It's about 300 cycles so at 6 years you are well past that. But leaving an old battery in there can fry charging circuits on the main board so I fear the news is grim. You can't tell if it's the battery without replacing it and you can't tell if the charging circuits are any good without replacing the battery!
But at 6 years we can easily write this battery should never be be in any machine.
Bob
I've got a 6 year old Dell Inspiron 9200 laptop. Its failing, but I don't have the money yet to replace it. When running it, it never seems to gain a full recharge, even then I leave it connected to the power adapter for hours at end. Does that mean the power adapter is bad, or is the battery going?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic