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  • Is my battery math correct?

    My system will have 16 Full River DC 224 6 volt batteries. Total energy stored 3584aH. So if I ran the batteries down only 30% I would have 1075aH. My inverters seem to max out at 65amps. Dividing 1075/65 gives me 16.5 hours of battery if I was maxing my draw.

    Roughly correct?

    Edit: I don't care if I'm off by a factor of 20% because of inefficiencies. I'm wondering if I am off by factor of 4 or 5 because I don't get the basics. There I just saved the nitpickers from some keystrokes.


    Thanks
    Last edited by kingofbanff; 05-31-2016, 02:27 AM.

  • #2
    Your math is off.

    How are you wiring those 16 x 6v batteries?
    If they are wired for 48volt you would have a 448Ah system. (2 sets in parallel of 8 wired in series)
    If they are wired for 24volt you would have a 896Ah system. (4 sets in parallel of 4 wired in series)
    If they are wired for 12volt you would have a 1792Ah system. (8 sets in parallel of 2 wired in series)

    When you wire batteries in series the voltage adds up but the Ah stays the same
    When you wire battery strings in parallel the voltage stays the same but the Ah adds up.

    Unless you have a 6volt system (all 16 batteries wired in parallel) then you would get 3584Ah system but at 6 volts.

    As for how much you will use a day, do not do the math in Ah but in Watt hours. Using Ah can result in the wrong usage amount.

    So what is your calculated daily (24hour) watt hour usage?

    Comment


    • #3
      No one can answer correctly because you have not said how the batteries are configured. There are 5 possibilities. Which one do you have.

      6 volts @ 3584 AH: All 16 batteries in parallel which would be very stupid
      12 volts @ 1792 AH 2 x 8.Extremely poor configuration
      24 volts @ 896 AH 4 x 4. Extremely poor configuration
      48 volts @ 448 AH 2 x 8, Doable but poor configuration
      96 volts @ 224 AH all in series. Correct configuration.

      Lastly you do not use Amp Hours to calculate usage. You use Watt Hours. Amp Hours means nothing and is just an end result.

      To answer your question, most likely your math is way off. Please tell us you are not operating at 12 volts.
      Last edited by Sunking; 05-31-2016, 09:58 AM.
      MSEE, PE

      Comment


      • #4
        Thanks for the quick reply. I was working in kwH but all the batteries are quoted in aH so I was using lazy cackle to convert.

        It is a 48 v system. So 448AH at 48 volts is 21kwh. 30% is 6Kwh. Interestingly my estimated usage in a grid down situation is 6kwh per day. (TV, cold beer and some fans).

        Comment


        • #5
          Originally posted by kingofbanff View Post
          Thanks for the quick reply. I was working in kwH but all the batteries are quoted in aH so I was using lazy cackle to convert.

          It is a 48 v system. So 448AH at 48 volts is 21kwh. 30% is 6Kwh. Interestingly my estimated usage in a grid down situation is 6kwh per day. (TV, cold beer and some fans).
          While 30% may be a little high to get the most out of those batteries your math is correct about it being 6kWh.

          Now remember what you take out of the battery system you have to put back and more due to efficiency and losses. So depending on where you live, a 5 hour insulation time period would require about 1800 watts of solar panels to generate that 6kWh.

          Comment


          • #6
            Originally posted by kingofbanff View Post
            Thanks for the quick reply. I was working in kwH but all the batteries are quoted in aH so I was using lazy cackle to convert.

            It is a 48 v system. So 448AH at 48 volts is 21kwh. 30% is 6Kwh. Interestingly my estimated usage in a grid down situation is 6kwh per day. (TV, cold beer and some fans).
            You did right by using Kwh. Amp Hours is just the end result. So if you calculated the battery size to be 21 Kwh and ran 48 volts then, 21,000 watt hours / 48 volts = 437.5 AH.

            Where you went off track is using 224 AH cells. You want to use only 1 string of batteries. So you should have been looking for 400 to 450 AH batteries. Would have been half the hardware and about doubled the life of the batteries. Would have been a lot less expensive in the end.
            MSEE, PE

            Comment


            • #7
              hmmm...the mav vdc input for my inverters is 64. So I could still run it in at 48 volts but the batteries are just higher capacity? I'm wondering if there is still time to change the order. Do you have a suggested battery? Thanks

              Comment


              • #8
                Originally posted by kingofbanff View Post
                hmmm...the mav vdc input for my inverters is 64. So I could still run it in at 48 volts but the batteries are just higher capacity? I'm wondering if there is still time to change the order. Do you have a suggested battery? Thanks
                Not sure I follow you. I thought you were running 48 volts?

                A 48 volt inverter should operate from 42 to 64 volts as that is the range of a 48 volt battery.

                There is one other catch and I don't think it should be a problem. To run 48 volts means your total panel voltage needs to be at least 72 volts, and your controller capable of 48 volts. You did not say what kind and how many panels you have, or what kind of controller.

                So I am not sure what you are asking.
                Last edited by Sunking; 05-31-2016, 03:38 PM.
                MSEE, PE

                Comment


                • #9
                  Sorry that was not clear at all. I was referring to your point that 96 volts would be the best configuration but I don't think my Outback Radian can handle that as input. I will have 13kw of SunPower panels. My question was instead of the FullRiver dc224 which battery would you suggest?

                  Comment


                  • #10
                    Originally posted by kingofbanff View Post
                    I will have 13kw of SunPower panels. My question was instead of the FullRiver dc224 which battery would you suggest?
                    OK you have some challenges ahead of you. I assume you will use the Full River batteries.

                    You really only have 1 option, 48 volts. However with 13 Kw of panels is an issue. The largest controller you can buy is 80 amps. With 80 amp output limit also limits how much panel power you can input vs battery voltage. Those limits are:

                    1000 watts @ 12 volt battery
                    2000 watts @ 24 volt battery
                    3000 watts @ 36 volt battery
                    4000 watts @ 48 volt battery

                    You have 13,000 watts, and at 48 volt battery 13,000 watts / 50 volts = 260 amps of charge current. That gives you two big problems.

                    1. It will take more than one charge controller, more like 4-65 Amp controllers. You would have to divide the panels up into 4 equal systems out puting into a common battery. That is doable as there are controllers to operate as Master-Slave. But very expensive. Want a 12 volt system? Then you need 13 very expensive 80 amp controllers. Do you have any problem with that? I would pissing all that money away. .

                    2. Is the Big Problem. Regardless of what voltage you configure the batteries, you have way to much panel wattage for the batteries. Anyway you do it is a charge rate of C/1.7 Lets just say C/2 for simplicity. The most you want to hit those batteries with is C/6. In other words the maximum panel wattage you should run is 4000 watts with 80 amps of charge current @ 48 volts. You would need 3 times the battery capacity to run 13,000 watts of panels. You have only made a 33% down payment so far for batteries. Is that a problem for you? Don't forget replacing all of them in 5 years.

                    Sorry but you have some issues to work out. There are only two battery types you have access to that can handle a C/2 charge current, AGM and Lithium. The bad news about that is it will cost as much as buying 3 times more of the same battery you have. You are screwed either way. Sorry! You other option is to get rid of the extra panels.

                    You need to set back and figure out just how many Kwh in a day you really need. Keep in mind anything you take off-grid is going to cost you many times more than buying power from the power company. Just in battery cost alone with replacement cost is going to run you 60-cents to $1 per Kwh. Not sure where you live, or how much you pay the utility, but where I live we pay 7-cents per Kwh. If I were to go off-grid would cost me 10 times more than I pay.
                    Last edited by Sunking; 05-31-2016, 04:51 PM.
                    MSEE, PE

                    Comment


                    • #11
                      The plan is an Outback Flexpower Radian 8048 acting as master and a 4048 as slave. Between them they have 3 80 Amp charge controllers. Does that solve problem1?

                      In your last sentence you mention AGM can handle a C/2. I think the DC224-6 is AGM...does that mean I'm ok there?

                      As always, thanks!

                      Comment


                      • #12
                        Originally posted by kingofbanff View Post
                        In your last sentence you mention AGM can handle a C/2. I think the DC224-6 is AGM...does that mean I'm ok there?
                        Is it AGM? If it is look in the specs and they should tell you the max charge rate. You may have dodged the bullet.

                        Is this a hybrid system?
                        MSEE, PE

                        Comment


                        • #13
                          Originally posted by kingofbanff View Post
                          In your last sentence you mention AGM can handle a C/2. I think the DC224-6 is AGM...does that mean I'm ok there?
                          The Fullriver DC224-6 can handle C/4. (They refer to it as C4 but that's almost certainly a typo.)

                          Comment


                          • kingofbanff
                            kingofbanff commented
                            Editing a comment
                            C4 would be able to discharge very rapidly!
                            Where did you locate the C/4?

                          • jflorey2
                            jflorey2 commented
                            Editing a comment
                            The data sheet indicates that the initial charging current for cyclic applications is &quot;<=0.2 x C20.&quot; That's C4. As I mentioned that doesn't make sense, but C/4 does.

                        • #14
                          I've been searching high and low for a charging rate for it but can't find one.

                          The system will be grid tied but have the ability to keep operating if the grid is down.

                          Comment


                          • #15
                            Originally posted by kingofbanff View Post
                            The plan is an Outback Flexpower Radian 8048 acting as master and a 4048 as slave. Between them they have 3 80 Amp charge controllers. Does that solve problem1?......
                            The integral controllers in the inverters, are most likely, AC chargers, from your genset. They are not Solar PV MPPT charge controllers.
                            Morningstar and Midnight make very good MPPT controllers, Outback also makes MPPT controllers, but I have no experience with them.

                            You will also need Solar PV combiners, and disconnects and you likely have overbought too many PV panels, and may be able to sell the panels to purchase the charge controllers.

                            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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