Harald, we use a small 1"x1"x1" cube relay rated for 30 amps that was
intended for use with high intensity driving lights. These can be
purchased for about $10. To be on the safe side, I added a 30 amp fuse
and 6 gauge wire in the circuit.
Larry
-86 Syncro Westfalia

harald_nancy wrote:

> To all westfalia camper owners, and especially those
> using a dual battery. If you bought a used camper with a "dual" batt.
> setup, it would be a good idea to check this out.
> I did some research at my VW dealer today about the different relays
> used in VW vanagons and campers.
> We came up with the following.
> 1.)
>  The vanagon westy camper with the 3 way refrigerator
>  (propane, 110 and the 12 volt while driving only)
> does not have a factory installed auxiliary batt. system.
>  It is equipped with a small shutoff relay located under
>  the driver's seat in the empty batt. box. It is a small
>   1" by 1" cube. This relay is called a :
>  Refrigerator 12-volt heater relay.
>  Part Nr. 231 069 555    price: 32.72
>  This relay is not designed to be used to charge a second battery.
>  To do so could damage your camper. In addition the wire to the
>  relay is only a 2.5 mm wire routed through the fuse panel.
>  This factory setup was not designed to handle the load to
>  repeatedly charge an empty battery.
>  Reference, Bentley 80-91 page 97.32b, 97.33b
>  2.)
>  Some vanagons did come equipped from the factory with a
>  dual battery system. Some of these are also called campers
>  in the Bentley, but appear to be weekenders. (no 3 way fridge)
>  VW does list a relay called:
>  Relay for 2 batteries.
>  Part Nr. 411 915 511 B    Price a hefty 60.39
>  ( maybe someone with a weekender camper with a factory
>    dual batt. setup can verify this part number.)
>  If the vanagon was factory equipped with a dual batt system, the
>  second battery is shown in the Bentley diagram for that vehicle.
>  The second battery is always conected via a
>  6.0 mm2 cross section wire, to prevent overheating and insure
>  efficient charging.
>  Reference, Bentley 80-91 page 97.34a, 97.222, 97.225
>
> In summary: If one wants an efficient dual batt. system in
> a full westy camper with the 3 way refrig., it is necessary to
> install a new system.
> You have your choice, one of the following:
> 1.) install a  manual switch, or 2.) install a 30 or 40 amp relay
> or  3.) install a batt. isolator.
>
> Also note:  Bentley sometime shows wire sizes in standard
> (awg) gauge, and in the later Bentleys mostly as mm cross-section.
>
> Harald
> 90 westy with a dual batt system.