Adafruit Feather Huzzah 8266 Battery Monitor
First off, it's not my circuit, it's based on a proposal by Mike from Adafruit, i just documented all the stuff.
There's an error in the Screenshot and .fzz file - it's a 1µF capacitor!
As i can't properly update the files here, take a look at the GitHub link - containing everything including some fixes!
Adafruit Feathers support LiPo batteries, but you'd like to monitor the battery level to know, when to recharge.
The existing projects only covered the battery itself but failed to distinguish USB power from battery, so i asked around and this is the result.
This circuit consists of two parts:
1. A voltage divider consisting of 10kΩ and 47kΩ resistors. This will get the voltage from the USB Pin down to safe levels for the ADC pin (max. 1V).
2. A 1MΩ resistor and a 1µF capacitor to smooth the reported values a bit. It's not necessary, but improves the results (see the part of the screenshot after the gap - before the gap it's without this).
Combined with the WiFi capabilities, you can draw nice graphs and add alerting.
 between 3.0V and 5.0V, depending if USB is connected and the battery charge