The battery has “sulphated” – can you save it?
Each time a lead acid battery is discharged, it sulphates.
It is a natural part of the chemical process
However, if the lead acid battery is deeply discharged and/or kept discharged, the chemical builds up coarse sulphate crystals that are difficult to dissolve (reset to lead again during charging). It is usually said that the battery has “sulphated”.
The only possibility to restore a “sulphated” battery is to use a battery charger with a Recond function. The battery charger then outputs pulse trains of charge currents to slowly dissolve these coarse crystals.
All Tystor battery chargers have this feature built-in for over the past 30 years.