Date: Fri, 14 Dec 2001 03:32:06 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: 2nd battery relays & the Hella in particular
In-Reply-To: <4c.3657e04.294af5b9@aol.com>
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At 01:27 AM 12/14/2001, Frank Grunthaner wrote:
>In a message dated 12/13/01 12:47:48 PM, dbeierl@ATTGLOBAL.NET writes:
>
><< but your finding that #10 wire requires a 20-amp
>fuse is consistent with the theory.
> >>
>
>David,
>
>I most strenuously object to your misuse of the word "theory" here. The
>restivity variation of wire by diameter was well established experimentally
>long before the birth of the parents of the listees. The theory of the
>variation of restivity with cross-section of the metal was postulated,
>promulgated and verified so long ago that the original archival journal paper
>is probably in need of chemical restoration (certainly well known in
>Maxwell's time).
<snerk> I agree "theory" is a bad choice. I was talking about the
allegation by the mysterious expert B. C. (who as we know works in a highly
technical position for Exide and is in a position to know this sort of
thing) that a 12-ga connection between the two batteries would result in a
worst-case charging current of 15 amps to the second battery, and usually
much less. This would be under the assumed condition of a stock Bosch
internal regulator, i.e. max 14.1 volts at the B+ terminal.
Now four feet of 12-ga wire (his assumption for gauge and mine for length;
I don't know what's in the Hella kit) at 20C should have a resistance of
6.35 milliohms, or 6.35 millivolts per amp. That means at fifteen amps we
get a drop of just under a tenth of a volt across said wire. Allowing an
equal amount (wild guess) for incidental resistances gives two tenths volt
difference between the starting and house batteries at a charging rate of
15 amps on the house battery.
So the question devolves to: Given an empty house battery and a full
starting battery, will the charging voltage of the house battery always be
within 200 millivolts of the starting battery? If not, then we're going to
exceed that fifteen-amp charge rate. Unfortunately for us all, our
regulation point is at the wrong end of twelve or fifteen feet of
various-gauge wire depending on model year (for the '84 that's a doubled
12-ga from alternator to the junction box, then 8-ga to the starter, then
2-ga to the battery), so how the (*#$#@(*^ is anyone supposed to know what
the battery terminal voltage is going to be under different charging regimes?
My conclusion: The Vanagon charging system is a fex [sing. feces],
cynically designed to minimum standards and based on the assumption that it
doesn't actually matter how the bloody battery charges since it is always
at "full" charge and never needs charging anyway; likewise it doesn't
matter that "full" charge is nowhere near full and any battery installed in
this miserable excuse for a system will permanently lose capacity, but
nobody will notice since we're only ever drawing a couple of amp-hours out
of it to start the beast and the rest of the time its major function is a
filter capacitor. All right, I'm exaggerating all over the place. Rant
off. Fact is, it's good enough -- barely -- for starting, but utterly
inadequate for charging a house battery that actually gets used; and a
regulator sense lead to the battery B+ terminal would be a big improvement
(short of putting in a smart regulator). Running humongous copper from
alternator to battery is a more expensive and less satisfactory way of
approximating the same result, i.e. having the battery charge voltage be
what the alternator thinks it is (but it has the desirable side effect of
maximizing available starter current).
So getting back to the question at hand... <g> -- under this Pleistocene
(Plasticine?) system, full of first- and second-order side effects, what is
the actual worst-case charge rate through a battery-coupling relay and
attached wiring as supplied in the Hella kit? And what is the "normal"
rate? Beats the heck out of me...I have a gut feeling that Mr. Cantrell is
off by a factor of two or so, but a gut feeling is all it is. Is it
possible, for example, that a fifteen-amp *fuse* has more resistance than
the whole rest of the circuit? Ok, I checked -- automotive fuses run in
the range of 50 - 125 millivolts at rated current, so it's a first-order
effect. I'd sure like to see some experimental data...
cheers
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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