Monitor the LBO pin on Adafruit Powerboost using a Feather.
The sketch can be found here.
14 of the Feather to the
LBO pin of the Powerboost with a diode in between.
Connect an adjustable power supply to the
GND pins of the Powerboost with a voltage of 3.2 VDC so that the low batter LED turns on.
Circuit made with Circuit Diagram.
task test:compile-upload NAME=lbo