Handheld battery tester that tries to be serious

[bateau020]

Just published the first workable version of my handheld battery tester.

Resumed specs:

• the regular AC 1kHz impedance
• time domain spectroscopy, allowing measurement of R_b and R_SEI. This is a faster and simplified alternative to Electrochemical Impedance Spectroscopy (EIS).

In 4 resistance ranges it goes from 150 Ohm full scale down to a 0.01 mOhm resolution (I did not say 0.01 mOhm precision, expect more like 0.1 mOhm).
Up to 20V (well, with small modifications 24V is possible).

All in a handheld format, for fast testing, and supporting SCPI over USB serial.

The goal was to have a fast and easy tool for Li-Ion cells and other common rechargeable and primary batteries.





It is equipped with the XS10 "avionics" plug also used by the YR1035+, so you can easily swap cables and even use it on test rigs.


All on github: https://github.com/hb020/batterymeter.

[bateau020]

Clarification, as I think that my original post did not explain the “why” of this tool.

Resumed:

The standard 1kHz measurement method does not go deep enough to my taste. At 1kHz, you only get 1 number. A number that even depends on a lot of factors. Other types of measurements can give much more detail, and can give insight not only in the aging of the battery, but also in the battery safety. Since Li-Ion batteries can be nasty beasts, I wanted that detail. This tool allows me to gain more insight in the state of charge and the state of health of batteries, while also being small and fast, allowing me to keep it easily accessible.


Long explanation:

Battery impedance is a complicated subject. A battery does not have one impedance, and even if you get a number, it can mean many things. Also, the battery impedance depends on a lot of different things, like state of charge (SoC), state of health (SoH), temperature, and of course the measurement method.

Various models of battery impedance exist, depending on the chemistry. For Li-Ion cells, the "Combined Randle and Warburg Circuit Model" is often cited as fairly complete, and, with some variations, also applicable to some other chemistries.



The EIS (Electrochemical Impedance Spectroscopy) method allows one to get a good view of the various components of the battery impedance. With this method an AC load of varying frequency is imposed on the battery, and the voltage swing over the battery is measured. The frequency sweep can go from the millihertz into the megahertz range, and can be used to generate a Nyquist plot like shown below. The image also shows the different related chemical processes.



For Li-Ion, the values of these components can be used as indicators.
•   R_b (aka R_s or R_Ω in other literature) = Bulk (or Solution) resistance. Increases when the battery ages.
•   R_SEI = Solid Electrolyte Interphase resistance. Is an indication of battery safety: it increases for example when the battery has been overcharged.
•   R_ct = Charge Transfer resistance. Increases when the battery is discharged, and when the temperature is lower. At a stable temperature and SoC, it is also a very good indication of battery health.

As an example, experimental results on Li-Ion batteries show that when the battery capacity drops from 3100 mAh to 2600 mAh due to ageing, the value of R_b rises from 44 mΩ to 51 mΩ, the value of R_SEI remains between 6 mΩ and 7 mΩ, and the value of R_ct rises from 6 mΩ to 20 mΩ.

The standard 1kHz measurement does not go deep enough to my taste. At 1kHz, you only get 1 number (in fact a sum of R_b and a part of R_SEI). Not enough to be able to distinguish simple aging from safety issues. Since I wanted that detail, I made this tool.