mine is a physical switch, to switch between battery 1 (main), battery 2 (aux) and both like Terry, and the set up i'm considering is virtually the same as he had.
It can use either battery (or both) depending on the switch position. So, unless the main battery died completely, the aux would (in my tiny mind) only be used to give it a bit of a kick to help start the engine?
Where was your aux battery Brian?