we got a new marine starting battery for our mariner. We thaught that the old starting battery would be good for the trolling motor in the canoe. We were wrong and at its highest speed the motor did not push the canoe. so now i just take the trolling motor battery out of the big boat when we need it for the canoe. so to answere your question, a car battery will probably be worse than a marine starting battery, which did not prove to be effective