Lead-Acid Electrochemistry: Fundamentals
The Pb–H₂SO₄–PbO₂ reversible cell reaction explained
Published February 1, 2026
The lead-acid battery is a secondary (rechargeable) electrochemical cell based on the reversible reaction between lead, lead dioxide, and sulfuric acid.
Discharge reaction (simplified):
Discharge reaction (simplified): At the negative plate: Pb + SO₄²⁻ → PbSO₄ + 2e⁻ At the positive plate: PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
Charge reaction reverses this process, converting lead sulfate back to active material and restoring sulfuric acid concentration.
Voltage characteristics. A fully charged 12V battery rests at approximately 12.6–12.8V. During discharge, voltage gradually declines. The end-of-discharge voltage for a 12V system is typically 10.5V (1.75V per cell × 6 cells).
Specific gravity of the electrolyte serves as a state-of-charge indicator. Fully charged: 1.265–1.285 g/cm³. Fully discharged: approximately 1.120 g/cm³.
Efficiency. Lead-acid coulombic efficiency is typically 85–95% depending on charge rate and temperature. Energy efficiency (watt-hour basis) is lower due to the voltage difference between charge and discharge.
References & Academic Standards
- — Linden's Handbook of Batteries, 4th Edition
- — IEEE 450-2010: Recommended Practice for Maintenance
- — ILA (International Lead Association) Technical Papers