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On 24 May 2004 06:11:19 GMT, (Gould 0738) wrote:

Thank you. It's good to see a little high quality science interject
itself into an otherwise degenerating discussion. :-)

All the better it supports what most of us have observed. Chuck's
electrons have become damp, slugish, and not a little bit stubborn,
having lived in the Pacific North West for so long.

Steve's hypothesis states that once the battery has achieved a full charge of
2.2 volts per cell, the decrease to 2.1
is due to an a change in the chemical concentration within the cell. Very
probable scenario.

Doesn't support any sort of position that the battery was never charged to
13.2, though, does it?

It may have been charged at 13.2, to 13.2, or whatever you want to call
it. But if you're taking the measurement right after charging, you're
reading an artificially high voltage. When the battery cools down and
the chemicals reach a constant concentration over the space inside the
battery, the voltage will drop to it's steady state full charge of well
less then 13.2 volts. The actual chemical voltage potential of the sum
of the half reactions of a lead-acid cell is between 2.05 and 2.06
volts.

The condition of a battery being fully charged is actually a chemical
condition and not a specific voltage. The battery is fully charged when
the negative plate is made fully of lead, the positive plate fully of
lead dioxide and there is no lead sulfate on either plate. The
electrolyte is sulfuric acid (around 25-35%) and the rest water. The
temperature and the concentration of the sulfuric acid can affect the
voltage of the fully charged battery. The maker of the battery will be
the one who knows what their designed full charge voltage is. They'll
also have to tell you how to measure it, i.e., what temperature, how
long to wait after charging, etc.

The voltage of the battery will drop as it discharges either through
self discharge or by being used because the electrolyte becomes diluted,
i.e., the concentration changes, when it breaks down into hydrogen and
sulfate and the hydrogen combines with oxygen to form more water. The
sulfate combines with the lead in both plates to form lead sulfate.

When the battery is fully discharged, both plates are covered in lead
sulfate and the electrolyte is almost all water. You don't want your
battery to get in that condition.

Merely explains with greater technical clarity than the term "self discharge"
why the voltage will eventually drop.

The voltage will drop eventually due to self discharge. But that's
something different then what happens in the few hours after removing
the charging voltage. The self discharge rate of lead acid batteries is
very variable. Sealed lead acid batteries lose maybe 3% of their
capacity per month. That also depends on temperature and the rate can
double if you increase the temp by like 20 degrees F.

However, you won't notice any self discharge in the few hours it takes
for the voltage to settle down after you remove the charger. I.e., if
the battery is fully charged and the float charger was at 13.2 volts and
you remove it and immediately measure the voltage as 13.2 volts, the
battery will still be fully charged (virtually no lead sulfate on the
plates and the same concentration of sulfuric acid in the electrolyte) a
few hours later when the voltage settles to a lower value.

Steve