Discharge-> sulfuric acid from electrolyte combines with lead plate (neg plate) and lead-oxide (pos plate) to create a soft mossy like lead sulfate on plate, leaving water behind in electrolye from the sulfuric acid missing its sulphur molecule.
Charging reverses process to put sulphur with water back to sulfuric acid in electrolyte and plates lead sulfate goes back to pure lead. This all assumes the batteries have not been laying around in discharge state so the lead sulfate is now hard crystals, This is why specific gravity of electrolyte drops during discharge and rises during charging based on percent of acid in electrolyte. (see table below)
Lead sulfate left for some time without recharging creates hard crystals that will not recharge. Best you can do is a controlled overcharge (equalization) and hope the vigorous bubbling and heating will knock off the hardened sulfate crystals so lead plate surface is re-exposed. The dislodged crystals work their way to bottom of cell. If lead sulfate is not fully hardened some of it will recharge back to lead and sulfuric acid with the equalization controlled overcharging.
This at least frees up the plate surface so it can participate in cell performance. Hardened sulfate coated plate surface is insulated so inactive to normal cell operation reducing cell capacity.
Any sulfate crystals that dislodge or are left on plates lock up sulphur that cannot be recharged back to sulfuric acid in electrolyte so electrolyte specific gravity will be lower than normal.
You should get a float based hydrometer. Not one of the crappy auto parts store plastic junk units.
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Before you start measure battery voltage and specific gravity for each cell. Record the readings.
Start with a normal charge cycle as manf. recommends but instead of stopping at absorb voltage (like 14.5v) keep going up to 15.5-16.5v.
This is the controlled overcharge called equalization. Leave battery caps off and expect a great deal of vigorous bubbling,. The bubbling is oxygen coming off the positive plates and hydrogen off the negative plates, so it would not be a good time to smoke a cigarette while doing the equalization. This eats up water from electrolyte so have distilled water to replenish it. Never let electrolyte drop below surface of plates as viewed down cell cap ports. As the bubbles pop they may spit some electrolyte acid around area of battery so be careful. Don't wear good clothes as they will have acid holes in them afterwards. I normally do for no more than 2 hour at the high voltage or until battery gets uncomfortably warm to touch. Let it cool back down than check specific gravity of electrolyte again to see if you have raised it any. If after three attempts you don't see any increase in SG you have gone about as far as you can go. Further equalization will just shed lead from plates contaminating electrolyte and ruining battery.
There is a semi-legit last ditch option but I am not a real believer in it. Epsons Salt will react with and dissolve lead sulfate crystals. It does contaminate electrolyte somewhat however. There are some Youtube videos with procedures for this process
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