Article

https://www.practicalmachinist.com/vb/material-handling-and-rigging/forklift-battery-back-life-303513/
How do desulfate batteries - [1]
Hydrometer measures specific gravity of electrolyte - [2]
Questions - how to knock off sulfate and replace battery acid?
Specific gravity of bat acid - [3]
Throw powder down each battery - is this just a base to react with sulfate? [4]
Use a battery load tester to test if battery is back up
Looks like a good instructional on Epsom salt method - [5]. Magnesium reacts with the lead sulphate, nonconductive crystals go away. The thing I don't understand - if the battery reaction involves sulfation - when does sulfation become irreversible? Only when too much sulfation happened, ie, you drained the bat too deep, and you can't get to the underlying metal?
It seems that nobody really understands why sulfation is not fully reversible [6]
This research paper shows micrograph of crystals. Still, it is not clear that sulfation is well understood. [7]

Video Series

Use epsom salt - mgso4 - [8]. This is great series - [9]. Half cup of epsom salt with water.
- Showed 3 mOhm resistance across 12v bat.
Instructional on using epsom salt to dissolve sulfate - [10]