I apologize in advance for my imperfect English. Could you advise me, please?

I've got my laptop for 7 months (Dell Latitude 3540 OS Windows 7 Professional ) and its battery keeps slowly draining even though I always keep my 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 again 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.

It's generally not a good idea to keep a machine running on mains power all the TIME and never use the battery as this is not very good for the battery's health.  You should ideally run the machine on battery from full to almost empty every couple of weeks at least to keep it in decent health.  What I'd do is charge the machine up full and run it on battery until it is TOTALLY dead then plug it in and charge it up again fully and see if this helps.Thank you for your reply  I will let it go on full drain next time. Just one thought: could it be that the battery in my computer is programmed to drain by itself? It happens every TWO weeks or so. Does it mean that I shouldn't worry about it? IMO you should not worry about it. 

Strange   behavior for a laptop battery and charger system have been reported. It is a common issue and does not prove any component is defective.
One of many:
Laptop battery odd behavior  - HP forums
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I'm using HPFanControl by troubadix at constant medium fan level so I know it's not heating issues. I also never go above 50c
I also ran the battery diagnostic in BIOS and it gave me no errors.
I also tried to discharged my laptop and it still happened.