How to Determine 25% State of Charge for 3V Coin Cell Battery in Embedded Device?

Hi,
I have an embedded device which is powered by a coin cell battery of 3.0 v. As per the requirement, the embedded device shall turn solid white color if the battery state of charge is less than 25%. I am trying to understand this requirement so I can test it.

Here are my questions related to the above requirement:
1. When the battery charge is mentioned as 25%, should it be mathematically calculated as 25% of 3V? If I am incorrect, please let me know what 25% charge mean.
2. Is there a way to drain the coin cell battery to less than 25% of charge? If so, is there a tool available for that?

Thanks in advance for your help!

Hi,
battery state of charge estimation (also known as SOC) is a very challenging task. If you are able to get the state of charge chart related to your battery, the calculation is straightforward. However, I realize it's not easy to get that information.
Otherwise you can use one of the two most used techniques: voltage translation or coulomb counting (or even a combination of them). Voltage translation assumes the voltage battery is linearly dependent on the state of charge. So, you just to measure the voltage through an ADC or MCU and then solve a simple linear equation (you also need to know the minimum battery voltage, which in your case I think it's about 2V, but you should check it on the datasheet). So, for instance, 3V=100% SOC, 2V (or whatever)=0% SOC. Voltage translation, however, can not be used for Li-ion batteries, which tend to keep their voltage constant. In that case you should use the second technique (you can find several tutorial online) which actually is based on current, instead of voltage, measurement.

Just to add to Stefano’s excellent reply-

If your battery technology operates with a diminishing voltage, you need to make sure your electronics will operate at the reduced voltage. As an example, If it is a linear decline, then the electronics must operate at below 0.75V supply.

As to your 2nd question: any load will discharge the battery. A resistor is probably best for a controlled discharge. If you want to test the system and If you are measuring voltage, and not current, then you can simulate with an adjustable power supply

Thanks so much for providing such detailed responses with very good information! That is very helpful!