The use of SLI batteries in deeper depth-of-discharge (DOD) leads to a pronounced capacity fading and too short life. The main failure modes of SLI type batteries are sulfation and shedding. Sulfation is related to the formation of too large crystals (or crystal agglomerates) of PbSO4. This salt is the product of both lead metal oxidation at the negative electrode and lead oxide reduction at the positive electrode. PbSO4 is not conductive and the larger crystals are not soluble during cycling. Evolution of large crystals of PbSO4 reduces the recyclable lead content, what degrades cell performance.
Due to physical, morphological and chemical changes ðPb⇔PbSO4 ⇔PbO2 Þ at the positive and negative electrodes, they are mechanically degrade, losing contact and causing short-circuits. Lead-acid battery chemistry is based on the following reactions