wvmarle:
...To get a voltage <3.3V at the A0 you need a voltage divider. For the ESP8266 (I assume the WROOM is the same), the ADC goes from 0-1.06V for 0-1023 reading. You have to reduce your voltage to about 1/4. So Vcc - 10k - A0 - 3k3 - GND will work fine for that. But then you also have a significant current leakage of 0.25 mA through those resistors - a waste when you're on battery power.
First time I used a divider using 15k and 4k7 resistors, with those values the voltage drop was way to high, as the reading on the A0 pin was ~200 .. the voltage was somewehere around 0.22v... so this is why I tried other methods.
I will make a voltage divider using 10k and 3k3 as you suggested and report back.