CR2032 Coin Cell Battery Monitoring Circuit using nRF52840

[Kumail_J]

Hello,

I am designing a battery monitoring circuit for a 3.0V CR2032 coin cell using the nRF52840, and had some questions. I am following the battery monitoring circuit used on the Thingy52 DK as a reference:

I understand how most of this circuit works, except for the values chosen for R20 and R21. How should I go about choosing these values? From the following thread, it sounds like we want to have this voltage divider so the battery voltage can be divided down to the bandgap voltage:  ADC to measure battery voltage 

I have been unable to find the bandgap voltage for the nRF52840 from the data sheet, and would like to know a bit more about the bandgap voltage and why it is useful.

Another concern I have is measuring the battery voltage as it drifts. Over time, the battery will obviously deplete (which is why we want to monitor it), and I would like to know how this will impact our ADC readings as the reference voltage will be changing.

I am open to any other suggestions regarding this problem.

Thanks in advance!

[hmolesworth]

First no external circuit is required to measure the battery voltage if using a CR2032 as the coin cell 3.4V drives the nRF VDD directly without using an external regulator; there is an internal VDD measurement option on the SAADC for this. Next the battery voltage will tell you almost nothing about remaining capacity of a Lithium coin cell due to internal cell resistance; applying a brief load and measuring voltage before and after the load is a much better fuel gauge for a coin cell.

For a detailed explanation and some coin cell fuel gauge code see my comments here