Measuring battery voltage at A0 (ESP8266), is a 100K resistor enough?

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1

I use a 3.7V battery (max 4.2V) and I need to measure its voltage (discharge). I use Wemos D1 mini pro. How much resistance to use? Will a 100K resistor be enough?

2

Yes.

A WeMos D1 mini already has a built-in 220k:100k voltage divider,
to drop 3.3volt down to about 1volt for the A/D of the processor.

A single 100k external resistor between battery and A0 changes that voltage divider to 320k:100k,
and input voltage range to about 0 - 4.3volt.
Leo..

3

Use a voltage divider calculator and do your calculation using the 100K as the bottom resistor. Then subtract the 320K from that value, the result is the value resistor you need. You should then use the next standard larger value rather then purchasing a expensive custom resistor.