Typically when SLA batteries are stored for a long period of time lead sulphate is formed on the negative plate of the cells. Keep batteries on a nice warm shelf (say 35 degrees) and the process can take as little as three months or so. Similar effect happens if you leave a battery deep discharged for a length of time.
However, sulphated SLA batteries that are from a reputable stable (i.e. the ones that actually weigh a bit, have real chemicals and lead inside and aren't 30% of the size/weight of the competition) can be recovered by careful steps.
Firstly, the sulphate layer increases the internal impedance of the battery to the extent it won't accept a normal charge. Trick here is to apply a much higher charging voltage to start to get some current to flow to break down the sulphate layer. Now the associated problem is monitoring for signs of runaway because once you start breaking the layer down the current will start increasing pretty quickly.
With the current market climate there are mountains of SLA cells all around the world that were bought in for internet farm applications and have yet to be installed. Left in the warm for 6-9 months and the sulphation will be excessive but if kept in the cold (0 degrees C)then you can get up to a year storage.
In summary there is no need to build anything too clever - apply high voltage (typically double normal CV charge), watch for the current to rise indication the sulphate is breaking down and then drop back to more conventional charging. Suggest cycling the battery a couple of times afterwards and if internal resistance is back inside limits then battery should be good to go.
