Okay im sure that theres a thread already on this i havent got the time to look at the moment. But my current setup is 4 12's, a mtx 421D thunder amp, and a 8 farad cap. I think the problem is the cap because for the last couple days my car has been stalling out when i play the music like any higher that 30. It only does it when i come to a stop...so im guessin that when the tailights light up its puttin enough strain on the battery to make it not have enough power to supply to the rest of the car. What i want to do is get rid of the cap cuz i think thats the main problem right there and replace it with a battery. Now my question is If I hook the second battery up in the spot the cap is will it drain my battery or burn up my alt? IMO I dont think it will hurt a thing unless theres a diff in the size of batterys but i just wanted to get some feedback from people that have run dual battery setups before so just let me know something soon thanks.