Batteries discharged too deeply or for to much time and older batteries create sulphide crystals. These sulphate crystals impede the flow of electricity from the battery plates, some crystals may even float on the surface. Sulphated battery plates is the primary cause of poor battery performance.
Non conducting sulphate crystals form when the battery charge is reduced and/or the discharge/ discharge cycles are not suited for the battery. The crystals insulate the flow of electricity in the battery, seriously degrading the capacity of the battery, reducing its charging and discharge capability. Normal recharging converts soft, recently formed lead sulphate back to its component materials; lead, lead dioxide, and sulphuric acid. As batteries age the sulphate crystals become larger and harder making conversion of lead sulphate more difficult during recharging. Temperatures over 21C (70F) degrees accelerate crystallization build-up as does discharging a 2 volt battery cell below 1.75V. Extended storage or use without a 100% recharge also creates the larger, harder sulphate crystals difficult to convert during charging . Repeated undercharging causes lead sulphate to harden or form larger crystals on the positive plates.
Over time as sulphate crystals cover more plate area they increase the electrical resistance within the battery reducing capacity through the electrical insulating properties of the sulphate crystals and through the blocking of the chemical interface between battery plate and electrolyte.
Sulphation doesn't just occur on the surface of battery plates. If battery charging is terminated before the charge cycle is complete only the surfaces of the battery plates may be cleared of lead sulphate the interior surface of will still contain the sulphate which will not be converted back to lead, lead dioxide and sulphuric acid. In other words, the inside of the plates still be at a low / lower charge level than the surface. The charge on the batteries will not last long. Test with a Hydrometer before taking the batteries off charge or you can use this technique to verify the batteries are charged.
A battery desulphator with advanced electronics will remove the sulphate crystals. A good desulphating unit uses sharp pulses of current at about 800 KHz to set up a resonance which "jars, crushes, grinds or dissolve" sulphide crystals through internal resonance, both mechanical and electrical, wearing down the sulphide crystals so they can be recombined into the sulphuric acid of the battery electrolyte. This action appears to occur at an ionic level whereby the resonance from the desulphator may act somewhat like an ultrasonic cleaner used to clean jewellery.
Sulphation is the leading cause of battery failure. However, a desulphator will not recover batteries that have other ailments such as, shorted cells, coating with other chemicals because other than distilled or de-ionized water was used to replenish the cells, lack of acid etc. As always there is an exception. Sulphate crystals especially in VRLA batteries, can form a lattice causing plates to short. I have seen this described as dendritic sulphation. A desulphator may sometimes be able to rectify this type of short.
Using the battery's own power our battery desulphator will work on 12 to 48V systems.
If you would like to learn more or are electrically inclined and would like to build your own desulphating device you can with a Google search Lead Acid Battery Desulfation and learn how.
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