Another questionable technology you might want to investigate is desulfators. There are lots of DIY circuits around. They're almost as controversial as Epsom salts! 
I had some success in flooded lead acid batt rejuvenating in the last years. Not in other battery types.
I found some rules what may help some of you.
Anyway, the basics are written down almost 100 years ago:
https://www.powerstream.com/1922/battery_1922_WITTE/battery_WITTE.htm#tocPulsers have to install on the battery in early stage, this is a prevention tech, not a cure (but sometimes worked)! Preventing to build up sulphate layer on the lead surface.
I found this company's products the best:
http://www.megapulse.de/.The main problem is the low charging voltage, what in some cars 13.8V. The charging should go up to 15-16V of course at low current, that can solve the sulphates on the lead surface.
The installed pulser only works when the batt voltage is >13.5V, and makes 20-50V short pulses what is handling the sulphates effectively.
One of our forklift is working for 10+ years and the battery is still fine, of course it needs maintenance.
One pulser can handle the battery of two cars, forklifts, if you change on the batteries regularly.
About dead batteries as mentioned earlier in this blog, mechanical or chemical damage can be the issue.
When no mechanical damage, there is some hope to rejuvenate if the battery has had low cycles.
The lead acid battery lifetime is about 700 cycles average.
But if the cycle data is not clear, I have found some tech to find out whether a battery have some hope or not.
1. Inspect mechanical status and if you can check cell voltages do it, if no visible issue, next step.
2. Use a CV, CC power supply, and set CC= Capacity/100 current or less (in case of an 50Ah battery, CC<= 500mA), CV= 15.5V, and charge the battery.
3a. If it reaches the CV within a week, there is a hope, discharge to 11V and repeat the charge>discharge 2-3 times, that will handle the sulphate, and the battery may works for additional 1-2 years, the pulser will help.
3b. If never reaches the CV then you can try epsom salt, or other miracles....
As I experienced the 10% of batteries can be rejuvenated, the younger the battery, the chance is higher.
It is forbidden by law to make rejuvenating commercially in my country, but as a private I can do.