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Query on my battery bank's state of charge

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  • Query on my battery bank's state of charge

    Hi all,

    It's been a while since I was here and thanks to those that helped with giving me advice. I've got my solar power system up and running in the last 2 days but I was wondering if someone could help me clarify a question I have about my battery's SOC.

    This is my system which I'm testing with x2 100W 12V panels:

    24V battery bank (x2 100Ah 12V Casil sealed lead-acid batteries)
    MidNite Classic 200
    Victron Phoenix inverter 24v 1200W (not running it anywhere near full capacity)
    x2 100W 12V solar panels

    The advice I received was that the majority of first-timers are going to ruin their batteries - so these two were picked up new but on the cheap (someone was moving so I got a good deal). Last time I was told that these were primarily for telecommunication purposes and not to expect too much out of them. I figured they would be good to learn on.

    In the Classic I entered that the battery was 100Ah and 24V. I have a Whiz Bang Jr linked to the classic and when I set it up it showed me that the SOC was 100% - this surprised me because the voltage (with only the classic being powered) was 24.2V. I had in my mind around 12.7V = 100% for each battery in the series giving 25.2V or so if full.

    This is the link to the battery pdf

    Also I'm using the default bulk, float and equalize on the midnite (28.6V, 27.2V, 28.6V respectively) - if anyone has any suggestions based on that pdf, I'm all ears. Thanks!

    This image shows the MidNite panel - it was under a small load at this point.

    Last edited by DavidH; 10-22-2020, 07:57 PM.

  • #2
    The only true way to tell state of charge accurately is to use a refractometer.. everything else is pretty much just an estimate based on external values and established constants.. Hydrometers can get you very close, but a refractometer is better. Of course, with a sealed battery, using either meter to test acid levels is not practical.

    I'm not sure about your wizbang device, but it might have a setting for battery voltage or need some kind of calibration. A lot of systems out there perform their own self-calibration based upon the first full charge/discharge cycle.

    Sorry, wish I could be of more help.


    • #3
      Is there a question? If you want to know your batteries SOC, read it with a hydrometer.
      MSEE, PE


      • #4
        "If you want to know your batteries SOC, read it with a hydrometer"

        Is that possible on an SLA battery?

        I too am looking for a way to more accurately know my battery's SOC.


        • #5
          Originally posted by Mattboyrdee View Post
          "If you want to know your batteries SOC, read it with a hydrometer"

          Is that possible on an SLA battery?

          I too am looking for a way to more accurately know my battery's SOC.
          Unfortunately a sealed battery's SOC can't be measured using a hydrometer. So the only way to get an estimated SOC value is to measure the voltage after the battery has been resting for more than a few hours after charging. An even then it is just an estimated value.


          • #6
            Bummer. I thought that would be the case but was hoping otherwise.



            • #7
              You can get a fancy meter that counts watts in and watts out, and if you use the right loss factor in the meter, you can get pretty close, but after a couple months watching a volt meter, you can figure in your mind, what the SOC is from the voltage. With my bank, (NiFe, not lead acid) I run the generator at 52V. Happy at 60V and full after absorb at 67V
              Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
              || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
              || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A