Hi @tim.nuwin there is a way to power the BBB thru the TPS pins (Take a look at the SRM)… However it does have a few gotcha’s that could easily wreak the board. USB is also disabled when using the TPS pin’s…
For years the BeagleBoard commuinty has just recommended getting a Power Cape from AndiceLabs:
Sadly it looks like they done: https://andicelabs.com/2020/10/andicelabs-is-closed/
another option would be to use this Power Booster to convert the battery 3.7v to 5v for the beagle:
I’ve used it in a few designs.
Adafruit has an awesome picture here, ( slide #3 )