Joe :
First of all, if the alternator is doing it's job, the power does not
"come from the battery and get replenished". At anything above idle,
the alternator should hold the system voltage high enough that the
battery does not supply any power, but is actually a load (being
recharged). My alternator turns a little slow (relative to engine
speed), but will keep up with either the fan or the headlights (not
both) even at idle. If your car has an ammeter, you can see this as the
needle never going to the discharge side (indicating the battery is
supplying power). If you have a voltmeter, the battery cannot supply
power anytime the system voltage is above 12.6 volts. (Note that, on
many cars, the voltmeter reads a little lower than the voltage at the
battery, because of voltage drops through the wiring and ignition
switch.)
Second, how much horsepower it takes to turn the alternator depends
directly on how much electrical power it is putting out. Under the
right circumstances (on a carbureted engine), you can hear the engine
rpm drop when you add a big load (like fan or headlights) to the
system. If you like, you can come listen to my generator set (which is
just a little Wisconsin 2-cylinder driving a biig alternator), which
will almost stall if you throw a big electrical load on it suddenly.
You can easily hear it open the throttle to pull the load (which most
modern fuel injected cars also do).
Randall
Joe Curry wrote:
>
> Would someone please tell me how an electric fan pulls power away from the
>engine???
>
> Sure it requires power to run, but that alternator is running all the time
>anyway. The electricity is coming from the battery and
> being replenished by the alternator.
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