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Discussion Starter #1
A large power draw pulls more current from the alternator, making it work harder. This extra electricity isn't free.

But how big is the penalty?

What I'm getting at is:

Will a car be faster with the stereo and/or headlights off? Or is the difference too minute to make a difference?

(I'm guessing it'll be too small a draw... but I want to hear others' comments.)
 

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Reading Topic: How much power does the alternator sap away?

I've always understood that the alternator's sole job is to recharge the battery, and that all power to devices in the car, including the spark plugs, comes directly from the battery, and that a regulator controls how much of the current being produced by the alternator goes to the battery (i.e. dependent on how much current is being drawn out of the battery -- and whatever power your battery doesn't need is basically wasted). After all, your lights and radio and everything still work, even if the engine isn't running, right? :) And your car will keep running for a while if your alternator dies, until the power output of your battery becomes too low to fire the spark plugs.

Ever notice how, if your battery is low and you can't start your car, you can often still operate things like the radio and the lights, and sometimes even the power windows? The amount of power required by those things is minimal compared to the power required to fire the spark plugs and crank the starter motor. Thus, I believe, to answer your question, having the radio and lights off would not give you any increase in power or speed.

Now, having the A/C turned off, on the other hand... :)
 

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Reading Topic: How much power does the alternator sap away?

Ever jump-start a car? The car you're drawing power from may have to rev it's engine to create enough juice. Surely then it's not all coming from the battery.

Besides, you can't recharge and draw from a battery at the same time- current only goes in/out one set of wires. It's either drawing or giving current.

Right?
 

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Reading Topic: How much power does the alternator sap away?

I thought alternators were in the ballpark of 14V. Everything is powered off the alternator when it is running. The reason the alternator puts out more voltage than the battery is so that it can provide voltage to electrical systems AND keep the battery fully charged. You don't pull from the battery when the car is running unless you overdraw the max current of the alternator.
 

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Reading Topic: How much power does the alternator sap away?

Alternators are in the 14V range, I test them day in and day out, 14.10 volts seems to be a common charging voltage.

As for revving a car to produce enough juice, if you have a dead/drained battery it is better to rev it up because at around 2,000 RPM, the alternator is putting out around 70% of it's peak amperage. As the RPMs gain, the pulley spins faster and faster, and will finally reach it's peak amperage. This is why I always recommend to a customer, after jumping them off, to drive down the highway for about 10-15 minutes.
 

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Replying to Topic 'How much power does the alternator sap away?'

Never really looked into detail at how an alternator works, but it was always my belief that the sole purpose of the battery was to start the car, period, everything runs off the alternator once the engines running, and put a laod on it and it will be a little harder to spin it. You can run a car indefinately on a dead battery (just dont shut it off); but it wont go very far with a bad altternator. Ive seen some numbers about how much hp it draws but I was never concerned enough to do the math myself. for example, turn off all acc, then watch the tach as you crank up the heater fan, turn on the headlights, and crank the stereo...unless they've got it really dialed in you'll notice a small sputter from the added draw of the alt, b4 the ecu levels off the idle. I dont believe this affects perfomance much unless you are looking for every hundreth of a sec you can get. Maybe some daring soul can bypass the alternator for a run down the quarter and post the diferences, my guess would be less than a tenth.
 

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Reading Topic: How much power does the alternator sap away?

Not sure if this will help. But I had a Dodge Intrepid where one of the battery wires on the positive lead coroded through. Found it out when the car wouldn't start one day. Had to have some hold the wire together to start it, once it started up, could unplug the wire altogether. So the alternator is probably directly wired into the system, instead of relying on the battery.
 
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