I almost hate to ask this question.  I am adding a relay and aux battery to
my passenger van.  The relay came from a weekender and I currently have my 2
batteries connected directly together and not using the relay.

So my question is can the relay tell the difference between battery voltage
and charging voltage (I wouldn't think so).  But if not what triggered the
relay to close only after the van was running.

Clarification is greatly appreciated, but please no wiring diagrams or big
words.  I liked the way the relay worked in my dual battery camper and I
would like to set it up to work the same way in my new van.

Thanks very much,
Dave in Denver