battery is about to die or your alternator is going bad. Same thing happened to me expcet that it was when i put on my turn signal on while i was running the AC. I ended up having to replace both though acuse the guy at autozone told me it was the battery and sold me a new one, then a couple days later, my batter was dead for no reason. Took it to an alternator shop and he had to rebuild mine. At any rate, replacing both took care of my problem. I guess its kinda a warning to your battery dying, isnt a bad thing really.