Date:         Wed, 5 Aug 2009 22:24:58 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Vitrifrigo trip results, wiring, battery life,
              charging issues, etc
Comments: To: Jeffrey Vickers <jeff@VICKERSDESIGN.COM>
In-Reply-To:  <605EDF24-2A23-493D-A4B7-AE335C484F24@VICKERSDESIGN.COM>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 07:56 PM 8/5/2009, Jeffrey Vickers wrote:
>I'm going to hold off on an wire improvements until I get my 3.3 SVX
>motor installed. It has a bigger alternator and might be oomph enough
>although the small charging wire problem is not lost on me...

Bigger alt won't help -- the problem is that as charging current
rises, a larger and larger voltage is developed across the charging
cable.  So let's say that at its current state of charge your battery
will accept 60 amps at 14 volts but only 20 amps at 13.5 volts.  If
your cable is dropping half a volt at 20 amps then the battery will
only see 13.5 even though the alt is putting out 14 -- and the system
is now balanced, charging at 20 amps even though it could potentially
charge at 60.  As the battery charges it will accept less and less
current at a given voltage; so the voltage drop across the cable
becomes less as the current drops and a new balance is achieved at a
slightly lower charging current and slightly higher battery
voltage.  This sort of thing is why Newton invented The Calculus of
Infinitesimals...  The voltage across the cable asymptotically
approaches zero as the battery voltage approaches the alt voltage and
the charging current approaches zero.  Note that the specific numbers
I pulled out of my hat, though they're well within an engineer's
order of magnitude of correct.*

*Engineers are pickier than scientists, factor of ~two rather than
factor of ten for "correct" answers...

  --
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"