I have a 1980 '37. I have added 3 group 27 batteries to the original one group 27 and one group 24 (under the galley step). One of the batteries is located in the storage area under the starboard saloon seat. The second is located in a compartment created by opening up the area under the galley stove. The third is in a shelf in the forward hanging locker. The first two batteries are connected to a typical 1,2,& all switch with the common output connected to the common pole of the original battery switch. The third is wired in parallel with the original #2 battery and essentially forms a single battery as it has no separate switch. The battery in the forward hanging locker is also the primary battery for my anchor windlass. My system is not bullet proof. It does allow each battery to be isolated from the system if desired by manually setting the switches.