Lead–Acid Battery Chemistry

Introduction

The lead–acid battery was invented in 1859 and still starts most petrol and diesel cars. It is heavy and stores only modest energy per kilogram, yet it is cheap, delivers very large currents and is recycled at very high rates. Its chemistry is unusual: both electrodes form the same product, and the electrolyte itself is a reactant. That makes it an excellent case study of how a cell's composition controls its voltage.

Core explanation

Construction. Each cell contains a negative plate of spongy lead (Pb) and a positive plate of lead(IV) oxide (PbO₂), both held on lead-alloy grids, immersed in sulfuric acid of density about 1.28 g/cm³ when fully charged (roughly 37% by mass).

Discharge reactions. At the negative electrode, lead is oxidised:

Pb + HSO₄⁻ → PbSO₄ + H⁺ + 2e⁻

At the positive electrode, lead(IV) oxide is reduced:

PbO₂ + HSO₄⁻ + 3H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Overall:

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

Both plates become coated in insoluble lead(II) sulfate. The standard cell potential is about 2.05 V, and a car battery connects six cells in series for a nominal 12 V.

Acid concentration changes. The overall equation shows that sulfuric acid is consumed and water is produced. As the battery discharges the acid becomes more dilute and its density falls, to around 1.10–1.15 g/cm³ when flat. This has two consequences. First, the density of the electrolyte (its specific gravity) is a direct measure of state of charge. Second, the open-circuit voltage falls with discharge, as predicted by the Nernst equation: the activities of H⁺ and HSO₄⁻ fall while that of water rises. The voltage changes by only a few tenths of a volt, because the dependence is logarithmic.

Charging. An external supply reverses both reactions, converting PbSO₄ back to Pb and PbO₂ and regenerating sulfuric acid. As charging nears completion, little sulfate remains, and the applied voltage starts to split water instead: hydrogen forms at the negative plate and oxygen at the positive. This gassing is kept small because hydrogen evolution on lead has a high overpotential; without that kinetic barrier, the cell could not be charged at all, since 2.05 V is well above the 1.23 V thermodynamic decomposition voltage of water.

Failure modes. If a battery is left discharged, fine PbSO₄ crystals recrystallise into large ones with little surface area. This sulfation makes recharging difficult and reduces capacity. Positive grids slowly corrode, and active material can shed from the plates. In freezing weather, a discharged battery is vulnerable, because dilute acid freezes at a much higher temperature than concentrated acid.

Safety. The electrolyte is corrosive, and gassing during charging produces a flammable hydrogen–oxygen mixture, so batteries are charged in ventilated areas away from sparks, with eye protection when handling. Lead compounds are toxic, which is why recycling is tightly controlled.

Formulae

Overall: Pb + PbO₂ + 2H₂SO₄ ⇌ 2PbSO₄ + 2H₂O; E° ≈ 2.05 V per cell; 2 mol of electrons per mole of Pb reacting.

Step-by-step reasoning

To explain why electrolyte density indicates charge:

1. Write the overall discharge equation. 2. Note that H₂SO₄ is consumed and H₂O is formed. 3. Less dense water replaces denser acid, so density falls. 4. Therefore lower density means a lower state of charge.

Visual explanation

Picture a tall glass cell with two dark plates. As the discharge arrow advances, a white sulfate layer grows on both plates while a float hydrometer in the acid sinks lower. On charging, the white layers shrink and the float rises again, with small bubbles appearing near the end.

Real-world analogy

The lead–acid cell is like a see-saw of sulfate. Discharging moves sulfate out of the liquid and onto both plates; charging lifts it back into the liquid. The liquid's "weight" tells you which way the see-saw is tilted.

Real-world example

Starter batteries must deliver several hundred amperes for a few seconds, so they use many thin plates with a large surface area. Deep-cycle batteries for boats and backup power use thicker plates that tolerate repeated deep discharge. Sealed valve-regulated designs recombine most of the oxygen produced at the positive plate back into water, so they need little maintenance.

Why?

Why does a lead–acid battery provide such high current? The electrode reactions have fast kinetics, the aqueous acid electrolyte has high conductivity, and the plates offer a large area. Together these give a very low internal resistance, so the voltage stays high even at large currents.

Common misconception

"Only the lead electrode is used up during discharge." Both electrodes react: Pb is oxidised and PbO₂ is reduced, and both form PbSO₄. The sulfuric acid is also a reactant and is consumed.

Worked example

Question: A car battery delivers 50 A h. What mass of lead is converted to lead(II) sulfate at the negative plates of one cell? (Pb = 207 g/mol)

Reasoning: Charge = 50 × 3600 = 180 000 C. Moles of electrons = 180 000 ÷ 96 485 ≈ 1.87 mol. Each Pb gives 2 electrons, so moles of Pb = 0.933 mol. Mass = 0.933 × 207 ≈ 193 g.

Answer: About 190 g of lead per cell (the same amount reacts in each of the six cells).

Quick check

1. What happens to the density of the sulfuric acid electrolyte as a lead–acid battery discharges, and why? Answer: It falls, because sulfuric acid is consumed and water is produced by the overall reaction.

Exam focus

Write balanced half-equations using HSO₄⁻ or SO₄²⁻ consistently and check both atom and charge balance. Explain the role of the hydrogen overpotential in allowing charging and the link between gassing and safety. Relate state of charge to acid density.

Advanced insight

Because both electrodes and the electrolyte are involved, the lead–acid cell is limited by acid diffusion into the porous plates at high rates. Adding carbon to the negative plate (in "lead–carbon" designs) improves charge acceptance and reduces sulfation under partial-state-of-charge use, as in stop–start vehicles.

Summary

In the lead–acid cell, Pb is oxidised and PbO₂ is reduced, both forming PbSO₄, while sulfuric acid is consumed and water formed. The falling acid concentration lowers both the electrolyte density and, via the Nernst equation, the voltage. A high hydrogen overpotential on lead makes charging possible. Sulfation, grid corrosion and gassing limit lifetime and safety.

Practice questions

1. Write the half-equation for the positive electrode during discharge. Answer: PbO₂ + HSO₄⁻ + 3H⁺ + 2e⁻ → PbSO₄ + 2H₂O. 2. Why does a 12 V car battery contain six cells? Answer: Each cell gives about 2 V, and six in series give about 12 V. 3. Explain why a fully discharged lead–acid battery is more likely to freeze than a charged one. Answer: When discharged the electrolyte is mostly water, which freezes at a higher temperature than concentrated sulfuric acid. 4. Why must lead–acid batteries be charged in a ventilated area? Answer: Near the end of charging, water is electrolysed to hydrogen and oxygen, which form a flammable mixture that must not accumulate.