@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