This topic is old, but I will reply if it is found through search functions:

I found a solution, what can be applied with small external hardware modifications. It is possible to take some measurements on HAT interface's pin "5VIN".

Measured voltages:

If powered by USB:
~4.7V

Powered by battery and switched off:
~0.7V

Powered by battery and switched on:
~2.4V

Using voltage divider with two same resistors, we get usable logic values for HIGH and LOW ( ~2.3V and ~1.2V). The measured values are not inside garanteed logic range but as a workaround: it works well. Maybe it helps someone else...

I checked again electronic circuit:
It is possible to take some measurements on HAT interface's pin "5VIN". Measured voltages:

If powered by USB:
~4.7V

Powered by battery and switched off:
~0.7V

Powered by battery and switched on:
~2.4V

Using voltage divider with two same resistors, we get usable logic values for HIGH and LOW ( ~2.3V and ~1.2V). The measured values are not inside garanteed logic range but as a workaround: it works well.

Maybe it helps someone else...

@ajb2k3
Thanks for the hint. Found a 3 years old thread. It is not possible to detect if it is powered from USB or battery...

Hello,

I bought the Core Ink for quick prototyping. I am very satisfied with the quality and the possibilities. What I'm missing: How can I tell if the device is USB or battery powered?
My firmware should provide an access point with a web interface, but only if the device is not running on battery power.

Current idea: I could use the battery voltage trend to determine whether the battery is charging or discharging. For this I have to check how accurate the internal measurement is. In any case, this implementation would be very cumbersome.

Maybe there are simpler options?