Normal battery isolators are a poor choice in a VW due to the forward
voltage drop. All power diodes drop over 1.0 volt when cool. As they
heat up, the drop increases. The vw senses and control the alternator
output internally so this drop is not compensated. Only 11.5 to 12 volts
actually charges the battery. Many American cars sense and control the
charging voltage remotely so the alternator output will increase to
overcome the resistance. The alternator can be modified to overcome
this. I feel it is not worth the effort. Virtually every RV manufacturer
uses relays to isolate-charge the "house" battery(ies). It is simple,
reliable, and low cost. It is also more efficient as you do not have the
heating effect with a relay.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Julian Burden
Sent: Tuesday, December 11, 2001 2:11 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Battery isolator

I was told that a regular battery isolator will not work in a Vanagon by
someone who was told by someone who . . .

I did not believe this as my understanding is that a battery isolator is
just a pair of diodes on a big heatsink.

But, then I looked at the JC Whitney website catalog - they list several
isolators, many of which are described as not suitable for all VW vans!

Why is this so, and what do I need?