An undesirable aging process in batteries in which too much lead sulphate is formed on the electrodes (coarse crystalline sulphation), which cannot be removed even after prolonged charging.
Sulfation reduces the performance of the battery and leads to its failure. Long service life, over-discharge, insufficient charging and acid stratification are causes of excessive sulphation.
Ideally, the chemical process runs as follows and can be completely reversed. In an uncharged battery, the combination of sulphuric acid and lead produces lead sulphate which is deposited as a white layer on the plates. When charging, the hydrogen that is produced removes the lead sulphate from the positively charged plates, and the oxygen combines with the lead to form lead dioxide. When the battery is discharged, lead sulphate is formed again on the plates.