OK, I've always wondered about this, but never really questioned it. They have starting batteries, which are not deep cycle, and they have trolling batteries which are deep cycle. Now I've got to replace my starting battery because it doesn't hold up the whole day when running the livewell aerator and the fish finders. Would it not be better to put a deep cycle battery there instead of a regular one. It would last longer on a charge, plus would have the staying power to run everything all day and still crank the motor. Tell me why this wouldn't work!