Came across this article from Elektor Electronics Magazine - November 2004 on the project for building a car battery desulfator.

http://notes.ump.edu.my/fkee/e-Magazine/Elektor%202004/Articles/E/e04b020.pdf

Extract from the article :-
Sulphation
The capacity of a lead-acid battery gradually decreases during its service life due to normal wear and tear, for a variety of reasons. However, the aging process is considerably accelerated if an uncharged battery is stored for longer than a...