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Unfortunately, no direct method of battery voltage measurement is built into the system.

But it should be quite easy to implement such a thing: Use a voltage divider on the battery to get a 3.3V max. For a 4.2V cell, this can be achieved using a 27k and 100k resistor.

But it should be quite easy to implement such a thing: Use a voltage divider on the battery to get a 1.1V max. For a 4.2V cell, this can be achieved using a 27k and 100k resistor.

Note, that the ADC of the ESP32 is set to 1.1V range by default! You can use the attenuation but this is not available on every platform.

^ V_bat

|

R1 = ~~27k~~

R1 = 100k

|

+----o ADC

|

R2 = ~~100k~~

R2 = 27k

|

GND

Now you can measure the voltage on the battery using the ADC. As the ADC is 12bit, you can get the actual voltage by multiplying the reading by ((27+100)/~~100~~) * 3.3 / 2**12, or in more general terms: ((R1 + R2) * V_ref) / (R2 * 2**Resolution).

Now you can measure the voltage on the battery using the ADC. As the ADC is 12bit, you can get the actual voltage by multiplying the reading by ((27+100)/27) * 1.1 / 2**12, or in more general terms: ((R1 + R2) * V_ref) / (R2 * 2**Resolution).

Super nice would be charge monitoring. The TL4054 can do this, you need to hookup to the PRG pin. But this results in nasty bodges on the SOT23 chip...

== Battery ==

The battery can be connected via a JST PH2 connector.

They can be bought also from Conrad: [https://www.conrad.at/de/buchsengehaeuse-kabel-phr-polzahl-gesamt-2-jst-phr-2-rastermass-2-mm-1-st-740547.html ArtNr. 740547].

== Example Projects ==

* Temperature and Humidity measurement using collectd as datalogger: [https://github.com/reox/esp32_sht21_collectd ESP32_SHT21_collectd]

== Further Readings ==

* [http://esp32.net/ esp32.net]

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