Empiricial formula for state of charge (SOC) of NiMh battery

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1

Hi Guys,

I have a physical problem that calls out for the help of mathematicians, physicists or in general just smart people. I would like to measure the state of charge (SOC) value of NiMH batteries. The SOC value is strongly dependent on the open circuit voltage (OCV) and the temperature. I can measure both, the OCV and the temperature and I have empirically determined graphs for SOC as a function of temperature and as a function of voltage. I would like to come up with an empirical formula for SOC(OCV,temperature). Please have a look at the attached graphs. i digitized the values from the datasheet.

Thank you!

Batterycapacity-vs-temperature.JPG
Batterycapacity-vs-temperature.JPG939×734 59.6 KB

Voltage-Capacity_NiMH_battery.JPG
Voltage-Capacity_NiMH_battery.JPG925×756 57 KB

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The SOC value is also extremely dependent on the age, internal resistance and usage history of the battery, and an accurate model for it will, in principle, require modification with every charge/discharge cycle.

You can buy "fuel gauge" ICs that estimate remaining battery charge. Take a look at the data sheets to see what factors they take into account. Some smart battery packs have a microprocessor that keeps track of current, voltage and time to create and maintain an accurate model for the state of charge at all times.

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jremington:
The SOC value is also extremely dependent on the age, internal resistance and usage history of the battery, and an accurate model for it will, in principle, require modification with every charge/discharge cycle.

You can buy "fuel gauge" ICs that estimate remaining battery charge. Take a look at the data sheets to see what factors they take into account. Some smart battery packs have a microprocessor that keeps track of current, voltage and time to create and maintain an accurate model for the state of charge at all times.

Thank you a lot for your reply! I have come across some fuel gauge circuits already:

There seems to be a standard one for LiPo:
https://www.exp-tech.de/module/stromspannung/5824/sparkfun-lipo-fuel-gauge?gclid=Cj0KCQjwrMHsBRCIARIsAFgSeI2wFplBWOlEwGY22USpKlzhBusMxYOt8ofEsAzJi2MDM2z8GL3IbUMaAigaEALw_wcB

The MAX17043G+U.

This seems to be worth a try for NiMh:

What do you think?

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I have not worked with any of those ICs.

You can generally assume that they will perform as described in the data sheets.