Just went through this recently and talked with some elecronics specialists. As the above thread states, it can be very difficult/expensive to totally isolate your two batteries. You do not always need a starting battery to start your motor/engine. If your starting draw is relatively small a regular deep cycle will do just fine. It was pointed out to me that the most important thing to have were two large(29-31) series batteries both in equal condition. If either battery is in a weeker condition it will draw down the other battery constantly and you will end up with two shot batteries. My .02 is to buy matched batteries and a battery strength indicator. This setup has worked very well for me and has taken all of the guess work out.