Hey Larry, Bill, Beckett etc.

I've done some digging into connecting 2nd-battery relays, possibly at the
alternator's "output".

If you look at the alternators and their electrical wiring diagrams
closely, there are TWO alternator outputs, marked B+ and D+ on wiring
diagrams.

The main output B+ is directly wired to the battery positive, starter, etc,
as pointed out by Larry.  No battery current flows back into the alternator
because it has rectifying diodes inside.  Their main purpose is changing
the alternator's generated AC power into pulsating DC power, similar to
what commutators and carbon brushes did for generators in pre-alternator
times.

The auxiliary D+ output serves to drive the alternator lamp and to drive
the Westy's auxiliary battery relay.  A-HA!  This is clearly shown in my
Bentley book in the camper wiring section.

Hope this removes some of the mystery, for all of us.

Stanley