I built one of those from junkbox parts.I'm not sure that the coils are correct but it does seem to help,it takes some time though.
I found this http://hmin.tripod.com/als/andysm/docs/Lin_Jao2.pdf that discusses the circuit from the Home Power magazine.
I'm too am looking for a battery desulphator circuit to build - just wondered if anyone has built this design and how successful it was or perhaps can comment on the circuit design.
Thanks