Measure Battery Voltage using SAADC and Convert to Battery Percentage

[epice500]

I am developing a small rechargeable device using an nrf52810. The device is powered using an LIR2032 coincell battery at 3.6 volts. I have been using the SAADC example as a guideline for how to take measurements on VDD. I would like to know how to convert this value into something that I can use to calculate the percentage left in the battery. I have been using a calculation that I found in another post to preform this, it is as follows: BatteryInMilliVolts = (((ADC_VALUE)(REF_VOLTAGE)) / 1024) * 6, but I am unsure if this is giving me an accurate result. Are there any examples which show how to preform this? 

Thank you

[hmolesworth]

Measuring remaining capacity on a xx2032 coin cell is not simple; there are some fundamental steps for getting a useful voltage measurement on a coin cell to infer capacity. Out of interest a (say) 270mAH coin cell which has supplied (say) 220mAH - so most of the capacity - can recover to 2.95 volts or higher after a significant period where the coin cell does not supply significant current (say few uA), a period known as "rest". When this cell is used to power a circuit which measures the voltage an erroneous reserve power indication can result, despite monitoring temperature and using compensation curves. The reason is the changing internal cell resistance as the chemistry of the battery changes over time and discharge.

One solution is to identify a part of the circuit which presents a predictable load each time it is turned on, perhaps a simple resistor driven by a port but also perhaps a small motor or voice coil or even a fixed code execution sequence such as code CRCC. Applying a fixed sequence of these battery load pulses during battery measurement allows measurement both under a known load and under relatively little load (the latter by sleeping during the SAADC sample). The difference between loaded and unloaded measurements allows inferring the approximate internal resistance of the coin cell, and thus better estimation of remaining battery life using the temperature and curve fitting already presented. Simply put, the loaded measurement returns a much lower battery voltage than the unloaded measurement. Pulse time is affected by total reservoir capacitance, which props up the coin cell battery voltage; the more the capacitance the longer the required load test pulse.

Edit: Note numbers I quote here are for non-rechargeable CR2032, not the rechargeable LIR2032, but the principle is the same.