Laptop battery plugged and draining

(I apologize in advance for my imperfect English)L Dell Latitude 3540 OS Windows 7 Professional battery indicates that it is plugged in and charging, but also shows that it is draining.

I have got my laptop for 7 months and its battery keeps slowly draining even if I always keep the computer plugged in and never run it on the battery. When the battery goes down about 50%, I simply shut down the laptop, take out the battery, put it back, turn on my laptop and then the battery starts to charge and goes up to 100%. It remains fully charged about a week or so and then starts slowly draining again. Is it a bad sign? What should I do?

I've checked BIOS and it shows that there's a 65W adapter on the computer and the battery works normally.