@hpro This only a community site, with mostly me.
You could try their GitHub repository and raise an issue:
https://github.com/Xinyuan-LilyGO/T-Display-S3/issues
Lithium batteries do not lose voltage as they are used up, They all of a sudden die!
My suggestion was to calibrate the voltage at USB conditions and anything less than that, accounting for ADC drift would indicate that it was on battery only, no USB.
-Terry

@dr3x Hi!
Are you using Arduino IDE v2.1.0?
If so, I consider this a confusing and annoying bug in the application.
The app kindly and incorrectly identifies your board type!
You still need to type in ESP32S3 Dev Module as shown below on my computer!

Screenshot 2023-06-27 at 12.24.32 AM.jpg

Hope this helps, Terry

@krueg
Last year I got a LilyGO T-Display S3 and had trouble even getting it to work, even with my Arduino and M5Stack experience!
So I created a github repository here:
https://github.com/teastainGit/LilyGO-T-display-S3-setup-and-examples
I will check out your repository !