From: Ronald Wong (ronwong@inreach.com)
Date: Tue May 28 2002 - 23:53:08 PDT
Message-Id: <l03102801b918d853f600@[209.209.18.204]> Date: Tue, 28 May 2002 23:53:08 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: pinhole battery questions
Neil said:
>In a car battery, the electrodes are large and very close together, hence they
>can deliver very currents of around 100 A. ...
By "very large", Neil means that the plates of the electrodes have a large
surface area. By making the surface area as large as possible and placing
them as close together as one can without having them come into physical
contact, the battery manufacturer minimizes the amount of internal
resistance the battery has and, as a result, maximizes the amount of
current available to the car. This is due to the fact that the resistance
of a conductor (the electrolyte and lead plates) is inversely proportional
to the cross-sectional areas of the conductor (determined by the plates)
and directly proportional to it's length (the distance between the plates
of the electrodes as well as their thickness).
Different cars have different needs and manufacturers supply 12 V batteries
with different "cold-cranking" amperage ratings by adjusting these
parameters. The ratings can be as little as 300 to as much as 800+ amperes.
My old diesel had a battery rated at 600 CCA.
When you consider the fact that a typical household circuit can supply 30 A
at 120 V. before the circuit breaker kicks in - amounting to 3.6 kW of
power - then you can see that you should be VERY careful when you are
working around an automobile battery. 600 A at 12 V is 7.2 kW!! For a very
brief period of time, my battery could supply twice as much electrical
power as a typical household circuit and it could do this repeatedly for a
number of cycles before it gave up the ghost.
This battery could also supply considerably more for a very small period of
time if someone accidentally shorted out the terminals of the battery. At
this point, as Neil said,
>the sulfuric acid solution in the battery will boil and the battery
>will explode.
It does happen.
So be very careful when you work around a car. I once worked on my car and
forgot to take off my watch with it's band of large, metal links. It
shorted out the electrical system at the voltage regulator. There was a
tremendous discharge and the amount of current was enough to vaporize part
of my watch's metal band. Fortunately,the battery stayed intact. This was
probably due to the fact that the short occurred where there was enough
electrical resistance in the circuit to prevent the current from reaching
destructive levels. I ended up with a welt around my wrist that looked
exactly like my watch band. It lasted for a couple of weeks. Ooooooh... 8^(
ron
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