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A way to accurately measure LiFePO4 SOC?

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  • A way to accurately measure LiFePO4 SOC?

    I'll be taking delivery of 400ah of 48 volt lifepo4 in a couple months, so I've been studying as much as I can in advance. Based on what the consensus seems to be regarding maximizing cycles, my plan is to charge to no higher than 80% SOC and discharge to no lower than 20% SOC, I understand that deducing SOC from voltage is a challenge due to the extremely flat charging curve.

    So here is my question: Given that charge current drops as the battery voltage approaches the charge voltage, doesn't this provide a good way to deduce SOC? For a given charge voltage, that is, doesn't the current (the battery's "acceptance") reflect the battery's SOC pretty accurately? I've seen specs that say, e.g. "for a 90% SOC, terminate charging when current falls to .005 C." For my 400ah bank, that would mean halting charging at 2a (with charge voltage set at manufacturer's recommendation). So doesn't it follow that stopping at 80% SOC would mean stopping at some higher amperage, e.g. 4a? If so, I wonder why there aren't tables that let you estimate SOC based on current at given charge voltages and for given Ah capacities.

    I'm not an EE, so I realize this mightn't make sense. Happy to be educated! Thank you.

    BTW--I found PNjunction's 07-12-2019, 05:08 PM post on bulk/absorb/CC/CV extremely enlightening, and I hope he weighs in on my post.

  • #2
    Hi openplanet!

    Well, the deed is done I suppose - it is questions like these that make me always recommend a "practice" battery first, something more down to earth like a simple 40ah / 12v setup before going big. Nevertheless ..

    Voltages are deceiving as an SOC indicator. It is the *at rest* voltages (which take about 4-12 hours) where it is *slightly* more accurate. Not practical if you are doing daily solar cycling!

    In other words, if *after a charge and a rest / no load*, your cells are sitting close to 3.45v each, then you are fully charged. Not that you want to go there often in reality, but this is just discussion.

    If after a rest / no load condition, your individual cells are sitting close to 3.0v, then you are approaching the bottom 20% knee.

    WAIT! You mean there is only about a 0.45v difference between 80-100% and say 20% at the low end - *when measured at rest*? Yep. Better have an accurate metering system in place, with VERY good low-loss wiring infrastructure. That means good FLUKE multimeters for you diy types.

    This is why for a large system like yours, you'll need to be watching coulombs, (aka a/h in out) in addition to voltage extremes. Doing experiments based on voltage solely with solar garden AA's, and hobbiest cells are a whole different ballgame from a practical aspect of a large bank with solar.


    • #3
      So you want an *at rest* SOC chart for LFP?

      Believe it or not, here you go. From Shorai. LFP motorcycle battery manufacturer.

      See the chart in this FAQ, in the "Can I use a lead-acid battery charger or tender" section for the chart. Uh, yeah, don't use a wall-wart motorcycle charger on your 400ah bank - but the voltages are right anyway:

      Divide the 12v chart by 4 for individual cell voltages, and apply to your 48v bank. (the 12v batts have 4 cells inside, right so divide by 4...)

      As valid for a 2.5ah LFP motorcycle batt as it is for a 400ah bank.

      So - if an individual cell is sitting at rest (4-12 hours no load) 3.585v, then it is 100% full. Don't let it sit like that for long!
      And, we can see that once a cell has reached 3.21v, *at rest*, it has reached the 20% knee you don't want to slide past.

      So if you want to go by *RESTING* voltage alone, and take your own monitoring into account, this is handy rough estimation for SOC.

      Yes, the Shorai's have a slightly different LFP chemical makeup than yours, but at least this is *something*.
      Last edited by PNjunction; 08-05-2019, 07:51 PM.


      • #4
        Thanks very much for this info.
        I have Midnite Classics and a Whizbang Jr. so I'll be monitoring charge in/out very diligently.
        Seems like whoever comes up with an accurate SOC-o-Meter for LiFePO4s will do well...


        • #5
          I just looked at the chart you provided a link to. Must say I'm surprised that 57.36v resting is considered 100%. That seems high.


          • #6
            It is a little bit high due to a slightly different chemistry. But close.

            If you split the difference between the 100% and the 90% voltage, one ends up very near 3.45v resting - that would be my maximum at rest. But again, you don't want to be sitting at that for very long.

            Sure, they make LFP voltage based soc meters. The Tecmate-Optimate TS-127 meter mainly targeted at 12v motorcycle users gets one in the ballpark with LFP / agm / flooded. I compared it against my Fluke, and it's close enough for shade-tree observations as the led floats around the outer edge of the meter.

            Still, at the end of the day, voltage alone, be it ANY battery chemistry is not always the best for determining true SOC (or health), but there ya go. Better than nothing.
            Last edited by PNjunction; 08-09-2019, 07:45 AM.


            • #7
              I have a large 16s LFP pack I use daily for my house power. Consequently, it is always under some load or charge.

              The slope of LFP is not very large, but it is there, and it is quite linear between 20% and 80% SOC. Moreover, at 16s, it is much easier for regular meters to read it with enough precision to be useful.

              Under typical house loads, as long as the demand has been pretty stable for a little while, I can tell you the SOC of my battery just by looking at the pack voltage readout, to within 5% or so.

              Many of the other comments here are also good ones, but don't let the conventional wisdom get you too worried. With common sense, and some time invested up front, you too can become a human battery SOC meter.


              • #8
                I have been trying to monitor my dropins staying between 20-80% with voltage alone & completely gave up. It varies too much with loads so not worth it imo. This was with an accurate DMM at terminals. I found the 20-35% region to be more flatter than the rest of the flat curve.
                Another issue is when charging off solar the rate is not solid like AC charging & so at lower rates I have found it too difficult to know where 80% is, you can easily skip past it & be at 90%+. The battery always absorbs it's way to full quite easy even with lower voltages & rates as I understand it.
                So I just use an AH counting monitor & program it well. I figure at worst it will be like a cars fuel gauge, not super accurate but gives a good idea.

                I found charging at lower voltages just means the battery takes longer to fill up & doesn't just stop at some magic region. 13.4V float voltage seems to be enough to fill the battery up (high 90%) with enough hours.


                • #9
                  Originally posted by Jman View Post
                  Another issue is when charging off solar the rate is not solid like AC charging & so at lower rates I have found it too difficult to know where 80% is, you can easily skip past it & be at 90%+. The battery always absorbs it's way to full quite easy even with lower voltages & rates as I understand it.
                  Yeah, it's pretty tricky to use voltage-based settings to stop at 80% or lower when at a low average rate like you might have with solar. You need a big solar array or a small battery to hit the CV threshold "hard" enough, and that can tough (or impossible) to do. Using a counter and/or a BMS that has something similar built in is the way to go then, for sure.

                  (And it's also probably the way to go even if you do have a big charger and a very reliable CC/CV regime you can follow.)


                  • #10
                    Forgot to say the one thing about amp hour monitors like my victron bmv 712 is you have to get back to 100% full often (maybe every 10cycles) to manually reset & prevent it from becoming inaccurate.