There is a current thread over at Bob Is The Oil Guy in which a member attempts to debunk the whole desulphation theory. It's a good read. In fact, I guess I will cut and paste it here if that's o.k.: credit "Rolf" for his "opinion"
"Here's what little I know about both the electrical and additive desulphators.
Sulfite crystals form when the battery charge is reduced and/or the discharge cycles are not well suited for the battery. These sulfite crystals act as insulators to the flow of electricity in the battery, seriously degrading the capability of the battery to have electricity flow in it, reducing its charging and discharge capability.
This takes places because the sulphate ions can bond with the lead at three successively deeper energy levels.
Level one bonds occur during normal charging and discharging. After a month or so at level one, some of the bonds form level two bonds wich require more energy to break.
After more than a month at level two the sulphate ions form level three bonds with the lead and will not conduct DC, therefore no amount of recharging will break these bonds.
So, the best desulphation is prevention: don't let the battery remain discharged and don't undercharge in use.
The desulphating lead acid battery additives contain EDTA (ethalene diamine tetra acetic acid). EDTA works by forming a complex compound with the lead sulfate layer on the plates. While this removes the insulating layer, the active material that had formed lead sulfate is not reconverted to lead.
The insulating layer of lead sulfate is removed, but active material is sacrificed in the process. Battery life is extended temporarily with the removal of the lead sulfate crystals and increase of reaction surface area, but the lifespan of the battery overall is not increased nor is capacity restored.
The thinking behind the electronic desulphators is that the crystals formed by level three bonds act as a dielectric and form a capacitive connection between the plate and electrolyte. Thus a sufficiently powerful transient pulse will pass energy through the dielectric and break apart the crystal, returning the battery from its 'sulphated' state.
The electronic desulphators pulse the battery with a high voltage. If it's going to work, it will probably take a few weeks. However, it can evaporate off the water and cause material to flake off the plates, shorting a cell which then ruins the battery.
Also it's not clear that the process restores the battery to its former capacity. Like the chemical approach, it just extends the life a bit.
Here's an odd fact: often you can restore a dead battery to life by shaking it vigorously. What happens is that the "mud" in the bottom of the cells goes back into solution or dispersion temporarily, allowing the battery to take a charge.
It then settles back down to the bottom and shorts the battery out again.
Personally I'd just buy and maintain good batteries and replace them when necessary."