TTGO T-Display esp32-s3 backlight off when running on batteries

[MartinNohr]

I'm not convinced this issue belongs here, but maybe somebody has run into this issue. When I run a brand new S3 from a lithium cell the backlight works. As soon as I load my code using TFT_Espi the backlight stays off. The software is running, I can see it doing things on the output pins, but the green led and the backlight are off. As soon as I connect the USB power the led and the backlight turn on.
I am totally mystified. Any of you experts have any idea?
I was using pin 38 with PWM to control the brightness, but even when I remove that code the backlight stays off until the USB power is connected. It's obviously not a normal feature since the backlight stays on with the factory test code.

[MartinNohr]

I found the problem, it had nothing to do with TFT_Espi. In order for the backlight to turn on when running on battery port 15 must be set to output and set to 1. It works fine now.