Design And Function: Discharge

WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Battery Discharge

Courtesy of VOLVO CARS OF NORTH AMERICA.

The process during discharge

Negative plate: Pure lead is converted to lead sulphate
Electrolyte: The sulphuric acid is converted to water
Positive plate: Lead oxide is converted to lead sulphate
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 .