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My setup in my 15' is the cranking battery is exclusive to the engine, and my second battery runs everything console forward including the trolling motor. The batteries are hooked to a Perko switch so I can charge either one while underway. The instructions provided with the switch specifically stated that the device is a "make before break" switch so I can crank with the good battery, switch to the weak one and charge it while underway. If it didn't close the circuit before swapping batteries there is a chance of frying the engine charging circuits. I did this because of the trolling motor and worries of it killing the battery. If you don't have circuits that draw a decent amount of current, like bait well pumps, CD player and the aforementioned trolling motor, I would just put a on off switch under the console to cut the power when not in use. Otherwise a second battery might be a good idea.