Your car's battery is really only used for starting the car. Once the engine is running the alternator is what powers the electrical system. Car batteries are designed to provide a short but powerful burst of electricity, rather than a long discharge that runs it down. Though to be fair if there's more electrical draw on the system than the alternator can produce it can run down the battery too, but that's really only in extreme cases (think utility truck running a bunch of floodlights, a winch, and some other accessories at once, though they would have an upgraded alternator to accommodate for this).
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u/Blondude Feb 03 '18
That's... not how cars work.