Don't use a "car" battery. Use a deep cycle made for trolling motors. Get one with a large reserve capacity, preferably 200. You can lengthen your time with 2 deep cycles that are hooked up parallel. That means run a jumper wire from + on battery 1 to + on battery 2 and another wire from - on battery 1 to - on battery 2. They both need to be the same size and age for good performance.