Bobby, which model are you running/considering, I would suggest there might me a difference between...say xb2003 and xb21. Like JWolff said, unless you are are pushing your trolling system to the maximum, many of us are getting away with 24v systems, on the other hand, there are people who are used to big Rangers and Skeeters, and demand ample juice. I would say that Allison owners are always performance oriented and thus resist the option of adding a third battery.