I agree with Mark,  I installed the Surepower 1315 separator/relay
(purchased from a reseller in NJ) on my 72 Westy over a year ago and
it's worked flawlessly.

It was easy to install and it's automatic -- just install and forget
about it...and no messing around with the alternator or voltage
regulator (on a '72) wiring.

My notes on the installation:
http://www.pauldottrip.com/vw_bus/battery_separator/

>SurePower makes a special relay device that may be the simplest and most
>straight forward way to wire in a second battery for a Vanagon.
>
>http://www.surepower.com/Products/batsep.asp
>