On 5/15/2012 2:54 PM, Eric Kort wrote:
> So, this morning I jump started our van (kids wanted to finish
> listening to a story on the CD player when we got home last night, and
> a person who shall rename nameless forgot to go back out later and
> take the keys out of the ignition which was then left in "Acc"
> position all night).  This brought to mind a question I have long had
> about jump starting.  The battery in my car starts my car immediately.
>   But when I connect that same battery to another car (the one I am
> jump starting), that car will not start immediately...I need to wait
> several minutes with the cables connected until it will start.
>
> My question: why is that?  I will share some hypotheses I have, mostly
> for their entertainment value:
>
> 1. The resistance of the cables is too high to deliver the needed
> current to start the dead vehicle.  (It seems if that were true,
> someone would be out there selling uber-cables that don't have this
> problem, but I don't recall seeing such cables.)
It isn't so much the resistance of the cables as it is the poor=20
connections at each end. Measure the surface area contacting both ends=20
and compare it to the size of the cable, if it isn't at least the same=20
amount of surface area then it doesn't matter how large the cable in=20
between , it is like connecting a 100 ohm resistor to a 2 inch thick=20
copper cable, the limit will still be the resistor and increasing cable=20
size isn't going to change that.

> And while we are on it, why does the owner's manual suggest connecting
> the negative end of the cable to the engine block of the dead vehicle
> (doesn't the engine have continuity with the negative terminal on the
> battery--making it simpler from a procedure point of view to just
> connect the battery terminals on both vehicles)?
>
> Thanks,
> Eric
This is all about safety.  Hydrogen gas can be an issue but the real=20
reason for not placing both cables on the battery is because the  clamps=20
easily come loose from the terminals. If the two ends touch you now have=20
an  arc  of current equal to that of an arc welder  with the clamps=20
bouncing around from the arcs and the user trying to stop the shorting=20
out,  plastics, wires, and anything near by can easily catch fire from=20
the high temps the arc creates. I have seen what happened when a=20
mechanic dropped a wrench and it fell across battery terminals, not a=20
pretty sight. Most other surfaces are painted and don't pose as much a=20
threat from the positive contacting the frame.

Mark


--=20
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