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A way to accurately measure LiFePO4 SOC?

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  • #16
    Originally posted by Ampster View Post
    Did you charge your pack to 29.2 volts (3.65 volts per cell) and count the Amphours that went into the pack?
    I agree with you that 50% SOC is the flattest part of the curve, but it isn’t totally flat. Accurately measuring to 1/100 of a volt, under reasonably constant conditions seems, with a known accurate meter, after sufficient “rest”, seems to be working for me.

    As far as methodology, I derived 26.32 volts by fully charging the pack, in the proper manner, four times. Each time, I then discharged half of the pack’s rated amp hours, while measuring discharged amp-hours simultaneously with 3 different measuring devices. Each time, the rested pack voltage settled very close to 26.32 volts, after discharging half the rated amp hours. I also did one test each discharging to 51% and 49%. Reading pack voltage to 10 millivolts with my Fluke DVM convinced me that 26.32 resting volts is repeatable enough at 50% for my needs. Now, that is always after at least 10 hours of rest. It has been accurate enough that tests of various Charge Efficiency Factors, and Peukert Exponents always gave sensible results.

    Of course, as you well know, trying to determine SOC vs voltage, while charging or discharging is many, many times more meaningless.

    I also never charge or discharge at any rate other than between about C8 to C14. Perhaps greater extremes of charge and discharge current might introduce wider variation, but I don’t need to go there.

    Your point is well taken, but I’m satisfied with this methodology. In a few weeks, I intend to take some time to re-confirm by again charging to 100%, and counting electrons removed to 50.0% of capacity.

    Another advantage of using 15% to 85%, or 20% to 80%, is that small errors are not significant, in terms of harm to the system.
    Last edited by JohnCFII; 04-11-2021, 09:03 PM.

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    • #17
      Originally posted by JohnCFII View Post
      ..........
      Another advantage of using 15% to 85%, or 20% to 80%, is that small errors are not significant, in terms of harm to the system.
      Thanks for the thorough explanation. Yes, it is nice to have a margin of error.
      9 kW solar, 42kWh LFP storage. EV owner since 2012

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      • #18
        The one thing I didn't take into account is real-world use, like aging. Or murdering the battery a few times.

        I'm afraid that the only way to accurately judge is by coulomb-counting. Using voltage alone is rough, but as things age, what used to be generally 50% can now easily be 30% in a few years as things age.

        So yeah, voltage alone is not really ideal. Like all batteries, we just can't wing it if we're really into them.

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