Battery Opinions Daniel I. Applebaum Revision 1994.A 31 October 1994 The conclusions I've reached given the information presented in the previous mail message are that true deep-cycle batteries are impractical for gas-powered automotive use. The choices then are starting batteries, marine/RV deep-cycle and gel-cell. Due to expense, the gel-cell is probably unnecessary except for exotic reasons, like the 90 degree rotated, trunk mounted battery in the Miata. In addition, the discharge rate limits of the sealed cell make it singularly ill-suited for winching. For general automotive use the marine/RV deep-cycle seems the best choice, if you can live with somewhat lower starting current. For a dual battery system, I'd use a marine/RV deep-cycle as the primary and an automotive starting battery as the secondary. Normally, all loads, including the starter, would be sourced off the deep-cycle battery. The starting battery would be left on float unless the deep-cycle was fully discharged or unable to cope with a difficult starting situation, like in cold climates. A gel-cell would be reasonable as the secondary, if the cost was acceptable. It's longevity might overcome the higher initial cost. I saw over the weekend that Motorcraft has a series of marine/RV deep-cycle batteries available. I saw them at Auto Parts Club. I noted that Marine Cranking Amps are rated at 32F whereas Cold Cranking Amps are rated at 0F. There was perhaps a 100 amp difference between the two values, so beware. The batteries were also labeled as "vibration resistant" so their use in offroad and rally vehicles would seem reasonable. In practice, the marine/RV battery in my car has proven very dependable for the last 3 years. It's been more than 80% cycled several times and has repeatedly suffered severe vibration on washboard roads.