Date:         Tue, 3 Feb 2004 07:42:23 -0800
Reply-To:     laurasdog@WEIRDSTUFFWEMAKE.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Steve Delanty <laurasdog@WEIRDSTUFFWEMAKE.COM>
Subject:      Re: Why do JC Whitney Battery isolators have this VW caveat?
In-Reply-To:  <586160AC-55ED-11D8-ADDD-000A959B3796@knology.net>
Content-Type: multipart/mixed; x-avg-checked=avg-ok-3B224E2F;

At 06:04 PM 2/2/2004, Jim wrote:

>â—¦       For all vehicles (exc. '86-01 Chevy, GMC, Toyota and all 
>Volkswagen vans with alternators and vehicles with Motorola 35-, 37-, 55- 
>and 62-amp alternators that do not have red terminal plate—used on '73-80 
>AMC Jeep® and IHC vehicles).
>        â—¦       70 to 165 amps.
>        â—¦       Run accessories in vehicle without draining battery
>
>Why can't I run an isolator in a vanagon?
>
>Jim


Jim,
The problem is that it would cause the battery charging voltage to
become excessively low...

Battery isolators typically contain 2 large diodes to isolate the
batteries. These diodes have a voltage drop across them of roughly
0.6 volts, so the batteries charge at a voltage 0.6 volts lower
than the alternator output.
"Real" charging systems have a separate wire from the voltage regulator
to the battery so that it can sense the actual battery voltage and
adjust the charging output accordingly. This serves to compensate
for any voltage drop in the wire from the alternator to the battery
and will also compensate for the voltage drop across a battery isolator.
This setup makes for a nice stable charging voltage that isn't greatly
affected by changes in load current.

However, there are some cheesy charging systems that do NOT
measure the voltage at the battery, but instead measure it right
at the output of the alternator. The GM "1-wire" alternators and
the Bosch alternator use in the Vanagon are notable examples.
It's a truly pathetic way to charge a battery and the only reason
it is done is to save a buck. It generally leads to very poor battery
voltage regulation, especially if the wire from the alternator to the
battery is long and thin. (Vanagon!)

The good news is that it isn't too difficult to add an external Ford
style regulator to the stock Vanagon alternator, although it does
require removing and partially disassembling the alt.
I did it on mine and it made a big difference in charging performance.
There's a Volvo related website that explains how to do it if you
are interested.

If you try and use a diode type battery isolator with the stock
Vanagon system you will likely end up with an extremely low
charging voltage at the battery

Steve
EJ22 -> '86 Westy "Escape Pod" 


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