Quick Update: It is due to the use of Nvidia card The fact that the problem appeared at the same time on Linux and Windows is just a coincidence: - on windows, (maybe) somehow after the Windows 11 update, it was using the Nvidia card although power saving mode (on demand) was checked in the settings, but in my case just changing and going back again to that mode did the thing and I could get again 8+ hours of battery - on linux (mint 21): I think a kernel update or nvidia update broke something and i think only the nvidia card was being used. Conclusion: I was wrong, nothing due to a firmware or bios update. (Using a Linux Live USB helped me solve my problem as I saw I was getting back my 8+ hours of battery, because by default it was using the intel integrated graphic card)
I bought my XPS 15 9500 laptop 1,5 years ago, and i've been using it in dual boot (Linux/windows) since the beginning. In both OS, I was getting 8/9 hours of battery. As I am not using Windows a lot, I decided to log in Windows and proceed to available updates (Windows 10 to Windows 11, Bios Update, Firmware and drivers updates). Right after the update, I could only get maximum 2/3 hours of use in both Windows and Linux. As it's affecting both Linux and Windows, I guess the problem is due to the Bios update or one of the Firmware updates. I tried to downgrade the bios to the 1.11.0 version (minimum I can go) but the problem remains. For that reasons, I'm pretty sure the problem is due to one of the Firmware updates, what can I do about it, I heard it's not possible to downgrade firmware, is it really impossible ? What solution do you have for me ?  
DELL XPS 15 9500 Battery drain after firmware update