It is difficult or perhaps even impossible to determine the SOC on LFP from voltage measurements alone. At least to do it with any accuracy. You can measure the temperature on each cell in addition to voltage to _perhaps_ get a better handle on the SOC, but still you can't get a very accurate SOC.

From a practical point of view, it is easier and cheaper to count the amp-hours in a single place, than to make very precise voltage and temperature measurements on every cell in a high-voltage pack. That is why it is done that way universally in OEM cars.


Bill D.

At 10:15 AM 8/8/2017, you wrote:
paul dove via EV wrote:
Sure you can. Voltage is the only way to discern SOC.
You just can't measure accurately under load. You have to measure open circuit voltage.
www.mdpi.com/1996-1073/9/11/900/pdf

This paper mainly discusses the higher-voltage Li-Ion cells, which have a larger voltage change between full and empty.

For the LiFePO3 cells being discussed here, the voltage change is much smaller. As Bill says, the change is so small that it can't be reliably measured outside of laboratory conditions and precisions instruments. Other factors, such as age, temperature, manufacturing variations, and the accuracy of the equipment play a larger factor.

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