Originally Posted by Kweisner
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.
On paper, this is how the solenoid would work. One thing to consider is that when 2 batteries are combined, they will want to equalize. Meaning the one with the higher charge will flow into the one with the lower static charge. So if one is real low or has totally taken a chit, then combining them through the solenoid means you could end up with 2 batteries too low to crank the engine. `
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?
A traditional dual-battery switch is 100% manual. It will allow you to choose to draw/charge from/to battery 1 or battery 2 or combine the banks and draw/charge both. Adding in a solenoid or ACR makes it a manual/passive system.
The ignition key switch only disconnects some of the boat loads from the engine, leaving others to be a parasitic draw. Having an actual battery switch will allow for ALL loads, except the auto bilge, to be isolated from the batteries when off.