Me personally I would go 3 bank and minimum of 10 amps per bank especially if you need the batteries charged overnight a 5 amp takes way to long to charge the trolling motor batteries. The reason I say three I would include the starting battery unless you are making long long runs the starting battery doe`s not get charged enough running electronics bilge aerator radio etc.. My outboard has a 65 amp alternator on it and by the time I get home and plug in my charger the starting battery takes a lot of charge.