Design and function: Discharge

Fig 1: Discharge Process Diagram

Courtesy of VOLVO CARS OF NORTH AMERICA.

The process during discharge 

1. Negative plate: Pure lead is converted to lead sulphate
2. Electrolyte: The sulphuric acid is converted to water
3. Positive plate: Lead oxide is converted to lead sulphate
4. Power consuming components.

During discharge, the lead in the negative plate is converted to lead sulphate (PbSO₄ ).

The lead dioxide (PbO₂ ) in the positive plate is also converted to lead sulphate. During the discharge process, sulphuric acid (H₂ SO₄ ) is consumed while water (H₂ O) is created. This reduces the density of the electrolyte.

The density drops throughout the discharge process and can be gauged to determine the condition of the battery. The electrolyte in a fully charged battery has a density of 1.28 g/cm³ . The density of the electrolyte in a fully discharged battery is 1.10 g/cm³ .