1525 battery (problem?)

I have had my 1525 since the end of January and haven't used it much (maybe 4 times a week) I got the 9 cell battery at a heavy upgrade price because i wanted longer life and i have gotten that up until a few days ago. I usually get about 5 hours out of it. The other day I had it fully charged and was using it for about 30 mins when i checked the battery status it said i had 4 hours and 20 mins left. Ten minutes later it pops up and says it is down to 4 percent and then shut my laptop down... I thought it was just vista being silly but it did the same thing again today. The battery monitor is in constant flux. It goes from 2 hours to 4, then back down again. Today it said i had 4 hours left just minutes before it said my battery was almost dead. What is going on? Is it vista? Is my battery faulty? If so will Dell replace it? (I dont have an extended warranty) Please help this is killing me I dont want to have to replace a battery that Ive barely used. They are pricey

P.s. Forgot to mention that I only charge it when it is almost dead. I do not leave it charging when it doesnt need it (if that helps)