How to measure Lithium battery voltage with low-power mode 1.8V

Question

Hey everyone. Found this thread about measuring the voltage on lipo battery:
 devzone.nordicsemi.com/.../how-to-measure-lithium-battery-voltage

Stefan Birnir Sverrisson · Accepted Answer

Hi Michal 
 Thank you for your question. 
 Actually, with 1/1 prescaling and reference voltage set to VBG = 1.2V, the range of the ADC is 0V - 1.2V. 
 If you choose 8-bit resolution, which is 256 bits, you get 1 bit output change when you change the input voltage with 1.2V/256=0.004688V.
If you choose 10-bit resolution, which is 1024 bits, you get 1 bit output change when you change the input voltage with 1.2V/1024=0.001172V.
When the input goes above 1.2V, the ADC output will saturate and show value 255 for 8-bit configuration, or 1023 for 10-bit configuration. 
 For the Lithium battery setup and 8 bit resolution, ADC will output following value when the battery is flat: 0.54/1.2 255=115
For the Lithium battery setup and 8 bit resolution, ADC will output following value when the battery is fully charged: 0.76/1.2 255=161
Result is that you will have 161-115=46 bit resolution in the Lithium battery voltage range when using 8-bit ADC configuration. So you could in principle utilize up to 46 different battery levels. However, I assume that the discharge curve of the Lithium battery is not linear between 3.0V and 4.2V, so you would most likely need to implement a transfer function to convert the ADC output to battery level percentage. 
 For the Lithium battery setup and 10-bit resolution, ADC will output following value when the battery is flat: 0.54/1.2 1023=461
For the Lithium battery setup and 10-bit resolution, ADC will output following value when the battery is fully charged: 0.76/1.2 1023=646
Result is that you will have 646-461=185 bit resolution in the Lithium battery voltage range when using 10-bit ADC resolution. 
 Let me know if you have further questions on the topic.

How to measure Lithium battery voltage with low-power mode 1.8V

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