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What and Why Sulfation

As we all know, the lead acid battery positive plate is PbO2 , and negative plate is soften metal of Pb, the electrolyte inside is H2SO4 . Since the battery discharged, redox reactions were carried out on the positive and negative plates respectively forming PbSO4 and H2O.

Lead Acid Battery

It can be seen that lead sulfate PbSO4 is the inevitable substance of lead-acid battery discharge or self-discharge, and with the increase of discharge depth, the amount of lead sulfate PbSO4 will increase, it will adhere to the plates surface to form lead sulfate cover. During the normal charging cycle and charged properly, PbSO4 crystals are reduced to lead. If the battery used and maintained improperly, such as: long-term undercharged and stored in warehouse, always deep discharged , water replenished not in time, etc., a thick and hard PbSO4 crystals will gradually be formed on the cell negative plate. This kind of PbSO4 crystal (PbSO4 ●5H2O) is inactive and low solubility, so that the Internal Resistance(IR) of the battery increases, leading to the decrease of charging ability. The traditional charger is difficult to reduce and dissolve it. As it charging, it mainly reacts with the water electrolyzed to H2 and O2, leading to the battery's further dehydration and sulphation accelerated. That is called "Battery Permanent Sulphation", which is the main cause of the early decrease of SOC and battery failure.

In theory, batteries should last many years, but they usually don't because of a series of detrimental problems caused by "excessive sulphation buildup" related to the natural and necessary formation of sulfate crystals on the surface of lead battery plates.


  • Battery used and maintained improperly
  • Long-term undercharged and stored in warehouse without recharging
  • Always deep discharged or over charged
  • Water replenished not in time

As a battery ages through use or sits unused for periods of time, these PbSO4 crystals enlarge and can build up "excessively" to the point where they create a physical barrier across the plate. Before long, this buildup (PbSO4 ●5H2O) can become so dense that a battery will no longer accept or release energy.

The real question is, if all of the lead sulfate crystals are not turned back into lead, how long does it take before they become so hard that they can not be converted? The answer is that varies--it might be weeks or months and depends on a number of factors such as the quality of the lead, temperature, plate chemistry, porosity, Depth-of-Discharge (DoD), electrolyte stratification, etc. The longer sulfation occurs, the larger and harder the lead sulfate crystals become. The positive plates will be light brown and the negative plates will be white.

Thus, sulfation is a huge problem for lead-acid batteries not being used, idled on a dealer's or DC's shelves, or in a UPS system standby deep discharged due to power cut, especially in hot temperatures leading to water lost.


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