16
Rev.12/31/97
No visible change occurs when the cell is discharged a normal amount. If
the charging program is correct, sediment accumulates very slowly and
should never be white or lumpy. The charging program may require
changes to produce a very
the charging program is correct, sediment accumulates very slowly and
should never be white or lumpy. The charging program may require
changes to produce a very
scanty,
fine, dark-brown sediment. The electrolyte
should be at the marked level, midway between top of the case and top of
the plates.
the plates.
2.4 CHEMICAL CHANGES
2.4 CHEMICAL CHANGES
A fully charged cell has brown lead peroxide on positive plates and gray
sponge lead on negative plates.
sponge lead on negative plates.
On discharge, electric current converts active materials of positive and
negative plates to normal lead sulfate and uses up sulfuric acid to
manufacture lead sulfate. This process leaves the acid weak at the end of
negative plates to normal lead sulfate and uses up sulfuric acid to
manufacture lead sulfate. This process leaves the acid weak at the end of
the discharge. Lead sulfate is white in color but cannot be seen on plates
unless the cell is over-discharged, which produces over sulfation. This
condition makes the plates first lighter in color and finally mottled white in
unless the cell is over-discharged, which produces over sulfation. This
condition makes the plates first lighter in color and finally mottled white in
patches or white all over.
Charging the cell reverses this process, converting lead sulfate in the plates
to lead peroxide and sponge lead and producing sulfuric acid, which restores
the specific gravity to normal. Chemical reactions in a storage cell are:
to lead peroxide and sponge lead and producing sulfuric acid, which restores
the specific gravity to normal. Chemical reactions in a storage cell are:
Battery Discharged
Battery Charged
(+ plate)
(- plate)
(solutio
n
(+ plate)
(- plate)
(solution)
PbSO
4
+
PbSO
4
+
2H
2
O
PbO
2
+
Pb
+
2H
2
SO
4
(lead sulfate)
(lead sulfate)
(water)
(lead peroxide)
(lead)
(sulfuric acid)
As the charge nears completion little lead sulfate remains to convert to lead.
The charging current begins to separate water into oxygen and hydrogen,
which bubbles to the top of the electrolyte and forms a mixture of very
explosive gases.
The charging current begins to separate water into oxygen and hydrogen,
which bubbles to the top of the electrolyte and forms a mixture of very
explosive gases.
Quantity of ampere-hours available from a cell decreases with an increasing
rate of discharge. Available ampere-hours are much less at rapid rates of
discharge (see fig. 3). The ampere hours also decrease for cells with weaker
specific gravity (see fig. 4).
rate of discharge. Available ampere-hours are much less at rapid rates of
discharge (see fig. 3). The ampere hours also decrease for cells with weaker
specific gravity (see fig. 4).
2.5 INTERNAL SELF-DISCHARGE AND EFFECT OF IMPURITIES ON FLOATING
2.5 INTERNAL SELF-DISCHARGE AND EFFECT OF IMPURITIES ON FLOATING
VOLTAGE
VOLTAGE
VOLTAGE
Lead-calcium and lead-selenium cells have the advantage of low internal
losses, which remain constant for the life of the cell. Lead-selenium cells
losses, which remain constant for the life of the cell. Lead-selenium cells