I took an Arduino UNO, and turned it into a low battery indicator as well as an automatic safe-shutdown circuit when the battery is too low. For that, connected everything on a breadboard and used a lab bench power supply to simulate a battery and get my readings I needed. I got my voltages by recording a video of me playing on a full battery untill my battery was totaly flat, so I could see at what minutes it is safe to shutdown and 10 minutes before that it is safe to say that the battery is almost done for.

My battery lasts for around 2 hours and 11 minutes in total.

My readings are:

Battery full: ~4.16 Volt (not needed, though)

Battery low alert (red LED): ~3.54 Volt

Battery empty, need to shutdown NOW: ~3.20 Volt

After everything was working, I made my arduino smaller by just using the atmega as itself with a quartz, and a voltage divider as battery input and checked that everything works like that: