Date: Thu, 13 Dec 2001 12:07:59 -0500
Reply-To: The Bus Depot <vanagon@BUSDEPOT.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: The Bus Depot <vanagon@BUSDEPOT.COM>
Subject: Re: Battery Isolators vs. Relays
In-Reply-To: <21517B66105AD211BE7C00A0C9E580EE044C698B@avenger.dsdc.dla.mil>
Content-Type: text/plain; charset="iso-8859-1"
> I thought the reason for going through the significant efforts to install
an isolator
> was to avoid a substantial drain placed on the starting battery by a
drawn-down aux
> battery when the van is driven after excessive aux battery use. By using
> the isolator, you keep the batteries separated so that the aux battery
> doesn't pull on the starting battery in a natural attempt to equalize.
The Hella kit (my part # 1738) will keep the two batteries separate when the
ignition is off, so no amount of discharging your camping battery can
possibly affect your starting battery. When you start the van, the Hella kit
(when installed properly and with the included wiring) will limit current
flow from the alternator to the camping battery to about 15 amps. This
means that no matter how dead your camping battery is, it will not draw more
than 15 amps from your alternator. So, no, it will NOT place a substantial
drain on the starting battery, even if your camping battery is completely
dead. Because it doesn't "slam" the camping battery with a ton of current
right away, but instead gradually recharges it at a constant and moderate
rate until it's fully charged. The "deader" your camping battery is, the
longer it will take to fully recharge, that's all. It's a very simple,
tried and true setup. That's why VW/Westfalia used it themselves for 20
years on those campers that were equipped with two batteries, as opposed to
more elaborate systems. Sometimes simple is better (and cheaper).
A relay based system has several advantages over an isolator system, not the
least of which is cost. Another advantage is the lack of voltage drop caused
by the diodes in an isolator. Thirdly, a failure of the system is unlikely
to leave you stranded with a dead starting battery, whereas the failure of
an isolator-based system often will. In the event of failure, a relay-based
dual battery system is most likely to fail such that the two batteries are
separated; this may mean that your camping battery won't recharge but it
also means that your starting battery won't discharge. By comparison, if the
diodes in an isolator fail, you are much more likely to discover the failure
when you go to leave and find that your starting battery is dead. (Believe
me; I know. It happend to me once, leaving me stuck in the middle of
nowhere.) Not that I'm bashing the isolator approach, which does work quite
well if you use the right product. But to me, it's unneccessarily complex
and expensive, given that a simple relay works perfectly well.
As for the advantage of using the Hella kit versus modifying your fridge
relay to do the job, I can't comment on the latter as I have never done it.
Others have, though, so obviously it works. For that matter, you could also
go out and buy your own relay, fuse, wiring, etc., and assemble a kit out of
pieces yourself. But why mess around when the whole kit is less than $20
anyway? For $18.95, you get a tried and proven kit designed specifically
for the job, with properly matched wiring and fuse protection to prevent
both under and over charging. No guessing, no risk, no hassles. Simple,
cheap, and reliable. This is a no-brainer.
- Ron Salmon
The Bus Depot, Inc.
(215) 234-VWVW
www.busdepot.com
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