Battery Application & Technology All lead-acid batteries operate on the same fundamental reactions. As the battery discharges, the active materials in the electrodes (lead dioxide in the positive electrode and sponge lead in the negative electrode) react with sulfuric acid in the electrolyte to form lead sulfate and water.
Voltage of lead acid battery upon charging. The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead to lead oxide. As a by-product of this reaction, hydrogen is evolved.
Such applications include automotive starting lighting and ignition (SLI) and battery-powered uninterruptable power supplies (UPS). Lead acid battery cell consists of spongy lead as the negative active material, lead dioxide as the positive active material, immersed in diluted sulfuric acid electrolyte, with lead as the current collector:
In addition, the large size of lead sulfate crystals leads to active material disjoining from the plates. Due to the production of hydrogen at the positive electrode, lead acid batteries suffer from water loss during overcharge.
The formation of this lead sulfate uses sulfate from the sulfuric acid electrolyte surrounding the battery. As a result, the electrolyte becomes less concentrated. Full discharge would result in both electrodes being covered with lead sulfate and water rather than sulfuric acid surrounding the electrodes.
On recharge, the lead sulfate on both electrodes converts back to lead dioxide (positive) and sponge lead (negative), and the sulfate ions (SO 42 ) are driven back into the electrolyte solution to form sulfuric acid. The reactions involved in the cell follow. At the positive electrode: At the negative electrode: Over cell: