I am thinking about installing a second battery for my car audio system. I've been reading about Marine Deep Cycle Batteries and I hear they are good because you can pull large amounts of power from them for long periods of time and you can extremly drain the power out of them and not mess them up. I want to set my suv up so that while the truck is running i can listen to my system on my original battery, but also i want to set it up so when the truck is off, the power switches over to the deep cycle battery to run the amplifiers and subs themselves. Is this possible? and if so, is there a way that the alternator would recharge both batteries while the truck is running?