The Lead–Acid Battery

Introduction

Lead–acid batteries remain useful where high starting current, mature manufacture and recycling matter more than low mass. Both electrodes form lead sulfate during discharge, while sulfuric acid is consumed. Charging reverses the intended chemical changes, but repeated deep discharge or prolonged low state of charge can make reversal more difficult.

Core explanation

The negative plate contains spongy lead, Pb. During discharge its simplified oxidation is Pb + SO4²− → PbSO4 + 2e−. The positive plate contains lead dioxide, PbO2, which is reduced in acid: PbO2 + SO4²− + 4H+ + 2e− → PbSO4 + 2H2O. Electrons flow externally from the lead negative plate to the lead-dioxide positive plate. Both equations make PbSO4, but their starting lead oxidation states differ.

Adding the two half-reactions gives Pb + PbO2 + 2SO4²− + 4H+ → 2PbSO4 + 2H2O, often written Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O. Thus sulfuric acid concentration and electrolyte density generally decrease during discharge. Their relation to state of charge is useful but affected by temperature, mixing and design.

A lead–acid cell has a reversible voltage of roughly two volts under common conditions. A nominal 12 V automotive battery has six such cells connected in series. Larger plate area lowers resistance and supports high current, while the heavy lead components lower specific energy relative to lighter chemistries. Practical voltage varies with SOC, temperature and load.

Charging applies an external voltage to drive the reverse reactions: PbSO4 returns toward Pb at the negative plate and toward PbO2 at the positive plate, while sulfuric acid is regenerated. The reverse process requires overpotential, and excessive charging can drive water electrolysis and gas evolution in some designs. Charge management, ventilation or recombination design and temperature control matter.

Ordinary PbSO4 formation is part of normal cycling. “Sulfation” in an adverse sense refers to persistent sulfate deposits or morphology changes that reduce accessible active area and are difficult to reconvert, often associated with prolonged undercharge. It is inaccurate to say any sulfate appearance means the battery is damaged. Corrosion of positive grids, water loss and shedding of active material are other aging processes.

Step-by-step reasoning

Write Pb oxidation and PbO2 reduction with sulfate and hydrogen ions, balance two electrons and sum. Track acid consumption and water production. For charging, reverse the overall reaction and add the need for external electrical work. Relate voltage of a series battery to cell count but separate voltage from capacity and mass.

Visual explanation

Draw two plates in sulfuric acid: Pb on the negative side and PbO2 on the positive side. During discharge color both surfaces progressively as PbSO4 and show acid ions entering products while water increases. Above the diagram draw an external electron arrow; below draw a reverse charging arrow.

Real-world analogy

Two different workers begin with different tools but both finish holding the same temporary material. Charging returns each to its original tool. If the temporary material hardens into an inaccessible form, reversal becomes difficult. This captures normal sulfate versus harmful persistent sulfation without representing the exact crystal chemistry.

Real-world example

A vehicle starter battery can deliver a high burst of current because it has many plates with large reaction area. After the engine runs, an alternator recharges the cells. If the vehicle is stored partly discharged for a long period, persistent sulfate and other aging can reduce available starting current.

Why?

The two half-reactions transfer two electrons and convert chemically distinct lead phases into PbSO4. Acid ions participate stoichiometrically, so electrolyte composition changes with state of charge. The cell's voltage comes from the Gibbs-energy difference between charged and discharged compositions, while high power depends strongly on electrode area and resistance.

Common misconception

Both plates are not initially metallic lead in a charged cell; the positive plate is PbO2. Normal discharge sulfate is not automatically irreversible sulfation. Also, a 12 V battery's six cells add voltage in series, not electron number per lead atom.

Worked example

Question: A fully balanced lead–acid discharge consumes one mole of Pb and one mole of PbO2. How many moles of H2SO4 are consumed and electrons pass through the external circuit?

Reasoning: The overall reaction is Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O, so two moles of acid are consumed. The lead oxidation half-reaction releases two electrons per mole of Pb, matched by the two-electron PbO2 reduction. The circuit therefore carries two moles of electrons for this reaction extent.

Answer: Two moles of H2SO4 and two moles of electrons.

Quick check

1. What solid forms at both electrodes during idealized discharge? Answer: Lead sulfate, PbSO4.

Exam focus

Balance sulfate, hydrogen, oxygen and two electrons. State that acid is consumed on discharge and restored on charge. Differentiate normal reversible PbSO4 production from persistent damaging sulfation, and remember roughly two volts per cell rather than per six-cell battery.

Advanced insight

Acid concentration can vary within a large cell because of stratification and limited mixing, so one bulk density measurement may not represent all plate regions. Nonuniform states promote localized aging, illustrating how mass transport and device geometry affect a chemistry that looks simple in a net equation.

Summary

Lead–acid discharge converts Pb and PbO2 to PbSO4 while consuming sulfuric acid and producing water. Charging reverses these intended changes using external energy. Six cells produce a nominal 12 V battery, while persistent sulfation and other morphology changes limit cycle life.

Practice questions

1. Which charged positive-plate material is reduced during discharge? Answer: PbO2. 2. What happens to sulfuric acid concentration during ideal discharge? Answer: It decreases as acid is consumed. 3. Why can a 12 V lead–acid battery contain six cells? Answer: Each cell is roughly 2 V and series connection adds voltages. 4. Is PbSO4 always evidence of irreversible failure? Answer: No. It is the normal discharge product; persistent hard-to-reconvert deposits are the concern.