Sulfation

Sulfation refers to the process whereby a lead-acid battery (such as a car battery) loses its ability to hold a charge after it is kept in a discharged state too long due to the crystallization of lead sulfate.

Lead-acid batteries generate electricity through a double sulfate chemical reaction. Lead and lead oxide, which are the active materials on the battery's plates, react with sulfuric acid in the electrolyte to form lead sulfate. When formed, the lead sulfate is in a finely divided, amorphous form, which is easily converted back to lead, lead oxide and sulfuric acid when the battery is recharged.

Over time, lead sulfate converts to the more stable crystalline form, coating the battery's plates. Crystalline lead sulfate does not conduct electricity and cannot be converted back into lead and lead oxide under normal charging conditions. As batteries are "cycled" through numerous discharge and charge sequences, lead sulfate that forms during normal discharge is slowly converted to a very stable crystalline form. This process is known as sulfation. Sulfation is a natural, normal process that occurs in all lead-acid batteries during normal operation. Sulfation clogs grids, impedes recharging and ultimately can expand and crack the plates as it accumulates, destroying the battery. Crystalline lead sulfate is resistant to normal charging current, and does not re-dissolve completely. Thus, not all the lead is returned to the battery plates, and the amount of usable active material necessary for electricity generation declines over time. In addition, the sulfate portion (of the lead sulfate) is not returned to the electrolyte as sulfuric acid.

Sulfation also affects the charging cycle, resulting in longer charging times, less efficient and incomplete charging, excessive heat generation (higher battery temperatures). Higher battery temperatures cause longer cool-down times and can accelerate corrosion.

Sulfation in proteins
Biochemical sulfation is a phase 2 enzyme reaction. This biotransformation process uses its cosubstrate 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to transfer sulfonate to a xenobiotic. Most of the time this is effective in rendering the xenobiotic pharmacological and toxicological less active, but sometimes it plays a role in the activation of xenobiotics (e.g. aromatic amines, methyl substituted polycyclic aromatic hydrocarbons).