OK first I have ever heard of this, but there are many webpages supporting this
Scheme Including one from Rutgers Univ which has a PDF up for this process.
The page above claims that it works but you need to dump the MgSO4 solution and replace it with fresh battery acid. Which is 30% H2SO4 IIRC. Not sure where you'd buy that but maybe someplace like a Interstate Battery shop. Be cautious handling the acid since it will burn you or blind you. Wear gloves, kitchen gloves would work, and eye protection.
You can buy more or less 90% or so sulfuric acid for drain cleaning at hardware stores but if you don't know how to dilute strong acid LOOK it up and PREVENT Disaster. I'm not going to tell you how because if it goes wrong I don't want to be responsible. Also that acid is 'technical grade' which is the least pure form sold, not sure if that will work in a battery.
Looking up battery acid it looks like auto parts stores still carry it, about $10. So don't try to dilute it.