I would love any suggestions about how to tackle this question -- my sailboat has a 3 batteries and a standard off/1/both/2 Perko switch. Battery 1 is used as the starting battery, even though it is a deep cycle (It was on the boat that way when I bought it). Battery 2 is actually 2 deep cycles (group 27, 830 mca), almost brand new, in parallel. They run the lights, etc fine, but when I turn the switch to battery 2, the starter switch will buzz, but then it will not provide enough juice (it does some, just not enough) to actually start my engine (an old 30 hp diesel). I don't do that except to test that #2 will actually start the engine in case #1 fails, which has not happened lately, so this is not an emergency situation, but I cannot figure out what may be happening. Shouldn't my deep cycles be enough to start the engine? I have considered a wiring problem, but am not an expert and can't logically figure out why, if everything else runs ok on #2, why the engine won't start on it?? Anyone have any ideas of what I should be checking? Is it possible that a loose connection or something means enough power is getting out to handle the lights etc but not the engine? Thank you very much, and I won't be offended if anyone thinks I am misunderstanding basics or missing an obvious issue!