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Date:         Wed, 3 Nov 1999 10:04:06 -0500
Reply-To:     Bulley <gmbulley@BULLEY-HEWLETT.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Bulley <gmbulley@BULLEY-HEWLETT.COM>
Subject:      Re: Is battery isolator better than a relay?
Comments: To: "Michael A. Radtke" <m.radtke@ELM.AZ05.BULL.COM>

Michael-

Yours is a concise explanation of what the guy at the camper store took 10 minutes to (almost) explain. Thanks!

G. Matthew Bulley Director Bulley-Hewlett & Associates www.bulley-hewlett.com Cary, NC USA 888.468.4880 tollfree

-----Original Message----- From: Michael A. Radtke [SMTP:m.radtke@ELM.AZ05.BULL.COM] Sent: Wednesday, November 03, 1999 9:35 AM To: vanagon@GERRY.VANAGON.COM Subject: Re: Is battery isolator better than a relay?

Cary,

The battery isolator is a set of two diodes that steers the charging system output into the least charged of the two batteries. If the battery voltages are the same (i.e.., equal charge level) the charging current is divided equally. In addition the charge from one battery cannot dump into the other, discharging the first.

On the other hand, the relay simply connects both batteries in parallel while charging (ignition on).

The bad news is that Vanagon's have (dumb) alternator regulators that regulate the alternator output voltage as measured at the alternator terminal. Since the battery isolator uses diodes, and diodes have about a 0.6 volt conducting voltage drop, the result is that the batteries don't get a full charge. The more usual way for an alternator regulator to work is to sense battery voltage with a separate lead. In that case, real battery voltage can be sensed and the alternator happily puts out the additional 0.6 volts.

I have heard that some folks have located the remote sensing connection in the alternator regulator and added a wire to remote sense.

The good news is that the relay is not as bad as it first sounds. Lead acid batteries have charging characteristics that allow them to charge in parallel and share the charging current fairly well. And, connecting a discharged battery across a charged one isn't all that dramatic either. Again, they quickly adjust to one another and as long as you drive long enough to charge the low battery there should be no problem. Besides, VW owners have been using relays for years without any big issues.

Mike '84 GL '58 Isetta '60 Vespa --- Phoenix AZ

---------- From: CARY CHIANG[SMTP:chiang1@USA.NET] Reply To: CARY CHIANG Sent: Wednesday, November 03, 1999 2:59 AM To: vanagon@GERRY.VANAGON.COM Subject: Is battery isolator better than a relay?

Listees: Regarding the posts yesterday about aux. battery charging via a relay, I was wondering what the benefits were of a battery isolator versus a simple relay? Is it that the isolator can alter charging current to one battery over another (or something like that) while a relay can only charge both batteries equally, or what? Don't both setups charge both batteries while the engine is running and disconnect (and prevent discharge from) the main battery when the engine is off?

Cary

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