The cell generates a voltage (potential difference)
when two different types of metal are in the electrolyte.
The two metals in a lead-acid cell are lead peroxide
(PbO
3
) for positive plates, and sponge lead (Pb) for
negative plates. See Figure 3. A potential difference of
approximately 2 volts per cell is generated. The
potential difference does not vary according to the size
of the cell.
Figure 3. Fully Charged Cell
During the discharge of the cell, lead peroxide and
sponge lead mix with sulfuric acid to make lead sulfate
(PbSO
4
) on both plates. See Figure 4. This action
decreases the voltage in the cell. When the sulfuric
acid is removed from the electrolyte, the specific
gravity of the electrolyte decreases. See Figure 5. The
potential difference of a discharged cell is
approximately 1.75 volts.
When a direct current is applied to a discharged cell,
the lead sulfate is changed into lead and sulfuric acid.
The lead goes to the positive plate and stays as lead
and to the negative plate and stays as lead peroxide.
See Figure 6. The concentration of sulfuric acid in the
electrolyte increases. The specific gravity of the
electrolyte increases as the concentration of sulfuric
acid increases. In a fully charged cell, the positive
plate again contains the lead peroxide and the
negative plate contains the sponge lead.
Figure 4. Discharging Cell
Figure 5. Discharged Cell
2240 YRM 0001 Chemical Reaction in a Cell
3
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