I would advise against using the same wiring and battery for the graph and trolling motor. You will more than likely get interference when you are using the trolling motor. You can buy an inexpensive battery to run your graph like a small motorcycle battery. The problem is they are typically not a deep cycle battery. Running it down and charging all the time will kill the battery over time. I just bought a cheapy deep cycle battery from the local parts store for $50 or less with a warranty. If it dies I take it back and get a new one for pretty cheap. Walmart sells deep cycle batteries for pretty cheap also. I think less than $40 for the smallest and cheapest battery. Run some small gague wires and you're done.