Sulphation

A battery will be completely discharged if it is left unattended for a long time and the specific gravity will become less than 1.100. This may result in sulphation on the cell plates.
To find if a battery has been "sulphated", pay attention to its voltage and current when charging it. As shown in the diagram, if the battery has been "sulphated", less current and higher voltage may be observed in the initial stages of charging.
A sulphated battery may sometimes be brought back into service by means of a long, slow charge, 12 hours or more, followed by a battery capacity test.