I have been reading about desulfators for while and finally decided to build one. Have a few old UPS and car battery that need reviving. This is also good way to get alarm panel backup batteries to last longer. My Long term plan is to convert UPS battery as battery pack for power tools for larger job.
Most of simple design use 555 pulse generator, and some newer hi-power models even use PIC controller.
What is desulfator:
Sulfation, or lead acid crystal buildup on your battery plates is by far the most common cause of battery failure. Desulfator prevents sulfation on your battery and reverses the existing lead sulfate crystals on the plates. The desulfation process can double or triple your battery life span.
On ebay you can find different voltages and models that have integrated chargers (maintenance chargers). It is good idea to use these with trickle charger for 2-3 days.
Some general info
PDF schema http://homepower.com/article/?file=HP77_pg84_Couper