I have three batteries on my boat - two on the original system and added one for just the electronics that are isolated from the rest of the system; the charging system charges all three. I added a solar panel through a split charger; one side to the electronics battery and one side to the original system. The diodes in the split charger prevent 'intermingling' of the systems. The solar panel keeps enough of a 'trickle' on all the batteries to keep them from discharging. Since they are fully charged at the end of each trip, they pretty much stay that way.