Date:         Thu, 20 Dec 2001 20:39:02 -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
Comments: To: The Bus Depot <vanagon@BUSDEPOT.COM>
In-Reply-To:  <IBEMIMEBBBEOIIGIKKAHEENDDDAA.vanagon@busdepot.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 07:35 PM 12/20/2001, The Bus Depot wrote:
>is some risk associated with doing so. Say you used a relay/wiring of
>infinate capacity (it would pass as much current as the aux battery wanted
>it to).  If the aux battery were totally dead or shorted, as soon as you
>turned on the ignition, it would try to pull everything from the starting
>battery all at once, and the relay would let it. The result of that sudden
>massive current transfer could be a fire or explosion.  No, it is not a huge
>risk, but it is a real one, just as there is that potential when you try to

It's really not that simple, Ron.  Batteries are active (sort of) devices
and don't behave like simple resistive loads.  If the aux battery really
was completely shorted your "15-amp" 12-ga setup would pass something like
one thousand amps until something melted, so that issue is moot ISTM.  Any
paralleling system should no doubt be fused in keeping with its capacity --
however even the stock setup has high-capacity wiring running to starter,
alternator, relay/fuse panel -- and not a fusible link in the bunch, so to
a certain extent it's the pot calling the kettle black.  Even back in '84
the Japanese were putting fusible links (hi-amp fuses really)  on
everything in the harness except the actual starter wire.  And incidentally
the factory aux battery wiring was 6.0 mm^2,  slightly larger than 10 AWG
--  the same size they ran from the battery to the fuse/relay panel, rated
by DIN to carry 54 amps at 30C and 38 amps at 50C.   A good deal heftier
than the kit you sell, I believe.

david


--
David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"