I had this, and it eventually burned out my laptop. (I needed a new one anyway :-) And when I realized, it was also happening to my cellphonne. Additional symptom: your battery starts getting fat - physically fat. So my cellphone back cover kept coming off, and the laptop started wobbling like the feet weren't level (the battery was that fat, and it was sticking out the bottom).
Here's the problem: Lithium Rot. Actually I just made up that name, but it's a situation with the Li-ion battery where, I dunno, lithium builds up or something. It's irreversible, you need to get a new battery and don't use the old one to avoid a FIRE HAZARD and a BURNED-OUT-ELECTRONICS problem. It'll just keep getting worse.
The problem comes from leaving the charger plugged in all the time, even when it's charged up. I used to leave my laptop plugged in all week long and only go out on weekends. And I'ld plug in the cell every nite, all nite.
So now I'm always unplugging the (new) laptop when I see it's all charged and sleeping so it doesn't go overnite or all week like that.