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 sulfation on the cell plates.
To find if a discharged battery has been "sulfated", pay attention to its voltage and current when charging it.
As shown In the image, if the battery has been "sulfated", less current and higher voltage may be observed in the Initial stage of charging.