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Lead sulfate (PbSO4) is created at both the positive and negative electrode plates during a discharge.
In principle, during the charging period, 100% of the lead sulfate transforms to the positive plate (lead
dioxide), the negative plate (lead) and sulfuric acid. However, in real life, when PbSO4 (lead sulfate) is
left in the battery for a period of time, it crystallizes and becomes a hard sulfate that coats the surface of
the electrode plates. This phenomenon is called sulfation. Because hard lead sulfate is a non-conductive
material, when it coats the electrode plates, it causes a reduction in the area needed for the electro-
chemical reactions. It also reduces the batteries' active materials needed to maintain a high capacity.
In theory, there are three types of lead sulfate. One is soft lead sulfate that decomposes with a regular
charge. The second is hard lead sulfate that only decomposes during an equalization charge (controlled
over-charge done in industrial battery maintenance). The last is a very hard lead sulfate that fails to
decompose even equalization charging.
The worst sulfated battery can be created by just leaving a battery unattended for a long period of time.
A battery is like a bucket with a tiny hole. Electricity leaks little by little through what is called "natural
discharge". It may take three to six months to drain a battery completely, and sulfation created in this
process is quite possibly the worst kind.
* Equalization charge is a controlled over-charge to equalize the specific gravity among all of the
cells. It is a controlled high-voltage charge lasting for a duration of 5 to 7 hours. During this charge, the
battery gases out hydrogen and oxygen and creates heat. By performing this charge it is possible to
damamge the battery plates by excessive gassing and heat. The benefit of an equalization charge is
removal of some sulfation buildup and correction the stratification of the acid density by stirring the
electrolyte using the rising gas bubbles. Stirring is necessary to avoid high acid concentrations that can
corrode the lower portion of the plate faster than the upper portion where the acid density is lower.
** In industrial usage batteries the recommendation is to avoid discharging batteries no more than
80% of their capacity and then charing them to 100% immediately after the disachrge. If drained lower
than 80%, a quantity of lead sulfate will be created more than desired. Also, hard lead sulfate will start
forming within 2 days if batteries are left discharged. To avoid sulfation buildup, the battery must be
charged back to 100% right after discharge. Equalization charge should take care of the hard sulfation,
but there will be some sulfation left over that the equalization charging misses.
Sulfation is a crystallized lead sulfate (PbSO4) which coats the electrode plates and eventually causes
premature battery failure.
Sulfated electrode plate (picture on the left)
White sulfation (hard lead sulfate, the crystallized PbSO4) is covering the electrode plate. Lead sulfate is a non-conductive material. Coating an electrode plate with this non-conductive material causes a reduction in the surface area which is needed for the electro-chemical reactions. Sulfation also causes loss of component materials of the battery.
Condition of a new electrode plate (picture on the right)
The surface of new electrode plate is spongy. This porous condition increases the surface area for the electro-chemical reaction. When an electrode plate is new and not coated by hard lead sulafte buildup, it has more area for electro-chemical reactions and more component materials to produce electricity.
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