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February 7, 2023

Lead-Acid Battery Chemistry FAQ

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Lead–acid Cells

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.

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