Hi just lloking at this and cant see why you dont use 2 smaller 12volt batteries run is parralel? only the motor you are using is a lot to be cranking over 6v volt batteries may not have to required punch. also if you incase the in alloy batteries in there own compartments with a small gap between the tank and battery and puts an insulator like a sheet of rubber in the gap the you would have no problems, i would also put a jacket on the battery beteen it and its case as for 6 months other the year that battery is going to be sitting in the cold and unused this will help your battery last longer
(on average a car battery looses 1% of its charge a day, in 100 days battery should be flat. The colder it gets the less days you get)
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