Quote:
Originally Posted by Formula413
But an alternator doesn't shut itself off when the battery is full, it keeps right on spinning. As opposed to say the air conditioner which has a clutch that engages when the A/C is turned on and disengages when the A/C is turned off. Unless there is some huge gap in my understanding of how an alternator works.
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Yes, there is a huge gap in your understanding :-) The principle is similar to the A/C clutch you mention. The A/C unit is a compressor: when it turns the engine does work compressing the refrigerant gas. Therefore you have a clutch to disconnect it when you don't want A/C.
Now with the alternator the same principle applies, except that the "clutch" is actually an electronic voltage regulator. (If you've worked on old cars, you've probably seen electromechanical regulators.) It controls the amount of energy the alternator takes from the engine & converts into electricity. If there's no electrical load, it only takes enough to overcome friction - if you were cranking it by hand, the alternator would spin freely. When there's a electrical demand, the regulator sends current through the alternator windings, causing it to create more electricity, which makes it hard to turn.
Bottom line: the more electricity you use, the harder the alternator is to turn. The engine has to supply more energy to turn it, and burns more gas to do so. THERE'S NO FREE LUNCH!