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Date:         Fri, 12 Jul 2002 10:22:42 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Battery switches
Comments: To: Rob Gunter <rgunter@SEC-TN.COM>
In-Reply-To:  <KBEDLMCEHDJNHLFELCLJEEDECEAA.rgunter@sec-tn.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 08:57 AM 7/12/2002, Rob Gunter wrote: >I am not familiar with the VW setup, but my van uses a battery isolator to >automatically charge each battery when needed. The isolator acts like a >diode by preventing current from running back out of the batteries into the >charging circuit.

Rob, installing an isolator in a Vanagon is not completely simple because the voltage drop of the isolator causes charging problems which are aggravated by the long cable from alternator to battery. It becomes necessary to bring the sense terminal of the alternator outside the case and downstream of the isolator (there are isolators with a special terminal for this, but you still have to modify the alternator and run the wire to the front. You then face the problem of which battery to sense...

There are various arrangements using a combining relay or an electronic "combiner" which alleviate this problem. The stock VW arrangement (uncommon) uses a relay with fairly heavy wire (?6 ga?) between the two batteries. Others use thinner or thicker wire, either of which affects the potential rate of charge of the aux battery.

> The problem of using switches (without an isolator) to do >this is you can get competition between (dueling?) batteries for the charge. >One battery will "charge" the other and they can switch back and forth >draining both.

I have to disagree with this, at least mostly. Because of the nature of lead-acid batteries, two batteries paralleled together while being charged will automatically apportion the charge so that the bulk of it goes to the less-charged battery. Charge does not flow from one battery to the other because both batteries are accepting charge from the alternator, albeit one may be only a trickle and the other a flood.

If the batteries remain paralleled while not being charged, some charge can flow -- however again because of the charging characteristics of lead-acid batteries it will be minimal unless the batteries are at very different states of charge. This is certainly not ideal, and batteries that are not hard-wired in parallel generally should not be routinely left paralleled while not being charged. The relay addresses this while the manual switch relies on the vigilance of the user.

cheers david

ps -- great pictures :)

-- David Beierl - Providence, RI http://pws.prserv.net/synergy/Vanagon/ '84 Westy "Dutiful Passage" '85 GL "Poor Relation"


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