Title: Module 2: The Discharge 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 discharge

Introduction

A lead-acid battery stores energy through a reversible chemical reaction. During discharge — when the battery powers the starter, headlights, or other loads — chemical energy converts to electrical energy. This lesson breaks down exactly what happens at each plate and how electrons flow through the external circuit to create usable current.

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

The Discharge Reaction

When a battery delivers current (powering the starter, headlights, etc.), a chemical reaction occurs at both plates simultaneously.

- The positive plate (PbO₂) and negative plate (Pb) both react with sulfuric acid (H₂SO₄)
- Both plates convert to lead sulfate (PbSO₄)
- The electrolyte weakens as acid is consumed and water is produced
- Electrons flow from the negative plate through the external circuit to the positive plate

At the Positive Plate

- Sulfuric acid (H₂SO₄) in the electrolyte splits into sulfate (SO₄²⁻) and hydrogen ions (H⁺)
- The sulfate bonds to the lead dioxide (PbO₂) plate, forming lead sulfate (PbSO₄)
- The H⁺ ions combine with oxygen from the plate to form water (H₂O)
- The plate accepts electrons from the external circuit

At the Negative Plate

- Sulfuric acid (H₂SO₄) in the electrolyte splits into sulfate (SO₄²⁻) and hydrogen ions (H⁺)
- The sulfate bonds to the sponge lead (Pb) plate, forming lead sulfate (PbSO₄)
- The H⁺ ions are released into the electrolyte
- The plate releases electrons into the external circuit — this is what creates current flow

Overall Discharge Reaction

Key observations:

- Both plates become lead sulfate during discharge
- Sulfuric acid is consumed — the electrolyte becomes weaker (more water, less acid)
- Water is produced — diluting the electrolyte further
- Electrons flow from the negative plate through the external circuit to the positive plate — this is the electrical current that powers your vehicle

3D Battery Component: Pick up the Cell 2 electrolyte container and read the DISCHARGE label. It illustrates this exact process — sulfate leaving the electrolyte and bonding to the plates.

Interactive Animation: Discharge Reaction

Step through each phase to see electrons flow, sulfate migrate to the plates, and the electrolyte weaken during discharge. Open in new tab ↗

Key Takeaways

1. Discharge converts both plates to lead sulfate and consumes acid
2. Electrons flow from the negative plate through the external circuit to the positive plate — this is the electrical current

Vocabulary

Term | Definition
Discharge | Chemical reaction that converts stored chemical energy to electrical energy
Lead sulfate (PbSO4) | The compound both plates convert to during discharge