Your friend is wrong. If you have a quality onboard charger then you will not have a problem. As Tom said, if you don't use a lot of aux amps and run your big motor often you will be fine. In my case, on tournament days, I start with nav lights and then run dual livewells, dual aerators, and dual electronics all day with very little time on the big motor. If I don't charge my cranking battery I get to paddle like Imoore said.