Take a look at www.surepower.com.  I installed their relay type battery
separator in our Westy over two years ago and it's worked flawlessly.  I
used the model 1314 for this purpose.  If installed with a heavy gauge wire
interconnect between the batteries it is possible to use the aux battery to
start the vehicle if your main battery is not up to the task.  If you are
interested I can give you some more details.

Regards,

Tom Moritz
'90 Westy

>Date:    Thu, 4 Dec 2003 17:10:34 -0500
>From:    Edward Maglott <emaglott@BUNCOMBE.MAIN.NC.US>
>Subject: circuit for bridging the batteries
>
>I was looking at a way to hook the aux and vehicle batteries of the Westy
>together.  I have a charger set up on the aux batt, so that it gets charged
>whenever the vehicle is plugged in.  I would like to connect the two so
>that the vehicle batt can share in that charging when needed.  Or let the
>aux batt boost the vehicle batt when needed.  So I thought, there's already
>a relay in place to do this.  How about a little switch to energize the
>coil on that relay?  I tried applying 12V to the blue wire and that did the
>trick.  So then I started to find a switch to install for the purpose.  Now
>I am thinking about where that 12V is going to go when it also travels back
>down the blue wire to the alternator.  Is this a bad idea?  Thanks for any
>input.
>
>Edward
>