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  • State of charge

    I'm sure there are thousands of posts regarding this, and no end to advice either! However I'm currently residing in the camp of those who aren't exactly sure whether the batteries are getting to 100% or not.

    I've recently purchased several SLA batteries so a specific gravity reading is not possible. The only clue is the voltage and current readings I'm getting.

    Battrery pack is 24volts and 400AH.

    Typical winter daily profile is 23.2 volts at sunrise under a 10Amp load, followed by a gradual reduction in load and an increase in current from the panels that takes it up to a net charging current of 23Amps at midday. The voltage at the batteries usually peaks at about 29.4 volts around 14:00 to 15:00 in the afternoon. By the evening they are on "float charge" at 27.3 volts. Evening peak current draw is no more than 25 Amps for a few hours followed by a reduction to about 8 amps until dawn. After the PV's stop producing the voltage falls steadily from 26 volts to 23.2 volts by dawn. Between 100 to 150AH are withdrawn overnight depending on how much TV we watch. Anything up to 150AH could be produced by the PV's depending on the weather this time of year. Battery voltages in the morning are always somewhere between 23.2 and 23.8 volts.

    Is this normal? I've heard before that letting the batteries go below 2v per cell (24v) is very bad for lead acid and will lead to premature failure!

    Any help would be much appreciated!

    D
    Last edited by Dave3011; 06-23-2014, 10:18 AM. Reason: typo

  • #2
    Originally posted by Dave3011 View Post
    I'm sure there are thousands of posts regarding this, and no end to advice either! However I'm currently residing in the camp of those who aren't exactly sure whether the batteries are getting to 100% or not.

    I've recently purchased several SLA batteries so a specific gravity reading is not possible. The only clue is the voltage and current readings I'm getting.

    Battrery pack is 24volts and 400AH.

    Typical winter daily profile is 23.2 volts at sunrise under a 10Amp load, followed by a gradual reduction in load and an increase in current from the panels that takes it up to a net charging current of 23Amps at midday. The voltage at the batteries usually peaks at about 29.4 volts around 14:00 to 15:00 in the afternoon. By the evening they are on "float charge" at 27.3 volts. Evening peak current draw is no more than 25 Amps for a few hours followed by a reduction to about 8 amps until dawn. After the PV's stop producing the voltage falls steadily from 26 volts to 23.2 volts by dawn. Between 100 to 150AH are withdrawn overnight depending on how much TV we watch. Anything up to 150AH could be produced by the PV's depending on the weather this time of year. Battery voltages in the morning are always somewhere between 23.2 and 23.8 volts.

    Is this normal? I've heard before that letting the batteries go below 2v per cell (24v) is very bad for lead acid and will lead to premature failure!

    Any help would be much appreciated!

    D
    150ah draw / 400ah capacity = 37.5% discharge = 62.5%SOC

    IMO,
    your battery pack is too small... and discharging 37.5% daily will lead to premature failure.
    20% or less daily is recommended.

    IF you are reaching float, then your panels are producing enough to replenish,
    but this is summer... what about winter and what about cloudy days?

    If you don't get full charge on a cloudy day, then you discharge another 37.5%, for a total of 75%... not good.

    Get a 750ah battery pack, or lessen your usage/draw to 80ah daily.

    Really, the only way you'll get a semi-accurate idea of SOC on your SLAs would be to disconnect your battery pack from charge and load @ 15:00 and wait to do a voltage reading @ 18:00... you need to let the SLA batteries settle 3 hours before guesstimating the SOC.
    [CENTER]SunLight @ Night[/CENTER]

    Comment


    • #3
      Originally posted by Shockah View Post
      150ah draw / 400ah capacity = 37.5% discharge = 62.5%SOC

      IMO,
      your battery pack is too small... and discharging 37.5% daily will lead to premature failure.
      20% or less daily is recommended.

      IF you are reaching float, then your panels are producing enough to replenish,
      but this is summer... what about winter and what about cloudy days?

      If you don't get full charge on a cloudy day, then you discharge another 37.5%, for a total of 75%... not good.

      Get a 750ah battery pack, or lessen your usage/draw to 80ah daily.

      Really, the only way you'll get a semi-accurate idea of SOC on your SLAs would be to disconnect your battery pack from charge and load @ 15:00 and wait to do a voltage reading @ 18:00... you need to let the SLA batteries settle 3 hours before guesstimating the SOC.
      I'm in the Southern Hemisphere 34°S so it is mid-winter as we speak. I use a UPS inverter which allows me to bypass the batteries when the weather is cloudy or if the PVs haven't produced enough to get to float charge stage. I have never removed more than 50% of the battery's charge in one cycle. Disconnecting the battery pack is impractical hence the reason I posted this thread. I've also heard one needs to wait more than 24 hours to take a state of charge reading by voltage. Not practical though...

      Comment


      • #4
        If you have commercial AC you should not even be using batteries period. SOC voltage are just about practically useless on a working system. It only works on fully rested batteries and even then is only a relative indication. With that said assuming a C/10 discharge rate (40 amps on your system) you can use this as a general rule.

        100% = 25.46
        75% = 24.74
        50% = 24.2
        25% = 23.4
        0% = 23.02

        So if you are seeing 23.2 volts with only a small C/40 discharge (10 amps) is telling you completely drained your battereis over night which is a recipe for very short battery life. Time to upgrade and replace your batteries.
        MSEE, PE

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        • #5
          Originally posted by Sunking View Post
          If you have commercial AC you should not even be using batteries period. SOC voltage are just about practically useless on a working system. It only works on fully rested batteries and even then is only a relative indication. With that said assuming a C/10 discharge rate (40 amps on your system) you can use this as a general rule.

          100% = 25.46
          75% = 24.74
          50% = 24.2
          25% = 23.4
          0% = 23.02

          So if you are seeing 23.2 volts with only a small C/40 discharge (10 amps) is telling you completely drained your battereis over night which is a recipe for very short battery life. Time to upgrade and replace your batteries.
          Thanks for the info. These batteries are obviously not performing as they should then as I have never seen those voltages hold that high over comparable discharge rates, even when the batteries were brand new. Perhaps it's because the temperature is colder due to the season? The room temperature is about 17°C. I would also be very interested to know how long a depleted battery needs to be on charge to be sure it's full. These batteries seem to have a high internal resistance because once the PV's are pushing more than 20 amps the voltage seems to shoot up fairly rapidly to the charge controllers maximum boost charge set point of 29.4 volts and the current gradually tapers off to about 5 amps over 2 to 3 hours before it switches over to float charge. My concern is that if the batteries are not fully charged yet why is the voltage going so high and th current tapering off so soon? Can only be that the resistance is too high due to damage or cold temperatures. However the batteries achieved a load test not dropping below 21.6 volts with a 1C load applied for 30 seconds.

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          • #6
            Originally posted by Dave3011 View Post
            These batteries seem to have a high internal resistance......

            because once the PV's are pushing more than 20 amps the voltage seems to shoot up fairly rapidly to the charge controllers maximum boost charge set point of 29.4 volts and the current gradually tapers off to about 5 amps over 2 to 3 hours before it switches over to float charge. My concern is that if the batteries are not fully charged yet why is the voltage going so high and th current tapering off so soon?
            You answered your own question. As batteries age and used the plates become sulfated which in turns raises the internal resistance. Sulfated batteries are 95% of all lead acid battery failures. Remaining 5% is corrosion. The symptoms are very easy to determine the death of a battery from insulation.

            1. When battery is put on a charger, voltage rises very quickly fooling the charger into thinking the batteries are charged up and shuts down.
            2. Open Circuit Voltage appears to be fine, but when a load is put on the batteries, the voltage collapses.

            Any of that sound familiar?

            A 24 volt 400 AH battery IR in good condition in a discharged state should be able to take 200 amps or more charge current to even rise the voltage of 1 volt which would be on the order of 1 volt / 200 amps = .005 Ohms. A battery that rises 2 volts with only 20 amps means the IR = 2 volts / 20 amps = .1 Ohms which means way beyond dead battery.

            Guess what time it is?

            Quit using batteries and go back on the grid. Grid power is much less expensive than batteries.

            Question back at you. Why have not you been using an AC commercial charger to charge your batteries or a generator? Every battery system is required to have a generator or AC power with a charger. It would have prevented pre-mature death?
            MSEE, PE

            Comment


            • #7
              Originally posted by Sunking View Post
              You answered your own question. As batteries age and used the plates become sulfated which in turns raises the internal resistance. Sulfated batteries are 95% of all lead acid battery failures. Remaining 5% is corrosion. The symptoms are very easy to determine the death of a battery from insulation.

              1. When battery is put on a charger, voltage rises very quickly fooling the charger into thinking the batteries are charged up and shuts down.
              2. Open Circuit Voltage appears to be fine, but when a load is put on the batteries, the voltage collapses.

              Any of that sound familiar?

              A 24 volt 400 AH battery IR in good condition in a discharged state should be able to take 200 amps or more charge current to even rise the voltage of 1 volt which would be on the order of 1 volt / 200 amps = .005 Ohms. A battery that rises 2 volts with only 20 amps means the IR = 2 volts / 20 amps = .1 Ohms which means way beyond dead battery.

              ...................
              ^^^ Priceless.

              Question for you Sunking: What is the procedure for measuring IR on an AGM/SLA Battery?
              [CENTER]SunLight @ Night[/CENTER]

              Comment


              • #8
                Originally posted by Dave3011 View Post
                The room temperature is about 17°C. I would also be very interested to know how long a depleted battery needs to be on charge to be sure it's full.
                On a side note, is your charge controller doing temperature-compensation, preferably at the battery terminals and not ambient? That would help.

                The other stuff we'd like to know is did you purchase new or used? How is your bank constructed - what is the parallel / series arrangment? 12v "blocs" or individual 2v cells? If this system is way out of balance, or was never balanced to begin with, that may be part of your troubles.

                Hopefully this isn't something crazy like series / paralleling 80 little 9ah ups-style agms.

                Comment


                • #9
                  Originally posted by Shockah View Post
                  Question for you Sunking: What is the procedure for measuring IR on an AGM/SLA Battery?
                  Pretty simple assuming you have the equipment to pull it off.

                  1. Charge the batteries up and let them rest for at least 6 hours, and measure the Open Circuit Voltage
                  2. Apply a significant load. The more load, the more accurate the test will be. Minimum load of C/4 and C/1 is even better. So for a 100 AH battery 25 to 100 amps.
                  3. Measure the voltage of the battery while on load.
                  4. IR = [Open Circuit Voltage - Loaded Circuit Voltage] / Load Current. Example for a 100 AH 24 volt battery. [25.5 volts - 24.5 volts] / 100 amps = .01 Ohms
                  MSEE, PE

                  Comment


                  • #10
                    And if you are needing more capacity then 400ah @ 24V, look to a 48V system
                    buy
                    But your first problem is to discover why your loads exceed your system, did it get designed for you ? buy it with the property ? keep adding more stuff ?

                    And if you have grid power, why are you using batteries daily instead of only in case of power failure ?

                    As others stated, quick recharge and quick voltage droop = sulphated/damaged batteries.

                    Sealed batteries should NEVER be paralleled, as they can experience thermal runaway in cases where simple flooded are fine.
                    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


                    • #11
                      Originally posted by Sunking View Post
                      Pretty simple assuming you have the equipment to pull it off.

                      1. Charge the batteries up and let them rest for at least 6 hours, and measure the Open Circuit Voltage
                      2. Apply a significant load. The more load, the more accurate the test will be. Minimum load of C/4 and C/1 is even better. So for a 100 AH battery 25 to 100 amps.
                      3. Measure the voltage of the battery while on load.
                      4. IR = [Open Circuit Voltage - Loaded Circuit Voltage] / Load Current. Example for a 100 AH 24 volt battery. [25.5 volts - 24.5 volts] / 100 amps = .01 Ohms
                      ^^^ Thanks!!

                      Originally posted by Mike90250 View Post
                      ...............

                      Sealed batteries should NEVER be paralleled, as they can experience thermal runaway in cases where simple flooded are fine.
                      ^^^ Good to know.
                      I have a set-up with 2 AGM/SLA batteries in Parallel... hopefully, thermal runaway won't become a problem.
                      So far, 3 months in, it's been operating flawlessly... another 33 months to go and I am home free!
                      [CENTER]SunLight @ Night[/CENTER]

                      Comment


                      • #12
                        FWIW if a battery list CCA or MCA it is telling you the IR at a specific temp of a new fully charged up battery.

                        CCA or Cold Cranking amps is the maximum current delivered from a battery at 7.2 volts (for a 12 volt battery) for 30 seconds at 0 degree F. So if the battery claims 650 CCA on paper the IR = 7.2 volts / 650 amps = .011 Ohms.

                        MCA or Marine Cranking Amps is done at 32 degrees F and should be about 30% higher than CCA. So a dual rated battery would look like something like CCA = 650 amps, and MCA = 850 amps.

                        DO NOT CONFUSE CCA and MCA with Amp Hours (AH) as they have nothing to do with each other. True Deep Cycle Batteries will not have a CCA or MCA spec. Only hybrid (marketing names like Marine, Golf Cart, RV, and Leisure) , and SLI batteries have a MCA and CCA specs. So when you folks come on here and say they have a 600 amp battery you know instantly they are screwed.
                        MSEE, PE

                        Comment


                        • #13
                          Originally posted by Shockah View Post
                          ^^^ Thanks!!



                          ^^^ Good to know.
                          I have a set-up with 2 AGM/SLA batteries in Parallel... hopefully, thermal runaway won't become a problem.
                          So far, 3 months in, it's been operating flawlessly... another 33 months to go and I am home free!
                          You are welcome.

                          I have no problem with AGM batteries, but they are niche application and have their place. Best used for very high charge and discharge rates. Problem is you pay twice for them. You first pay roughly 100% more for them at a given capacity. You then pay for them again with only half the service life of FLA. So in the end you end up paying some 4 times more for them. Shockya in your warm climate and excellent Solar Insolation screams for FLA. AGM is more suited to cold climates with extremely short winter Sun Hours of 2 and less where high charge rates are needed and extra freeze protection. Mobile applications are another good use for AGM where you use the alternator to do most of the charging at high rates and no spillage issues. AGM was invented for aircraft applications where it gets to 60 below zero and inverted.
                          MSEE, PE

                          Comment


                          • #14
                            Originally posted by Sunking View Post
                            You are welcome.

                            I have no problem with AGM batteries, but they are niche application and have their place. Best used for very high charge and discharge rates. Problem is you pay twice for them. You first pay roughly 100% more for them at a given capacity. You then pay for them again with only half the service life of FLA. So in the end you end up paying some 4 times more for them. Shockya in your warm climate and excellent Solar Insolation screams for FLA. AGM is more suited to cold climates with extremely short winter Sun Hours of 2 and less where high charge rates are needed and extra freeze protection. Mobile applications are another good use for AGM where you use the alternator to do most of the charging at high rates and no spillage issues. AGM was invented for aircraft applications where it gets to 60 below zero and inverted.
                            I agree with you 100%, Sunking...

                            The one and only reason I use AGM is because I am not able to maintain FLA batteries in all of the installations.
                            Most are atop posts... where it is not advisable to be removing caps and checking acid levels

                            I am currently designing another project for a Used Car Lot... those batteries are going to be mounted at the base of the posts... and the lot is located near me. I'll be using FLA on that one.

                            For the smaller systems that require 18ah - 35ah, the price for AGM is reasonable enough.
                            [CENTER]SunLight @ Night[/CENTER]

                            Comment


                            • #15
                              Originally posted by Shockah View Post
                              The one and only reason I use AGM is because I am not able to maintain FLA batteries in all of the installations.
                              That is one of the Niche applications I did not mention. I just want to make sure you make an Informed decision and know what the pros and cons are.

                              Good luck to you.

                              BTW the coffee in Panama is every bit as good as Kona. It is really cheap here as I live right in the middle of the farms that grow it. Originally I was not much of a coffee drinker until moving here. Personally I put Kona Hawaii, Jamaica Blue Mountain, and Boquete Panama coffees best in the world. Those are the only three I really like enough to drink every day.
                              MSEE, PE

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