Adding another battery will just put more strain on the alternator because, now the alternator has to charge up 2 batteries instead of one. I have 2 caps in my car and an optima power pumper 750 deep cycle battery, I still have alil light dim when riding around, nothing too serious, I'm happy with it. Personally I expect it to drain power from it considering how much wattage is being put to my subs. I just recently installed a 134 amp alternator, charges the battery real quick. My opinion is upgrade the battery you have to a deep cycle, and upgrade the alternator. See if that helps, if not, try caps.