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  • Battery Storage

    Hi all

    How many kW does my battery bank store? Would someone explain battery storage to a rookie?

    I'm asking because I have 690Ah battery bank @ 24v that I decided to take for a spin before my panels are installed. My understanding is that 690ah x 24v = 16.56kW of energy to full depletion, 8.3kW at 50% depletion. However, per prior watt meter tests there's no way I've used 8.3kW since 4pm yesterday (I'd estimate < 1kW), but my battery voltage was at a critical 23.6v when I woke up this morning (Deca indicates 50% depletion is indicated at 24.2v). What am I missing?

    edit: math
    Last edited by -TX-; 02-16-2017, 10:53 AM.

  • #2
    Your calculation is almost correct but you have the labels wrong.

    A battery is rated in Amp Hours so multiplying that value by the battery voltage gets you Watt Hours not Watts.

    A 690Ah 24v battery has the potential of 16.56 kWh and 50% of that can be calculated to deliver 8.3kWh.

    The problem you are seeing is that while the math on paper works in real life that battery may not have be fully charged even if the voltage indicated so.

    Next thing is that while it may be able to deliver 50% it can't do that every day and still perform as expected. Most manufacturers provide the longest life span at a daily cycle of 20% - 25% DOD.

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    • #3
      Thank you for your response

      The batteries were fully charged per my inverter's charger controller.

      Are you saying my expectations are correct? Any idea what else could lead to such a discrepancy between 8.3kWh expected but only 1kWh delivered? Being a bit vague here, but since the trial started I've used 960wH of direct DC and a bit less of AC with coffee (hour), microwave (5 minutes), TV (3 hours).

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      • #4
        Originally posted by -TX- View Post
        The batteries were fully charged per my inverter's charger controller..
        That does not mean anything. Only a hydrometer can tell you what the true state of charge is. Secondly even if the hydrometer reads 100% SOc does not mean the battery has its rated capacity. Batteries loose capacity with every charge cycle and age.

        Lastly battery capacity is specified at the 20 hour discharge rate. For a 690 AH battery that would be 960 AH / 20 Hours = 48 amps. That same battery is discharged at say 10 hours or 96 amps only has roughly 700 AH.

        MSEE, PE

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        • #5
          Ok, I've ordered a hydrometer to get me a better idea on how good my inverter's charger is.

          I don't know why but your "960 AH / 20 Hours = 48 amps" just had me finally understand that AH rating... I think. So if my test period was 16 hours and my bank holds 690AH, that means I should expect 960 / 16 = 60 amps to total depletion, 30 amps to 50%? Is that right?

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          • #6
            Originally posted by -TX- View Post
            Ok, I've ordered a hydrometer to get me a better idea on how good my inverter's charger is.

            I don't know why but your "960 AH / 20 Hours = 48 amps" just had me finally understand that AH rating... I think. So if my test period was 16 hours and my bank holds 690AH, that means I should expect 960 / 16 = 60 amps to total depletion, 30 amps to 50%? Is that right?
            Amp Hours = Amps x Hours

            From that using 5th grade math we know

            Amps = Amp Hours / Hours
            and
            Hours = Amp Hours / Amps.

            However physics comes into play and there is a Law call Peuket Law which states a battery capacity becomes lower the faster we discharge the battery. So you have a 960 AH battery and that is at the 20 hour rate of 960 AH / 20 H = 48 amps. If you discharge at the 16 hour rate is 960 AH / 16 H = 60 amps. If you did that then you wil not measure 960 AH, it will be less because you are discharging at a faster rate.

            Not many manufactures publish discharge rates, only good manufactures do that. Example Rolls does a good job of it. Take a look at this spec and look at the Discharge rates. At 20 hours the battery is rated at 893 AH. Now look up and down. Example at the 15 hour rate is 830 amps (55.37 amps), now look at the 1 hours rate and it is down to 304 AH or 304 amps for 1 hours.

            So I can market the Rolls battery anyway I want. To consumers I call it a 893 AH battery. To Industrial user I call it a 697 AH battery because they rate Industrial batteries at the 4 and 6 hour rate. I can spin it anyway I want as a manufacture. Only quality manufactures like Rolls and Trojan publish discharge rates. It is a number game and you had better understand it.

            Last comment do not buy any ole hydrometer liike from Walmart or auto parts store, they are junk. You want a Temperature Corrected battery hydrometer like this one for $5. from DEKA.

            EDIT NOTE:

            Nothing is wrong with your charger. The mistake most people make is they use voltage set point and think that means it is the right voltage. Wrong, dead wrong. Voltag eset point for a battery is a WINDOW or a range of voltages. Example most batteries use a range of 2.3 to 2.5 volts per cell. Most would just use 2.4 volts per cell and think that is right, WRONG. That maybe to high or low. On a 24 volt battery that would be a range of 27.6 to 30 volts. What is the right voltage for that day? Only your hydrometer knows the answer.
            Last edited by Sunking; 02-16-2017, 02:04 PM.
            MSEE, PE

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            • #7
              Thanks. I just canceled and ordered your recommended unit instead

              My inverter's fast (bulk) charge is set at the high side of your range @ 29.2v. And thanks for further explaining the math. I still need to know why my test was so far off though. My battery bank (GC-15 4S3P) is 2 months old, never really being used except for DC, and my inverter, which is a pretty decent one, indicated full charge. I got very little out of this battery bank I installed which I thought was of good size for my purposes.

              Comment


              • #8
                What battery manufacture? Is it Deka? If so I think that is a 215 AH battery so 3P is 645 AH.

                One mistake you are making is parallel batteries. Learn how to use a Temperature Correction Hydrometer. I cannot answer why you are not getting the results you expect because we do not know what the actual SOC is. Only way to do that is with a Hydrometer and applying a Peukert correction factor.

                There is only one way to determine a battery capacity which you really do not have the means to measure. You have to start with a fully charged and equalized battery. Apply a Constant Current load of C/6 until th ecell voltage reaches 1.75 vpc or 21 volts on a 24 volt battery. The kicker is the Constant Curren tload must be precise regardless of voltage. That is a very expensive piece of test equipment.

                Now you can use one of the Hobby Chargers that RC Modelers use. They do fairly accurate discharge test, but for a large Pb battery like you have would take days at 10 amps, or roughly 69 hours.

                When was the last time you Equalized the batteries? You cannot do a capacity test before that is done and it takes a hydrometer to do that. So when you get the hydrometer, EQ the batteries until the Specific Gravity is equal in all cells. Consult the battery owners manual for EQ voltage and Specific Gravity. Be patient as it can take 24 hours.
                Last edited by Sunking; 02-16-2017, 02:45 PM.
                MSEE, PE

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                • #9
                  There are (for solar operations) only 3 stages of charge,
                  3 for summer time,
                  2 for wintertime
                  Bulk: Where a preset voltage is targeted and as much power as your system can supply is dumped to the batteries.
                  Absorb: When the target voltage is reached, Absorb starts and power starts to decrease as batteries approach full. The battery mfg sets the target
                  amps that signal the end of absorb - If you still have sunlight to power the charger. usually at least 90 minutes
                  Summertime, you get to Float, where the controller decreases voltage to Float, where the battery continues to slowly charge till it is full.
                  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

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                  • #10
                    More like 21.5AH GC15

                    edit: Just noticed your edit: Parallel: Don't understand what you mean. Parallel connectivity is a necessary evil in solar battery banks. There's no other solution using practical lead acid.
                    Last edited by -TX-; 02-16-2017, 03:06 PM.

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                    • #11
                      Originally posted by -TX- View Post
                      More like 21.5AH GC15

                      edit: Just noticed your edit: Parallel: Don't understand what you mean. Parallel connectivity is a necessary evil in solar battery banks. There's no other solution using practical lead acid.
                      Do not be silly. Lead acid batteries range from 7 AH to 7000 AH. You buy what is needed. Hell I already showed you a 893 AH lead acid battery. Four of of them in series is more than what you have now.

                      How are you getting 21.5 AH? Me thinks you miss placed a decimal point. The Deka GC15 is a 215 AH battery @ 20 hours. A golf cart would not run 5 minutes on 21.5 AH.

                      EDIT NOTE

                      My dyslexia strikes again. I rolled 690 and 960. So with Deka 4S3P GC15 you have a capacity of 3 x 215 AH = 645 AH. About the same as Trojan IND13-6V battery with 695 AH. 50 AH more than you have with 3 parallel strings. All you would have need is 4 in series.
                      Last edited by Sunking; 02-16-2017, 04:34 PM.
                      MSEE, PE

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                      • #12
                        I was being facetious. Specs were linked, GC15, 230AH

                        Love the Trojan but it's not going to work here. This is an RV install and the battery bank and box I devised maxes out available space.

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                        • #13
                          Originally posted by -TX- View Post
                          Love the Trojan but it's not going to work here. This is an RV install and the battery bank and box I devised maxes out available space.
                          Well that changes things a bit. You could have used Trojans T-105 or even better T-105RE as both are the same BCI Jar (same dimensions). Trojans have 3 to 4 pounds more lead in them.

                          Anyway being in an RV is a bit of a different use. I assume you do not live in the RV and cycle the batteries every day? If that assumption is true keep your batteries on FLOAT when not in use and stored away. That will keep them equalized and reverse some of the effects parallel batteries. The down side is it wil shorten cycle life and when one battery fails, replace all of them. Look at T-105RE when replacement time comes.
                          MSEE, PE

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                          • #14
                            I sought T-105's but the shipping cost was prohibitive. The Deka's were local and recommended.

                            While on contract, yes I live in RV. But it's large enough to add 2k PV and with that be considered your typical home solar setup.So once the panels are installed I will be cycling everyday.

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