I have a Dell 9100 laptop and had same thing happen. I first noticed mine stopped charging back to 100% - it would only do 77% and then 57% and it would take a long long time. I was not able to get it back, mine eventually quit even letting me run off AC and failed completely. Mine is my work machine so its docked 8 - 10hrs a day and AC powered. I found a genuine Dell replacement on Ebay and it was about 30% of Dells list price. Don't buy one from Dell - you can get the same thing directly from a reseller or Dell supplier on EBay and it will look exactly like your orig, mine even had the same Dell label.

One thing no one ever told me - at least once a week or so, run on batt power and disch at least down to 50% and then let it recharge. Exercising the batt this way is supposed to make it last longer. Sitting on AC at 100% all the time will slowly degrade it.