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I thought I heard the guy at the store said 7 miles, but it could have been 7 minutes. What is average? I live about 4 miles from work and it takes around 10-12 minutes to get there depending on the stop lights.

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In general they say that an alternator produces its best charging at 3000RPM on Petrol cars, it is just a rule.. they already charge at lower ones... You need to know how much ampere your alternator is 50,60,80,120.. and read about how much power one turn off the engine can take in general,, calculate the drain of power and then the optimal recharge time based on the alternators power. You dont need to drive x miles to charge. You need RPM's -- also read about petrol generators and how they charge and invert power. Consider making your question more constructive –  ppumkin Feb 11 '12 at 11:09
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@ppumkin: Drop in some example calculation and that would be a great answer :) –  Krom Stern Feb 14 '12 at 13:27
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up vote 3 down vote accepted

There's too many variables to come up with any one particular number. It can't be too very long, what with cars that shut themselves off and then restart at each traffic light. :-) Also, note that today's alternators are MUCH more efficient at idle RPMs than the generators of years ago (some of which would discharge rather than charge at idle). I suspect that the real number is somewhere in the range of double digit seconds...

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+1 I think it will vary a lot –  Gabriel Mongeon Feb 10 '12 at 20:58
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