At 01:26 AM 5/19/2012, you wrote:
>I think we are really all saying the same thing. All I'm saying is
>this: what Spehro says MAY be true, but it also might not be true,
>depending on the circuit parameters.
>
>Have a look at this image to see what I am thinking:
>
>http://i.imgur.com/QmQZE.png
>
>There are four cases in this photo: lower right-hand one is a normal
>start (not a jump-start). The starter sees 11.7 Volts in this case.
>
>Upper right-hand is a jump-start with a battery which is so dead that
>it has a very high internal resistance. In this case, the starter sees
>10.8V.
>
>The two left-hand ones are where there is a dead battery with either
>10 times the "fresh" battery internal resistance, or 2 times. In both
>cases, the voltage delivered to the starter is from 100 to almost
>400mV less. Not much less, but not the same or more than without the
>dead battery connected. Note that in both of these cases, the dead
>battery is still charging while the cranking is happening. It does not
>begin sourcing current (in this example with these circuit
>parameters).
>
>My only point is that it is possible for the dead battery to be a
>liability (rather than a neutral player or a helper) when trying to
>jump-start a car. It would, in some cases, be better if the dead
>battery were not connected at all.
>
>Sean

Your approach is conceptually correct, however I think it would be
better if you put more realistic numbers in for the starter current
etc.

A typical car battery is rated to supply 875A with output voltage
equal or better than 7.2V, or 700A at 0=B0C. So, the internal resistance
of the fully charged battery won't exceed 5.5milliohms. Similarly, typical
voltage drops under normal cranking should not exceed around a volt
total. If we assume typical cranking current is 300A, then we have
3.3 milliohms for the wiring.

If, again, we assume a 300A cranking current then we have a
starter + wiring resistance of around 24 milliohms.

However, just by inspection we can see that the conjecture cannot be
true... if the starter is just barely cranking over, then the
voltage at the recipient vehicle battery must be less than the rated
7.2V (assuming the battery is the only problem!), so the "dead"
battery will be supplying some net current if it's internal voltage
is >10V.

Best regards,



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