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.

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

Vocabulary