How much torque is required to spin a standard car alternator in order to charge a standard car 12 volt battery?
by 
Junk Yard Dog
ANSWERS: 2
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It depends on the alternator, but generally not much. When not under load, you might measure it in foot-OUNCES to avoid a small fraction. Under load, electromotive force may pose some resistance but even then it will only be a small handful of foot-pounds. What saps things is that power is a product of torque and RPM, and the alternator is turning at many RPM, usually appreciably faster than the engine. At a cruising RPM of about 2600, a single foot-pound is about 0.5 HP.
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Torque is not really the right unit measurement for this since it is an instantaneous quantity. A better measure is horsepower or kilowatts. According to a recent article I saw, a modern car needs about 3.5 kilowatts to run its electrical systems (less for older, less electronics-intense cars). 3.5 Kw is about 5 horsepower. Note that this is a *peak* figure. If you're cruising down the highway with the headlights/radio/HVAC fan/etc. off, this figure is much less, probably on the order of 0.5 horsepower.
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