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Deep cycle marine batteries are actual a hybrid of deep cycle capacity and starting capacity. I did the same thing but went with two 6 volt golf cart batteries (in series) which are specifically designed for deep cycle use. Costco or Walmart has decent prices, but check the reviews. Two 12 volt batteries in parallel work well as long as they are the same type and age. Otherwise, one of the batteries could siphon charge from the other reducing your overall capacity.