why will battery that is not fequently used stop charging to full capacity just after 13 months of ownership?

My Inspiron 1525 battery only charges up to 38-40%. It will display "plugged in, not charging" after reaching this range. I noticed this 3 days ago. Can someone help me please? I have treid to remove it and put back. I also ran the battery down to 0% and recharge but it is still the same. Nothing that I know of seem to work.