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Well the battery isn't limitless, point is the alternator will have to do work at some point to charge/ run the stuff, and the more power it needs to make, the more drag it will be on the engine.. since it's only really spinning one speed, give or take a thousand rpm or so, in order to charge faster/more it ups the mechanical resistance in order to push more juice. Obviously more resistance = lower gas mileage.. maybe not by much but if you want to save as much as you can, it's just another thing to help.
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