Button cell monitoring

I use a RTC DS2032 with button cell CR2032; I desoldered the charging resistor.

Daily I want to measure the coin cell voltage via an analog input. Now I measure 3.325 V with a DVM and adjusted the gain of the ADC to get the same result in my Arduino Nano. I want to create a warning sign “change button cell”.

In the datasheet of the CR2032 i never can find such a high voltage for this cell

Two questions:

  • What is a save low level voltage for the DS3231 (it works between 2.3 and 5.5 V)?
  • What is a good formula to calculate the rest capacity of the coin cell?

This spec sheet shows the discharge characteristic of these cells , from this you can work out remaining capacity etc

http://www.cr2032.co/cms/prodimages/sony_cr2032_datasheet_13.pdf

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