Hi there,
You do know that It has an RGB-LED onboard and a charge LED that does that already. “charging indicator”
The issue is the board files SEEED engineering (marketing) puts out without thoroughly testing them. Different files support different non-standard configurations of methods to get the batteries voltage. In some cases additional voltage divider resistors need to be added to work and be able to read the voltage.
You’ll have to read the threads to determine what can work for you. I have several posts with video and code demonstrating different methods and results.
Here’s the hardware view
HTH
GL 
PJ