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Steven Shelikoff

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Battery Meter

On 24 May 2004 00:15:23 GMT, (Gould 0738) wrote:

Are you claiming that if you can ever read 13.2 volts from a battery it must
be
fully charged?
Yes or no answer please.

Must be?
No. You could get a false reading.

But you can get a 13.2 reading from a fully charged battery without it being a
false or artificial reading. That's the combined voltage of all six cells
before the battery self discharges to a lower voltage.

After a battery partially discharges, it will stabilize around 12.6. Partially
discharged is not the same as fully charged- although it will represent a
common state of charge for batteries that have been removed from
a float charer and allowed to self-discharge to that level.

Why do you suppose every battery charger mfg sets float voltage at 13.2 or
13.3?
Just an arbitrary number?

I'm amused to see that the laws of physics are suddenly suspended when they
don't support your tottering argument. :-)

If you want to talk laws of physics, you're all wrong.

Sure, you can
find web sites that support just about any voltage of a lead acid cell
from about 2 volts to 2.2 volts with most of them supporting a claim of
around 2.1 to 2.15 volts. But if you want to talk physics, why not
write out the half-cell reactions that take place at the cathode and
anode of a lead-acid battery. Add them up and that is the potential of
the battery.

I'd write them here but the characters are hard to reproduce in ascii so
I'll just say that the anode reaction is about 0.36v and the cathode
reaction is 1.69v for a total of 2.05v. You can look them up yourself
by googling pbs04 half-cell reactions. Now, to see why 2.05v can really
be anything "around" there due to differences in the concentrations of
the chemicals as well as temperature, google the Nernst equation, which
tells you how the potential of the cell varies according to temperature
and concentration.

The Nernst equation also explains part of why a lead-acid cell will have
a higher voltage when it's just taken off a charger and then settle to a
full-charge voltage a little bit lower and still be considered to have a
100% charge. It has to do with 1) the concentrations of the chemicals
at the plates equalizing some time after removing the charging voltage
and 2) the cooling down after charging which lowers the potential. It
nothing to do with self discharge or internal resistance.

Steve

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