I think you might mean solenoid?? It's controlled by the ignition. When the key is on / car running both batteries are in parallel. When the ignition is cut, the solenoid isolates a few circuits that are wired on the backup battery side of the solenoid.
This is what I have in my vanagon. I run the radio, fridge, interior lights and some 12v / USB outlets.
This is what I have in my vanagon. I run the radio, fridge, interior lights and some 12v / USB outlets.
http://www.gowesty.com/product-details.php?id=2418