I have twin engines, and a 20amp battery charger that supports two banks. I
want to add a stereo and an amp to the boat. I want to add a third deep
cycle battery just for the stereo, but I would like it to charge while at
the dock and charge while the engine is running. How can this be done?

Also, should I keep my batteries on All while plugged in to shore power? Or
should they be in the off position.

Glenn