Measuring state of charge in LiFePo

It seems to be a problem for solar based batteries with a 48V stack.Manufactureres say it's real tricky... temperature/charge rate/discharge rate blah blah. So why is it so hard to get an accurate SOC ? 0%= ~40/41V 100% =~57V ... A 12 bit ADC would yield 4mV resolution. Isn't that enough to get an accurate state of charge? My analogue experience is next to nil..If there is a real problem, how about 3 ADCs each measuring low middle and high voltage? i.e 40-45;

46-51;52-58 or is that a stupid idea ?
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EG 9.5 kW battery by Givenergy ( UK/China)

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You have to know the temperature of the battery which gets warmer when you pull current out of it or put current into it.

Ideally, you want a temperature sensor in the middle of the battery, which battery manufacturers don't provide. If you know the currents flowing you do know the heat being generated in the core of the battery, and can correct the temperature you measure just out side the cell, but it is tricky.

You need a second ADC to keep track of the temperature and you need to have a reasonable idea of the heat capacities and thermal resistances involved.

It's not trivial.

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Anthony William Sloman

Because SOC is not reflected in a straight line voltage change between full and flat.

Your LiFePo battery does not start with full charge at ~57v and linearly drop down to no charge at ~40/41V

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If you want more detail

Electrochimica Acta 60 (2012) 269– 273 can tell you more. the paper title is

Effects of temperature on charge/discharge behaviors of LiFePO4 cathode for Li-ion batteries

Authors are

Lixia Liaoa,b, Pengjian Zuoa, Yulin Maa, XinQun Chena, Yongxin Ana, Yunzhi Gaoa,∗, Geping Yina,∗∗ a School of Chemical Engineering and Technology, Harbin Institute of Technology, No.92, West Da-Zhi Street, Harbin 150001, China b College of Science, Northeast Forest University, No.26, Hexing Road, Harbin 150040, China

Figure 2 is informative - not as informative as a more intelligible exposition would have been, but it gives you some idea of what happens.

The Arrhenius equation shows up just once in the article and not in a useful place.

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is a bit more explicit but it confuses internal resistance with capacity, which isn't helpful.

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Anthony William Sloman

Make sure the cells are actively balanced. This removes single-cell dominance in the string, allowing generalizations to work in capacity estimation for varying discharge rates.


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