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Anode: Sponge metallic lead
Cathode: Lead dioxide (PbO2)
Electrolyte: Dilute mixture of aqueous sulfuric
acid
Applications: Motive power in cars, trucks,
forklifts, construction equipment, recreational water craft, standby/backup
systems
Used mainly for engine batteries, these cells represent
over half of all battery sales. Some advantages are their low cost, long life
cycle, and ability to withstand mistreatment. They also perform well in high
and low temperatures and in high-drain applications. The chemistry lead acid
battery half-cell reactions are:
| half-reaction |
V vs SHE |
| Pb + SO42- —>
PbSO4 + 2e- |
.356 |
| PbO2 + SO42- +
4H+ + 2e- —> PbSO4 +
2H2O |
1.685 |
There are a few problems with this design. If the cell
voltages exceed 2.39 V, the water breaks down into hydrogen and oxygen (this
so-called gassing voltage is temperature dependent, for a chart of the
temperature dependence click here  ). This requires
replacing the cell's water. Also, as the hydrogen and oxygen vent from the
cell, too high a concentration of this mixture will cause an explosion. Another
problem arising from this system is that fumes from the acid or hydroxide
solution may have a corrosive effect on the area surrounding the battery.
These problems are mostly solved by sealed cells, made
commercially available in the 1970s. In the case of lead acid cells, the term
"valve-regulated cells" is more accurate, because they cannot be sealed
completely. If they were, the hydrogen gas would cause the pressure to build up
beyond safe limits. Catalytic gas recombiners do a great deal to alleviate this
problem. They convert the hydrogen and oxygen back into water, achieving about
85% efficiency at best. Although this doesn't entirely eliminate the hydrogen
and oxygen gas, the water lost becomes so insignificant that no refill is
needed for the life of the battery. For this reason , these cells are often
referred to as maintenance-free batteries. Also, this cell design prevents
corrosive fumes from escaping.
These cells have a low cycle life, a quick self discharge,
and low energy densities (normally between 30 and 40 Wh/kg). However, with a
nominal voltage of 2 V and power densities of up to 600 W/kg, and safe and
inexpensive, the lead-acid cell is an adequate, if not perfect, design for car
batteries. |
