I got an answer on esp8266.com forum with a link to a blog where there are more details about this board. It seems the R1 is 220k and R2 is 100k, with those values it seems the A0 pin can only measure up to 3.3v .. I don;t get it, what's the point of measuring up to 3.3v when the battery maximum is 4.2v ? This is a battery powered board, isn't it logical to be able to measure the battery voltage with it ?
What you suggestions are regarding on what modifications I can make to be able to measure up to 4.2v ? Is there anything simpler than just replacing the R2 resistors ? I will look over to see what resistors do I have around 68k value, as this is what the voltage divider calculator reported to be the R2 value, to be able to measure up to 4.2v
Any more advices are welcome.
Thank you