performance problem

Hi, I have a BIG problem with my Acer Aspire E5-471-5715 laptop. The problem is that the laptop performance lowers when I have it plugged in but without the battery, when the battery is on and it’s plugged the performance is at its best, just as when I use it with the battery only. This problem it’s easier to be found when playing videogames:
In the first image it is plugged in to the charger and the battery, the second image is with the charger plugged in and no battery; one can clearly see the sudden FPS drop
http://imageshack.com/a/img674/133/nne0Ml.png

http://imageshack.com/a/img661/2333/mNBdCh.png

Because of this, I took it to authorized technical service, where it was for 5 days and they just format the laptop and didn’t take a look at the problem. I’ve tried the following:
1. To update video drivers
2. To update BIOS to the latest version given by Acer
3. Set up the energy settings to high performance (pci express unchecked and Intel® GraphicsPower Plan at its maximum performance)
4. To reinstall S.O
Nothing of above has worked. I’ve surfed in other Acer forums, and it seems the problem is the same within other laptops and other brands (the solution for the other brands was to make a new BIOS version)
In the light of these results, I wander what I can do.
Could acer provide me with a BIOS update?
Is there any way to see BIOS advanced settings (it’s locked by default) to examine energy settings?