Date: Wed, 31 Dec 2008 18:15:02 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Voltage Drop - Less than 3% - 15 feet, 2/0,
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At 02:55 PM 12/29/2008, M'obeechi wrote:
>I have copper connections, could also get marine grade copper
>connectors which are lined with tin I think. Only thing is, crimping
>the connection with a wrench seems kind of iffy, and doesn't give a
>me the "dimple" I see on the connectors someone else made for me.
Please! Get a proper crimping tool (costs ~ $50, you bang it with a
three-pound hammer until the ends meet, try marine stores) (It's a
spring-loaded seesaw with one side sized for terminals, the other for
lugs which IIRC are slightly larger -- set the spring to hold the
working side closed -- cavities on the bottom, points on the top) or
hire the job done by someone with the proper tools. You're going all
nuts for everything just exactly so, and then talking about skimping
one of the most vital parts of the job, it just doesn't make
sense. If you don't get a proper gas-tight connection at the
terminals then you're Damocles with humidity and salt air nibbling at
the thread. And with the current that setup can deliver, my guess is
it could fail by catching fire, not just wimping out.
Which reminds me -- you do remember you're basically building a
multi-thousand-amp welding rig there, so serious attention to proper
fusing should be large in your plans. You're going to be needing
interrupting capacities up in the 10-20,000 DC ampere range, I would
think. [Incidentally, in the early '50s the Navy sent my father to
MIT to try to build a fuse with 100,000 DC amps interrupting capacity
for submarines -- they were using 600 volt batteries at several
hundred amps working current IIRC, for propulsion. I don't think he
succeeded, but I know you can buy one now.] Here's a datasheet for a
suitable fuse (200,000 amps AC, 20,000 amps DC interrupting capacity)
in values up to 30 amps. The same company makes other series fuses
for higher current values.
<http://www.bussmann.com/pdf/f8f687cc-1fff-4389-be7b-f62931b45053.pdf>
The recent fad for ridiculous automotive stereos has made
high-blowing-capacity fuses easy to get at the FLAPS, but I don't
know whether or no they have appropriate interrupting capacity, as
that is much harder to achieve, *especially* for DC. AC is much
easier because the arc shuts off so many times per second regardless,
but DC just keeps on pushing.
Here's a teaser from an extremely interesting article:
>For comparison, we decided to directly short the battery with only
>the shunt and switch in the circuit. This would give us an idea of
>the maximum available current the batteries could deliver to the
>devices we had tested. The switch was thrown for approximately three
>seconds and then shut off. The meter recorded 6960 amps as the peak
>current. We repeated this three times, with each additional reading
>lower in value. During each test the 4/0 positive cable lifted up
>off the ground 4 inches into the air by the forces generated from
>the extremely high current flowing through the circuit.
The entire article, which is very readable and densely filled with
directly relevant information, is at
<http://www.ibiblio.org/ecolandtech/alternative-energy/homepower-magazine/archives/27/27p26.txt>
Your setup has somewhat smaller output capacity than theirs. For
comparison the Optima Group 31 battery has rated impedance of 0.0025
ohms, allowing a theoretical short-circuit current up to about 5,000 amps.
How far you actually follow all this in practice might depend on how
much of the setup is inside vs outside the cabin and no doubt other
factors as well, but you need to be informed to make such decisions prudently.
And just for giggles, here's what a truly serious AC arc looks
like. I can tell you that the sound doesn't do justice to it. I've
been a few hundred feet away when an ordinary street-pole breaker
opened, and it was EXTREMELY LOUD. That one arced for about a second
and there was no visible excitement.
<http://www.arcfault.org/video.htm>
Cheers,
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"
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