Your main motor should charge your starting battery with some throttle to the motor.A couple of questions.1.how many batterys do you have and how are they hooked up? 2.What do you have hooked up to these batterys(electric riggers,fish,finders etc.)I have a dual bank charging system that charges both my deep cycle batterys at the same time all I do is plug it in from the boat to a wall plug when it's done the charger stays on the boat and away you go it's always hooked up If you have room that would be the way to go.Here is what I have on my 19 footer it's the 100 model.Good luck you can't go wrong with type of charger.
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