I have a 12V 20Ah valve regulated absorbent glass mat lead acid battery. It was dead when I received it, and spent 4 years under a desk. It would not take a charge or deliver much current before voltage collapse, however it did hold its off-load voltage at about 11-12V
Here are my thoughts and processes followed in an attempt to recover the battery.
1. It's very badly sulphated, the long time at low charge has covered the plates in bad large crystal sulphation. It has a high internal resistance. Charging using a standard battery charger does not store any significant charge in the cells.
2. It's an AGM. It will be very badly starved of electrolyte, it might even be completely dry.
3. With some difficulty the recombination valve caps were removed so the internals of the battery could be viewed. The plates were not warped and the glass mats looked good. Each cell looked very dry. (During charging the caps were removed to allow gases to escape.)
4. Deionised water was added slowly to each cell so that the water level just reaches the top of the plates, but no higher. This will soak into the glass mat, and provide some excess electrolyte for the desulphation process. Gassing will be a lot more that usual for this process.
5. A power supply with variable voltage and variable current must be used to desulphate. The battery must be cycled to get the battery chemistry working again.
6. Power supply open circuit voltage set to 19V. Connect to battery. Set current to 300mA. Leave 10hrs, or overnight. It is wise to supervise the cell for temperature or fault.
7. Discharge into 8R to 11.6V or voltage collapse. Voltage after 1hr 12.3V. V collapse after 2hrs 20min. Capacity = 2.33 x 1.53 = 3.56Ah. This battery is coming back.
8. Bulk charge with standard battery charger until charge current falls to under 500mA. This took approx 1hr to 15.5V. My charger has Min and Boost setting. Started on Min for a little while, then switched to Boost for a charge current of 4A.
9. Power supply voltage set to 16V. Connect battery, set current to 300mA. Leave 16hrs, or overnight. Don't allow V to rise above 16V.
10. Discharge into 8R to 11.6V. Voltage at 12.5V after 1hr. Voltage collapse after 3hrs 40min. Capacity = 3.66 x 1.55 = 5.67Ah
11. Bulk charge until current falls to under 500mA. This took hr to 15.5V. Initial setting Min for a while then switched to Boost. Charge current 4.5A.
12. % Ah increase = 59% increase
13. V <= 16V. I = 300mA. 21hrs
14. Discharge into 8R to 11.6V. Voltage at 12.57V after 1hr. 5hr 30min. Capacity = 5.5 x 1.56 = 8.58Ah. 52% increase.
15. Bulk charge until I = 500mA. Charge current starts at 5A. 3 hrs.
16. V<=16.4V. I = 300mA 12hrs
17. Discharge into 8R to 11.6V. Voltage at 12.61V after 1hr. 6hr 30 min. Capacity = 10.14Ah. 18% increase