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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
Comments: To: "Warner, Jeff (DSIO-MS)" <Jeff.Warner@DSIO.DLA.MIL>
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

_____________________________________________ Toll-Free for Orders by Part # 1-866-BUS-DEPOT


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