I have played about with old batteries with various degrees of success. Epsom Salts method had about the best results. LINK
Generally just one or two cells that fail through sulfation. I found if you gently tip the acid in a plastic bucket, wash the sludge out of the cells with a hose then pour the old acid back in and top up with distilled water and deep charge it gives a bit more life.
They were just for my sons “field” car so didn’t relay matter if the attempt to rejuvenate them failed.
If it is a car you rely on then just go and buy a new battery for £45, probably save that in the change of cloths you will destroy from battery acid spills.
Oh and wear goggles if you decide to start popping the top off and pouring the acid in a bucket, it is not something you want in your eyes. 😯