Title: Module 2: The Charge Reaction - Lesson
Course: ASE A6 Battery Service > Section: Module 2: Battery Chemistry & Electrolyte

Objectives

By the end of this lesson, students will be able to:

1. Describe the chemical reaction that occurs during battery charging

Introduction

During charging — when a charger or alternator pushes current back into the battery — the discharge reaction reverses. Chemical energy is restored to the plates and the electrolyte strengthens. This lesson breaks down the charge reaction at each plate, shows how electron flow reverses, and summarizes the complete charge/discharge cycle.

3D Battery Labels Used: Cell 2 (Charge/Discharge)

The Charge Reaction

When a charger or alternator pushes current back into the battery, the discharge reaction reverses.

- Lead sulfate (PbSO₄) on both plates breaks apart
- The sulfate (SO₄) returns to the electrolyte, regenerating sulfuric acid (H₂SO₄)
- The positive plate is restored to lead dioxide (PbO₂) and the negative plate to sponge lead (Pb)
- Electrons flow from the positive plate through the charger to the negative plate — opposite of discharge

At the Positive Plate (Charge)

- Lead sulfate (PbSO₄) on the positive plate breaks apart
- The sulfate (SO₄²⁻) recombines with H⁺ ions from the electrolyte to regenerate sulfuric acid (H₂SO₄)
- Water (H₂O) is consumed — its hydrogen and oxygen are used in the reaction
- The plate is restored to lead dioxide (PbO₂) — its fully charged state
- The plate releases electrons through the charger

At the Negative Plate (Charge)

- Lead sulfate (PbSO₄) on the negative plate breaks apart
- The sulfate (SO₄²⁻) recombines with H⁺ ions to regenerate sulfuric acid (H₂SO₄)
- The plate is restored to sponge lead (Pb) — its fully charged state
- The plate accepts electrons from the charger

Overall Charge Reaction

Key observations:

- Lead sulfate on the positive plate converts back to lead dioxide (PbO2)
- Lead sulfate on the negative plate converts back to sponge lead (Pb)
- Sulfuric acid is regenerated — the electrolyte becomes stronger
- Water is consumed — which is why flooded batteries may need water added over time

3D Battery Component: Flip the Cell 2 container to read the CHARGE label. It shows the sulfate returning from the plates to the electrolyte.

Interactive Animation: Charge Reaction

Step through each phase to see the discharge process reverse — sulfate returns to the electrolyte and the plates are restored. Open in new tab ↗

Why This Matters for Technicians

The charge/discharge cycle is reversible — but not perfectly. Over time, some lead sulfate hardens and resists conversion back to active material. This is called sulfation — a major failure mode covered in Module 6.

Energy Storage Summary

Condition | Positive Plate | Negative Plate | Electrolyte | Energy State
Fully charged | PbO2 (lead dioxide) | Pb (sponge lead) | Strong H2SO4 (SG 1.265) | Maximum stored energy
Discharging | Converting to PbSO4 | Converting to PbSO4 | Weakening (water forming) | Releasing energy
Fully discharged | PbSO4 (lead sulfate) | PbSO4 (lead sulfate) | Weak H2SO4 (SG 1.120) | No stored energy
Charging | Converting back to PbO2 | Converting back to Pb | Strengthening (acid regenerating) | Absorbing energy

Key Takeaways

1. Charging reverses the reaction — sulfate returns to the electrolyte as acid
2. Electrons flow from the positive plate through the charger to the negative plate — opposite of discharge
3. The charge/discharge cycle is reversible, but sulfation can reduce capacity over time

Vocabulary

Term | Definition
Charge | Reverse reaction that restores chemical energy using external electrical current
Reversible reaction | A chemical reaction that can run in both directions (discharge and charge)
Sulfation | Hardening of lead sulfate crystals that resists conversion back to active material