In a lead-acid cell, discharge turns both the lead dioxide plate and the sponge lead plate into lead sulfate while sulfuric acid is used up and water is made, which is why the electrolyte weakens as the cell empties. By Faraday’s law each ampere-hour withdrawn consumes at least 3.87 g of lead, 4.46 g of lead dioxide and 3.66 g of sulfuric acid, so the real plates must carry more than that mass.
Part of Battery Building and Maintenance
The plate and electrolyte reactions of a lead-acid cell, what they imply for acid strength, gassing and sulfation, and the minimum active material per ampere-hour.
Discharge Makes Lead Sulfate At Both Plates And Uses Up Acid
The carrier article gives the discharge reactions as one line per plate. At the positive plate, PbO₂ + H₂SO₄ → PbSO₄ + H₂O. At the negative plate, Pb + H₂SO₄ → PbSO₄ + H₂O. Adding them gives the whole-cell reaction: Pb + PbO₂ + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O. Check that it balances: 2 lead on each side, 2 sulfur, 10 oxygen (2 + 8 on the left, 8 + 2 on the right) and 4 hydrogen on each side.
Two consequences follow directly. Both plates end up as the same compound, lead sulfate, so a fully discharged cell has two plates that look alike; and every unit of charge withdrawn removes acid from the solution and replaces it with water. That is the reason the electrolyte’s specific gravity falls with discharge and why a hydrometer reads the state of charge (Specific Gravity Control). Charging runs the reaction backward: the plates return to lead dioxide and lead, and the acid is rebuilt (Oxidation-Reduction describes lead-acid as highly reversible over hundreds of cycles).
The Voltage Comes From Two Standard Potentials And Depends On Acid Strength
Cell Voltage gives +1.69 V for the lead dioxide plate and −0.36 V for lead in sulfate, so E = 1.69 − (−0.36) = 2.05 V at standard conditions. The carrier lists 2.10 V at a specific gravity of 1.265 and lower values as acid weakens, so the resting voltage of a real cell moves with acid strength; the corpus does not give the formula that links them, and this article does not supply one. Because the voltage falls slowly until the cell is nearly empty, the hydrometer is the better gauge.
Charging Past Full Splits Water Instead
Once the plates are converted, extra charging current goes into splitting water into hydrogen at the negative plate and oxygen at the positive. Water electrolysis needs only 1.23 V in theory (Water Electrolysis), well under the 2.05 V of the cell, so a cell held at a charging voltage above its resting voltage will gas whenever the plates cannot absorb all the current. Gassing is why the water level falls and acid does not (Water Level Maintenance), and why charging is a hydrogen hazard (Hydrogen Gas Hazard).
Sulfation Is The Reverse Reaction Failing
Lead sulfate made in a recent discharge is fine and reacts easily. Left for a long time in a discharged cell it changes into hard crystals that the charging current cannot easily convert back. The corpus (Rechargeable Batteries) puts the rule plainly: do not discharge below 1.75 V per cell, and do not store a lead-acid battery discharged. Reconditioning methods are in Desulfation Techniques.
Worked Example: Minimum Active Material Per Ampere-Hour
Faraday’s constant is 96,485 C per mole of electrons (Faraday’s Laws). One ampere-hour is 3,600 C, so one mole of electrons is 96,485 ÷ 3,600 = 26.8 Ah. Each lead atom gives up 2 electrons, so one mole of electrons consumes half a mole of lead, half a mole of lead dioxide, and, from the whole-cell reaction, 1 mole of sulfuric acid (2 acid per 2 electrons).
The molar masses are general knowledge, not from the other articles: lead 207.2, lead dioxide 239.2, sulfuric acid 98.08 g/mol.
- Lead: 207.2 ÷ 2 = 103.6 g per mole of electrons, and 103.6 ÷ 26.8 = 3.87 g per Ah.
- Lead dioxide: 239.2 ÷ 2 = 119.6 g, and 119.6 ÷ 26.8 = 4.46 g per Ah.
- Sulfuric acid: 98.08 g, and 98.08 ÷ 26.8 = 3.66 g per Ah.
For a 50 Ah cell this is at least 50 × 3.87 = 194 g of sponge lead and 50 × 4.46 = 223 g of lead dioxide, and 50 × 3.66 = 183 g of acid, if every gram reacted. No plate reaches that: grids, support, unreacted paste and acid left in the solution all add mass. The corpus gives no utilization figure, so do not predict capacity from paste mass. Build one cell, discharge it through a known load to 1.75 V, and measure ampere-hours as current × time (Plate Manufacturing).
Common Mistakes
- Treating the theoretical figures as achievable capacity — they are floors for the mass of material, never a prediction
- Reading resting voltage as exact charge — it moves with acid strength and with recent current, so confirm with specific gravity
- Assuming gassing consumes acid — it consumes water, so the cure for a low level is distilled water, never acid
Safety
The reaction products of charging are explosive gas and corrosive liquid
Charging past the point where the plates can take the current splits water, and the hydrogen it produces explodes in air between 4% and 75% (Gas Hazards). The electrolyte is sulfuric acid at all times; add acid to water, never water to acid. Plates and paste are lead compounds, so keep dust off hands and out of the air.
What’s Next
- Learn next: Plate Manufacturing — making the two plates the chemistry needs
- Combine with: Specific Gravity Control — reading the acid strength the reaction changes