Although not regular flooded type, AGM batteries can also be restored by adding water if it’s not too far gone.
I have two 12V 17Ah AGM battery made by Hitachi that has been floating for the past 5 years, standing by in an UPS, and the capacity has reduced to about 6Ah each.
So instead of recycling them I tried to restore these batteries by adding water. The process is simple, however if you ever try to do this you do this at your own risk. Batteries are extremely dangerous, they release gases when charging and may explode if mishandled.
First pry open the lids and expose the cell cap. Remove cell caps.
When you look into the cell you can see the plate (black) and glass mats (white).
Next I added about 5ml of water into each cell.
After that I discharged and charged the battery. I used a iCharger 206B, which has temperature sensing for battery body. During the first charging cycle the battery has some temperature rise and released some gas.
The capacity has been restore to about 20Ah each battery when charged to 14.4V and 18Ah when charged to 13.8V. How long it will last is still unknown, so I wouldn’t trust these restored batteries for mission critical uses. But as standby power storage they should be plenty fine.