I have always, and still do use just one 29 series battery in my boat. All I have to power is a 12v 45# trolling motor, navigation lights, bilge pump, and to start the engine. I have never had any problems with this setup, but I am rewiring the boat and thinking about adding a starting battery. Could anyone tell me what advantages there are to running the extra starting battery? My engine is a 25hp Mercury with electric and pull start.