Let's Name the Electrical Problem, 12 Volt Question

[letsgetsailing3]

Bought 2 new batteries for a 2 bank system, then left the boat for a few weeks, with the main switch to "off". Disconnected shore power as well. Came back to one very dead battery (9.6V), but the other was still at 12.35V, so it seems that one of the banks was isolated from the problem.

Any really likely culprits? Shouldn't turning the main switch to "off" turn EVERYTHING off? Is something hooked up outside the system, and how to find it?

I'll be back at the boat tonight.

[Stu Jackson]

Do you have a bilge pump connected to one battery?

[Cheechako]

Things are often taken directly to a battery post bypassing the switch on many boats. You might check for that. There will be no wires except the cables coming off the posts unless someone has bypassed the switch. Often a bilge pump will be taken direct to avoid switching it off. Has it been raining a lot? Also, a wire can be attached to the back of your off/on switch in the same manner....bypassing the switch.

[letsgetsailing3]

There's a switch for the bilge pump, and I haven't heard it running, but that is a possibility that I'll look into.

[IdoraKeeper]

I found an anti mildew fan that was hooked up that way in the engine space....the PO was not great about electronic stuff. Spent the last two years removing stuff like that.