Bob -

Details on my auxiliary battery installation are here:  http://www.sittser.com

I used a solenoid (relay) connected to the alternator, like you described.  However, if I were to do it again, I wouldn't do it that way.  Instead, I'd use a "battery combiner", such as this Sure Power 1315:

http://www.cooperindustries.com/content/dam/public/bussmann/Transportation/Resources/instructions/BUS_CBT_INST_180074d.pdf

 
A battery solenoid only connects the batteries when the engine is running, and needs a signal from the alternator.  The separator has two advantages:

(1) The only required connections are wires to each battery's + terminal, and a ground (just a small-gauge wire to the body is fine).  No alternator connection is needed.

(2) Whenever either battery is charging, for any reason, it connects the batteries together.  Otherwise it separates them.  That means that if you run the engine, or charge either battery with a battery charger, the other battery will automatically get charged too.  (That's if you get a "bidirectional" separator, like the Sure Power 1315.   If you get a "unidirectional" like the 1314, it only checks one battery for charging.)

The Sure Power also can take an optional "jump start" switch to manually connect the batteries so you can jump your engine from the auxiliary battery.

At around $50, it's more expensive than a comparable 100 amp solenoid, but about the same as the GoWesty auxiliary battery kit.  Of course, with the separator you'd need to add you own wires and connectors for the battery connections.

I'm currently installing one of these on our other camper van (a Sportsmobile - you can never have too many camper vans), to replace its ancient failed diode-based battery isolator.  If I plug in the camper to 110, the power converter will charge both the camping batteries and the engine battery.  

               - Steven Sittser

 

-----Original Message-----
From: Robert Clemmer <n51219@COMCAST.NET>
To: vanagon <vanagon@GERRY.VANAGON.COM>
Sent: Thu, Apr 19, 2012 10:32 am
Subject: Aux Battery

I'm going to install an auxiliary battery under my rear seat in my 91

Vanagon to power my refrigerator and other appliances without killing the

service battery.  Any suggestions on where I can tap into for a trigger wire

that will power the solenoid that will connect the aux. battery to the

charging circuit only when the engine is running.

Bob C