Quote:
Originally Posted by JohnnyGrey
This doesn't make one bit of difference. The more amps you draw, the more torque and therefore power, the alternator will demand.
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I agree!
Some more insight can be gained by a simple thought experiment. Suppose that instead of the alternator powering headlights, it was powering an electric motor. Now suppose that electric motor replaced the engine and as such was powering the alternator. If the alternator indeed did not take energy from the motor, as would be the case if the alternator did not take anymore energy from the engine if the headlights were on, then the electric motor and alternator should then be able to run indefinitely without outside energy. This is the basis for a perpetual motion machine, which violates the conservation of energy law of physics. What we can learn from this is that when the headlights are on, the alternator must draw energy from the engine to supply the headlights with power so that energy is conserved.
The amount of energy is small, perhaps only reducing your gas mileage by 3% (see my calculation on this thread and the experiences of several in this forum), so it would be difficult to detect except with careful measurement.