Calculate battery percentage remaining

I think Gordon already mentioned this:

The MDBT42Q breakout?

Unfortunately the problem is it's connected to a voltage regulator, so it only 'sees' the regulated voltage. You'll be able to use code like that to see when the voltage starts to drop from 3.3v (meaning the LiPo voltage is near or below that), but by that time the battery is only 10% full or so.

NRF.getBattery() measures the voltage of the immediate source, which in your case is coming from the linear regulator which would then produce 3v3 for voltages higher than 3v3 (like your lipo for example).

There's also https://www.espruino.com/Tips+and+Tricks­#common-issues-and-questions

How do I measure battery voltage on devices that don't have E.getBattery?