Originally Posted by MLA
One advantage to having a traditional dual-bank battery switch, is the ability to use the house bank as a backup starting battery. Eliminating that switch in place of a continuous-duty solenoid, prevents this. Just some food for thought. That same solenoid can be used injunction with with switch.
I was under the impression the relay does the same, but check my thinking on this:
When the key is in the OFF or ACC position, the relay is open, thereby separating the batteries. When in the ON or START position, the relay closes, essentially combining both batteries. So when I start my boat, I am pulling 12v from both batts in a parallel circuit.
I guess where the switch differs is that you can engage or disengage the batteries individually vs. the relay which is either separate or combined. So does the switch allow the user to "substitute" either of the batteries into the starting circuit?
Even in that case I'm not sure of the advantage of substitution vs. combining.