Simple question, complex answer.
I will attempt to reply as succinctly as possible.
Sulphate crystal up has been known about since the Lead Acid battery was invented, which build up leads to over 70% of premature failure of lead acid batteries. However all batteries wear out with time and no device can prevent that happening.
Over the last 30 years or so battery manufactures have made great strides in combatting sulphate develop and improving the life of their batteries, by introducing modifications to the construction of the battery discs, and in ideal conditions, correct upkeep and correct charging, sulfation shouldn’t be a problem.
However we don’t live in a perfect world and sulphate build-up remains the major cause of battery failure.
It’s been found that a controlled overcharge of the battery will break down the crystals, and when used correctly will continue to keep the batteries in good clean condition, and prolong the helpful life of this battery.
In practice, made the exact same effect as the controlled overcharge except it was not regulated, so over the years these high voltage pulses damaged the battery plates. This negated the positive impact. The circuit board diagrams for this kind of device are publicly available online and several of the low-cost devices available today are constructed using these layouts. Short-term solution, long-term damage.
At the exact same time alternate solution were believed and experimentation showed that if a frequency pulse has been sent to the Efest 8 Bay Charger battery then it could divide the sulphate build up in case the correct frequency could be attained, nevertheless different frequencies were needed for different sizes of crystal that built up to the plates. So the devices using the single frequency system were and still are, hit and miss, some work some unworthy.
What was required was a device that could work over a variety of frequencies, voltages and size of battery. Until the last few years building this kind of apparatus was quite expensive (in excess of $1,000) and large (the size of a classic style video player). As no warranty could be provided to the end-user that sulfation was the issue with their battery no companies were ready to create them.
Dramatic drops in the size and cost of parts has allowed companies to produce small reasonable prices products able to clean out the plates of batteries of different capacities and voltages.
A small number of manufactures around the world today produce products that can wash the plates of batteries and halt the sulphate build-up on new batteries, so recovering them keeping fresh batteries clear of their build-up. In effect recovering many older batteries and extend the life span of any battery that isn’t billed or preserved in the correct way. This really is ALL they do and is the one thing responsible manufactures claim that their desulfators can do, they can’t recover batteries together with any internal damage, shorted cells or AGM batteries that have dried out.
As it isn’t possible for providers to determine the condition or defects of the batteries customers wish to use the device on you should look for suppliers, who provide clear advice of how to check for flaws within the batterylife, or provide a telephone or email support to help with any problems you might have with having a desulfator or with your particular battery installation.
In conclusion modern multi frequency desulfators aren’t a magic bullet which will fix all faulty batteries, but they can significantly extend the life span of your lead acid batteries, and also recover a large percentage of batteries that are sulphated and could otherwise be scrapped. This could greatly reduce you replacement battery expenses, which isn’t insignificant on applications like solar storage batteries, ride golf carts, fork trucks, big boats and motor homes.