> Here's the schematic
>
> http://wirewerksperformance.com/images/Schematics/aux%20battery%20charge%20kit.doc
>

That schematic has the right idea. The only thing I would do differently is
the relay ground. Instead of grounding the relay coil to the chassis, I
would connect it to the + side of the starter solenoid (ground is then
available THROUGH the starter, so long as the key is not turned to start).
The "hot" side of the relay coil being fed off the blue alternator wire
ensures that the secondary battery is only connected to the system if the
alternator is spinning. Grounding the coil through the starter solenoid
ensures that it'll ALSO wait until after the starter motor is no longer
drawing from the main battery. Without that, if the secondary battery is in
better shape than the main battery, the starter motor can draw some wicked
amperage from the secondary through what is usually a very small wire.

--
John Bange
'90 Vanagon - "Geldsauger"