Date: Thu, 20 Dec 2001 16:32:40 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: A twist on dual-battery configurations
In-Reply-To: <04c601c18994$a74c86e0$3b47fea9@grumpy>
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At 03:26 PM 12/20/2001, Julian Burden wrote:
>By now we all know that a standard battery isolator drops the voltage
>available to charge the batteries. In a Vanagon this leaves too little
>voltage for the battery(ies) to be fully charged.
Make that "in any vehicle using an internally-regulated alternator." It
may be a bit worse in Vgon because of long wiring runs...
>1. it takes a long drive to recharge a depleted auxiliary battery
>2. it is impractical to run all accessories from the auxiliary battery
>(which is what I've done on previous vehicles, headlights and all!).
#2 is curable by using a heavier relay/wiring setup.
#1 is partially curable by heavier setup, but really needs proper regulator
control.
Comment: Why would you want your running loads to come from the house
battery? The house battery is to run things when the engine isn't.
>Another solution is to modify the voltage rectifier on the alternator to
>compensate for the voltage lost through the isolator - although some have
>concerns about the alternator running hotter for two reasons:
The "mod" consists of running a sense lead from the regulator to the
battery so that it can regulate based on battery voltage instead of
alternator voltage (which makes sense since we're not trying to charge the
alternator...). This is a worthwhile mod even for a single battery.
>1. it is putting out a higher voltage
Specious -- the alternator can probably put out 120v into an open circuit.
>2. energy is lost in the battery isolator - this energy has to come from the
>alternator.
True.
>One other solution suggested in the archives is a dual output alternator -
>one with two sets of coils on the stator. The only negatives I've seen in
>the archives is the expense of these beasts and the fact that if one battery
>is fully charged half of the alternator's capability is going to waste.
I don't know for absolute sure, but I'd be astounded if they used two sets
of coils rather than two sets of diodes...
>Now this has got me thinking - surely a far simpler way of building a dual
>output alternator would be to have only the one set of coils, but two diode
>packs? And couldn't a standard alternator be easily modified to achieve this
>end?
See above.
>Unless I get advice which sinks this idea, what I'm going to do is to tap
>into the three-phase wiring prior to the diode pack. I'll have three wires
>coming out of the alternator and going into an external diode pack made up
>of three high current, low voltage drop, diodes. The cathodes of these
>diodes will be connected to a terminal serving as the alternator's second
>output - to be connected to the auxiliary battery.
Possible concern -- if your diodes Vf don't match Bosch's, you'll have
different outputs to the two batteries.
>I see this method as having several advantages:
>
>1. The main battery's charging system will remain unaltered.
True
>2. The two batteries will be properly isolated - reverse current will not be
>able to flow through either diode pack, so a flat auxiliary battery will not
>be able to discharge the main battery.
True
>3. The full output of the alternator will be available to either battery.
True
>4. Less current will be going through the existing diode pack, meaning the
>alternator will run cooler.
Mmmm...but you have to figure out how to cool the external pack. The
alternator has a bloody great fan sucking air through it...
>5. Current will only pass through one diode on its path between the
>alternator and a battery or accessory (avoiding the double voltage drop
>encountered when using a standard isolator).
True.
>6. At about $60 for the parts this is still reasonably inexpensive (although
>more expensive and much more work than the relay kit).
>
>So, fellow listees, why should I not proceed?
IMHO, because this does nothing to address the issue of regulation which is
more worthy of effort. The actual charging connection is easily addressed
with a relay or electronic combiner, either of which will allow an
externally-sensing regulator to charge based on whichever battery needs it
more -- which the diode isolator does not do. Your extra-diode solution
does address this, but is still looking inside the alternator, a very poor
place to find out the state of the battery(s). If you really want to have
and use a deep-cycle battery, grab the bull by the horns and set things up
so the sucker will actually charge. I know it's big bucks, but you'll be
happy later. :) http://www.amplepower.com/
cheers
--
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
