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No, they're not. There are two general type of these batteries, one is design to produce a lot of current for a short amount of time, like to start a car or boat engine. This is the type you describe with their capability measured in Cold Cranking Amps. The second type is a deep cycle battery and this is what you need. A deep cycle battery is designed to produce a low amount of current for a long time, like running an electric motor as we are doing here. You will want deep cycle batteries, ask for ones intended to run a trolling motor.
Really nice and clear instructions! Very well done. I love the upcycling idea.My only possible issue is I think the arms are easily going to come loose from where they are attached to the seat.