Why not just put a battery Isolator in. It will allow you to charge
both batteries without drawing power from on another. The problem
using a switch to select the battery is if you forget. That said I
don't have a need to use both batteries at the same time so there may
be a better way.

JFF

http://www.smithae.com/surepower.html

>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