I've also done successful desulfation on small 12V SLA batteries using a 30V HP printer wall wart and a pot in series.
Its a quick and dirty way to set a current that wont taper off with elevated terminal voltages. In a sulfated battery, one cell is almost always much worse off than the others, interfering with charge current. Placing the battery on a normal charger wont get you anywhere because the current tapers off to almost nothing when it sees the weak cell. The HV and pot combo allows you to keep the current flowing, without risking damage to any of the cells.
The guidelines I've been using for myself:
For small SLA (<5AH) I set it for between 2 and 5mA. C/1000 is the absolute maximum. Higher currents risk causing shorting by distorting or otherwise damaging the remainder of the plates that still exist, leading to dead cells. I almost always lose one when I get impatient and try to use too much current in the early stages. Start out lower for batteries that are in worse shape, and as the battery gains strength, the current can be increased by small amounts. These rates probably should even be determined somehow by what the battery is capable of delivering in a damaged state. More on that some other post...
Terminal voltage during charge in the early phases means almost nothing, unless it is close to your supply voltage, which means the internal resistance of the battery is so high, that it is likely dry, and isn't going to take a charge. The most successful restores have been the ones that read somewhere between 9 and 12V with the application of the initial rate. I've seen lower come back, but they usually end up weak, and take longer. Most have a shorted cell, which of course, means its a candidate for the core pile anyway.
Occasionally, I 'wake' the battery by hitting it with a small load, usually in the C/100 rate range for a few seconds, every few days. This seems to help (dispersing bubbling that has accumulated on the plates?). Here lately I've been considering lowering this during the first stages, maybe as low as say, C/500? Still experimenting with this. Terminal voltages will do all kinds of weird things immediately following this procedure. Usually, it dips radically during the load, and rises sharply and then begins to taper back to nominal after removal (charge current still applied). Like I said, experimental.
The key with desulfation is patience, patience, patience. It can take several weeks to a handful of months to get one back in order again. Remember, batteries don't sulfate overnight, they're not going to resurrect overnight either.
When you can put a C/10 load on for a minute or so without the terminals dropping below 12.0, bump the charge rate up to say, C/500, and let it sit for a couple of weeks. The run time should increase each time the battery gets cycled, but don't go over C/10 for your load, and don't let it go below 12.0V or so. This has a tendency to have a negative effect. As the battery gains health, be mindful of terminal voltages during charge as well - As charge currents increase, so does the risk of damage due to an overcharged cell. Terminal voltages at this point in time should be in the 12.8-13.2 V range at approximately C/500 rate, rising slowly after your 'wake' load. If it still jumps and then falls slowly after a wake, you need to go back to C/1000.
Occasionally, let the battery just sit. 24 hours for every week of charging seems to work well. I'm thinking it lets the cells stabilize and de-gas, further enhancing their ability to take a charge. Start out a little light at first when bringing the charge back, then return up to the 'next level'.
Toward the end, float them up to 14.5 at no more than C/250 or so, and then pull the battery from charge and let it sit. If all is promising, you should see it heading for the 12.6 range (but not lower) in the early finishing, and holding above 13 during later phases, after a 24 hour rest. At this point, normal charge could be used, and the battery can probably be placed in light service, but I usually keep them on charge, reverting back to the C/1000 rate for another couple of weeks for a final equalization type charge.
One final word, don't put them into service immediately following a charge for the first several cycles - I have found that the surface charge dissipates into the 'deeper' charge, and the batteries quickly try to revert to a sulfated state if pushed too hard too soon.
There are no hard code rules, as they all seem to behave differently, but I have resurrected a handful of them to more than 75% original rated capacity using this method. The idea is universal - lower the internal resistance of the entire battery.
How well does it work on FLA? No idea, but SLA seems to respond to it rather well.
