Try hitting the Dell documents on your model and see if there is a battery calibration feature. Then try that.
Remember that as batteries go past their 300 cycle rated life they can get weird. What confuses many is that past this 300 and 18 months there is no consistent pattern of failures or behavior. Those less confused know that inconsistency is normal for old batteries.
Today I opened the back of my computer, disconnected my fan, took it out and blew it out. put everything back together and have since had a weird issue with my battery. The computer says the battery is not charging. However, when I hover over my battery icon it says I have 5479 hr 00 min (82%) remaining. The percent hasn't change at yet, so I would assume that the computer is running off of the AC adapter and not off the battery, but I cant figure out why it would display the huge time remaining. Not sure where to begin to look to resolve this issue, any advice would help a lot. I can run computer off just the battery and no connection with AC adapter and I can run computer with just AC adapter and no battery in. I get no errors regarding the wrong type of AC adapter connection either.