Ugh, sorry - I guess I should have stated that when I hook to AC, it's using a dual battery charger (the 4/4 unit): **broken link removed**.
This charger distributes current to each battery "as needed" so that both are fully charged (when hooked up to AC). It has a standard US 110V AC plug (which I propose to "tap"). I turn off the duo-charge when I plug into AC, but I know I'll forget at some point, so the relay came to mind as a way to short the power to the duo-charge any time I plugged into AC. Sorry to make it more confusing than necessary! Thanks for the response.
Tom