Why two batteries need a switch
Wikipedia’s battery-isolator article describes the job: let one charging source feed more than one battery without letting a weak pack drain a healthy one. On a boat the common hardware is a 1-2-BOTH-OFF switch or an isolator/combiner. The starting battery should still be able to crank after a night of cabin loads. House on 2, start on 1, OFF when you walk away is the ordinary pattern.
BOTH is for combining, not for every Saturday
BOTH parallels the banks. Useful to start on a weak starter by borrowing house, or to share charge in a controlled way. Left in BOTH all week, a parasitic on the house side flattens both. Owners describe “both batteries died together.” They were tied. We find the switch position as well as the cells.
Isolators add drop; combiners sense voltage
Diode isolators prevent backfeed and cost some voltage between alternator and battery. Combiners close a relay when they see charge voltage. Either can fail. A failed isolator looks like a battery that never gets charged. We test the path, not only the CCA sticker.
Lithium plus lead on one switch
Different chemistries on BOTH is a messy voltage story. If the boat has LiFePO4 house and a flooded starter, the combiner and charger have to be chosen for that pair. The switch article does not make that pair legal by itself.
Gulf consoles
Grady-White and Scout dual-battery boats in Lee County usually have the switch in the console. Salt on the studs is as common as a tired cell. We clean, test, and replace what failed — battery, switch, or both.
Failed positions we see in salt consoles
OFF that still passes current, BOTH that is welded, and a 1 position that never sees the charger are common once salt is in the studs. We test with a meter, not by trusting the knob. Replacing the switch with the batteries is ordinary on Gulf boats. Leaving a new AGM on a welded BOTH is how both packs age together again.
Sources
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.