Hello. My Alienware 17 has a charging problem, it is in desktop mode, but I cannot remove the battery.

It is plugged in all the time, and after a couple of minutes the screen gets darker and the icon down in the left side indicates that it isn't plugged in, alltho it is, so unplug it and plug it in again, and the screen gets brighter and the icon down left incicates that it is plugged in, then after a couple of minutes the screen gets darker again, etc.

What is the problem, and how can I fix it?