and heater are on, the windshield wipers are working hard, the seat warmers are on, and the electric windshield defrosters are operating). When the vehicle is stopped and the engine is idling, the alternator cannot keep up with all of this electrical load, so the electrical system will draw energy from the reserve in the battery to meet the demand. When the vehicle is moving, the engine turns the alternator much faster, therefore generating enough electricity to meet (and exceed) demand—this is when the battery is recharged.
In modern vehicles, the amount of voltage supplied by the alternator is regulated by the engine computer where previously, a voltage regulator was built in to the alternator itself. Due to the growing number of onboard computer systems and the fact that newer vehicles need to maintain an ever-tightening envelope of voltage, the voltage regulation was transferred to the more powerful and sophisticated engine (or power train) computer.
How an Alternator Works
An alternator is a device that uses a rotating iron shaft that is wound with very thin copper wires into what is called a field winding. The ends of the windings are connected to a regulated voltage source. As the vehicle needs more voltage, the electrical system provides more electrical current, which is applied to the field windings on the rotating iron core (the "rotor"). This creates a magnetic field that induces AC voltage into a surrounding set of copper windings (called the "stator" because it is stationary). The AC voltage from the stator flows through a set of one-way electrical gates called diodes. The diodes only allow DC voltage back into the electrical system by way of the B+ or battery output terminal on the back of the alternator that is directly cabled to the positive terminal on the battery.