At 08:55 PM 16/05/2012, you wrote:
>I agree, Bob. The resistance of good clamps on fairly clean metal is
>going to be maybe 1 or 2 milliohms per connection. This would be 8
>milliohms max for the entire circuit. 4 AWG wire is about 0.25
>milliohms per foot so if the set of cables is 10 feet long, the entire
>path length is 20 feet or 5 milliohms for the wire. The total path
>resistance is then 13 milliohms. If you went down to 0 AWG wire, the
>resistance of the wire would be 0.1 milliohms per foot so the total
>contribution from the wire would be 2 milliohms making the entire path
>10 milliohms which is significantly less. Going to even larger wire
>would let you approach 8 milliohms total. Actually, 8 milliohms for
>the clamps is probably higher than the real value anyway, which makes
>the cable even more important.
>
>I think that people are not giving enough consideration to the current
>it takes to charge the dead battery. Assuming that it is merely low
>and not damaged permanently, it will probably draw 200 or 300 Amps
>when connected to the other car. This is higher than the starter
>current so it would likely be the more dominant factor in producing a
>large voltage drop across the cable and preventing you from starting
>the car right away. I wouldn't think that revving the engine on the
>"donor" car would make all that much difference since the alternator
>is probably not capable of supplying more than 100 Amps.
>
>Sean

The current taken by the charging battery isn't important, because
the net flow is OUT of the 'dead' battery when the engine is being
cranked. Unless the 'dead' battery is internally knackered (shorted
cell etc), it will have an OPEN CIRCUIT voltage that is more than 10V.
It's more the internal resistance that goes up as the battery discharges.

I suppose a _really_ cr*ppy cable will get significantly hot from the
charging current and its resistance will thus increase. A 25=B0C rise would
result in about 10% less current being available.

Experience says that revving the engine does help (a bit). It supplies a
bit of extra current and raises the voltage. It's also hard on the
alternator rectifiers.

I have a portable battery that has relatively short heavy cables on it
(similar to this one, but with LEDs in the alligators)
http://www.amazon.com/Clore-Automotive-JNC660-12-Volt-Starter/dp/B000JFJLP6=
/ref=3Dacc_glance_e_ai_ps_t_2
It works amazingly well (short AWG 4 cables).
(downside is that it takes overnight to charge, so if you are caught
with it discharged and a dead battery, and have to leave _now_, you're
scuppered.


Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m

Please help out: http://dailynews.yahoo.com/fc/US/Emergency_Information/=20


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