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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