People have a tendency to "overkill" the size of chargers they think they need. Over the years I have uses several different sizes of chargers except never more than 10 amp per bank. I eventually wound up using chargers with 5 amps per bank and I have never ran out of power for the trolling motor. This includes week long trips when we would fish 8-12 hours per day every day. The way you fish has everything to do with it. Our style back then was slow moving with a lot of poles out (spider rigged). Batteries were usually big Delco Voyager. The type that were sealed and you didn't have to worry about water level. I imagine that if I trolled every day at 2-3 MPH I would have encountered problems. The last set I had were about 8 years old when I sold the rig and still checked good with a load tester. It all boils down to how you fish and how soon you need the batteries recharged and yes I know how to compute how long it would take a given charger to charge certain batteries. Keep in mind that these new on-board chargers are pretty smart. For instance if you used a charger with 5 amps per bank, say 1 for the starting battery and 2 for the trolling motor batteries, the starting battery will get topped off pretty quick because it gets less use. Once topped off the charger will just supply a trickle charge to that battery. The excess amps will be supplied to the TM batteries so you will wind up with about 1 amp periodically going to the starting and 7 amps going to the TM batteries resulting in a faster charge. I guess the major factor in deciding what charger to get is peace of mind. I mean who (or is it whom) wants to go on a much anticipated fishing trip and then worry all day if you have enough power to get thru the day. I am not an expert but these are just some things I have learned, and can remember, in the past 75 years.