By now I already have had a professional make the pcb design called vortexe9000 on fiverr, we discussed the design, I asked him to add a switch (pin 15 to gnd for easy flashing of the ESP) and he gave me some great advice to make it more compact (not needing the I2C and stepdown module).
Once he finished the design for me I ordered a few prototypes at a pcb manufacturing and assembly company in the Netherlands called Proto Service, and they work great! I’m currently trying out the batterylife running it on 3AA batteries.
I use this code to read the voltage from the ESP8266:
ADC_MODE(ADC_VCC); //vcc read float vdd = ESP.getVcc() / 1000.0;
Since there is a regulator between the power and the ESP, it doesn’t measure the exact batteries voltage, but I think it will be enough to estimate when to replace the batteries, I’m hoping I will get it to run for 14 days for now on 3 AA batteries.
I hooked one up to thingspeak, to track progress: