Adding a second battery was one of the first things I did with mine. Mine came with an isolation switch for the one battery that came with it so I didn't install a singling switch between the batteries on mine as I have them connected to work together all of the time. They run down together and charge together. They will remain in the same condition over their life. I don't like the idea of switching back and forth to be sure they are fully charged. That would be my luck to have forgotten to allow it to charge periodically, then one day, run down the battery, turn to the backup and it be too low to crank the vessel.