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How to set 30% DoD

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  • How to set 30% DoD

    My solar setup is a 24V system.

    The Charge controller is by Famicare with built-in 2.5KVA inverter.

    The datasheet:
    http://www.famicareindustry.com/prod...--38758--.html

    I have two, 12V 180Ah AGM batteries, configured in series to obtain a 24V bank.

    How should the charge controller be configured for 30% DoD?
    What should the cutoff volt be?

  • #2
    Voltage is a poor indicator of state of charge.

    Why ? Because under a load, the battery voltage sags. Many inverters have a cut-off voltage, with a time delay, so the voltage needs to be low for 2 or 3 minutes . When the load is removed, the voltage rebounds and you have a good battery again ! NOT. The battery still had power removed from it.
    So, I think it's more of a judgement and evaluating your usage and then you can set the voltage you want shutdown at.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      Here are some links from Trojan

      http://www.batterysizingcalculator.com/
      https://www.trojanbattery.com/tech-s...y-maintenance/

      BUT, in a nutshell, you need to estimate / measure your consumed Ah based on your loads and run times. Then based on your desired DOD, scale the battery capacity to match.

      So for example. Given your 12V 180Ah example. Your two batteries in series gives 24V for 180 Ah. Based on a 30% DOD, you have 54 Ah @ ~24V available (0.30*180)

      So that's 1296 VA hours (let's assume a unity power factor). Assuming you're voltage doesn't sag too much, and assuming your inverter is ~90% efficient, that's roughly 1166Wh of usable capacity per cycle -- when everything is new and fully charged. So, for example, you could run a 100W light bulb for ~11 hours.

      Now, total Ah rating of batteries depends on the rate at which power is consumed, though depending on the battery it can be hard to find the current withdraw rate at which this capacity was actually measured. But usually it is VERY SLOWLY -- often slower than you would like to use it. The faster your draw the current from the battery and the older / number of cycles the battery has the lower the residual capacity will be.

      Of course, some of your consumption would be offset by the PV system assuming it's running during the day (and is sunny outside) and your PV array is able to generate sufficient voltage/current to actually charge the connected battery bank.

      They're battery calculator can walk you though this.

      Comment


      • #4
        Interesting. I have always heard about DoD, I thought it is a straightforward thing to set.
        The specs of the charge controller shows "Battery low level disconnect" as selectable 20V or 21V.
        I guess I will just select 21V and call it a day.


        Thank you both.

        Comment


        • #5
          Originally posted by fw12 View Post
          ...I guess I will just select 21V and call it a day.
          That will make your expensive battery the happiest.

          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

          solar: http://tinyurl.com/LMR-Solar
          gen: http://tinyurl.com/LMR-Lister

          Comment


          • #6
            Keep in mind that low voltage shutoff parameter is there as a "safe guard." It is not indicative of state of charge, per se. The voltage that the inverter sees is dependent upon:

            1) Battery chemistry
            2) Battery health
            3) Battery SOC
            4) Resistance to current flow in your setup
            5) Current draw from your load
            6) Charging current, if any, from your PV

            By the time your FLA battery string gets to 21V / 2 = 10.5V per battery, your batteries are already DEAD!

            Refer to this table (Last page #26)
            http://www.rencobattery.com/resource...vs-Voltage.pdf

            The voltage curves of FLA and AGM batteries are very flat. And of course, apparent voltage varies with load. As described by the various C/3 through C/100 discharge rate curves.

            You never said what your load actually is, how many watts or VA's it uses, and how many hours/days you hope to run it.

            Your 2.5KVA inverter, even running at 50% load (1250VA) is way too big for your two tiny batteries.
            1250VA / 24V = 52 Amps
            180 / 52 = C3.46

            So with a ~1000W load, you'd hit your 30% DOD in about one hour.

            Over which time battery voltage would have started at ~25V to 26V fully charged, and unloaded. And then fallen to ~23.5V to 24V under 1000W load at 0% DOD to ~22.5V to 23V at 30% DOD. ASSUMING your wire gauge and connections are able to support 50+ amps continuously without heating or other significant resistance.

            Without meaning to be condescending, this board is flooded with posts of folks trying to run loads that are way too big for their battery bank size, charging with too few/small PV arrays, and then wondering why their batteries barely last a year.

            The simplest thing to do is to buy a KillaWatt or similar
            http://www.p3international.com/products/kaw-chart2.pdf

            And then measure your actual consumption. Once you get to ~1166Wh (e.g 1.2kWh) you're done.

            How frequently are you cycling your batteries? Daily? How many days of reserve capacity due to you need to build in if it isn't sunny? Alternate means of charging?

            Comment

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