I have a second battery mounted in the the rear they are used a lot in Australia to enable the running of a car fridge. With the hot weather over here you need the fridge when you go camping as you end up in some fairly remote areas and a long way from towns or shops. i run a Redarc battery isolator solenoid, this is fitted between the start battery and the second battery. The batteries are connected together when driving however when stopped the solenoid seperates the batteries when the start battery voltage drops to 12.5v. When the car is started the solenoid stays open untill the start battery voltage reaches 13.2v then the solenoid closes and starts charging the second (deep cycle)battery. There is quite a good write up on it here http://www.fridge-and-solar.net/dual_bat.htm