I did this. I was given a 36 volt (18 cells, 2 volts each) forkilift battery. Mine read 5.6 volts when it arrived--REALLY DEAD!
After much effort, I got them out of the metal box, recabled the cells into 3 group of 6 in series = 12 volts/per group. I put a 40 amp charger on each group in turn and let them cook for days, adding water as needed.
then, I disconnected them all and read voltage on each cell individually. I found a couple that were hopeless, and several that were not so hot. I selected the best 12 cells by choosing the highest voltage ones, and made 2 groups of those = 12 volts each. Threw away the bad ones.
The remaining 2 groups were then run for weeks on a (too small) desufator charger, which got them up to around 12.4 volts, or 80% charge. These I kept on float for a while and they are now useful.
If you don't select out the bad cells, if any, then they will drag the rest down to their level.
Mine will get used with 4 sets of Harbor Freight solar panels for some lights and other light loads. Not a real prize winner, but they are still essentialy FREE, so no complaints here.