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  • Battery bank design under pressure of regular fridge

    Hi, I am thinking on an option in battery bank design which will be able to serve running afull size fridge, inverter will serve as UPS, rather then solar energy collection. In order to overcome the starting load of compressor of fridge, some people utilize a capacitor between b.bank and inverter (I remember someoneone, he was using 1 farad, 12v). I am planning to create a bank, which will be consisting 12v agm batteries, 100 ah each, probably 2S4P config.

    Here comes the idea. For providing support similar to that of capacitor (which I do not know a thİng about), what if I add, say, 150ah flooded (starter) battery in 2s config, to the frontline? I know Its not possible to charge such a battery bank due to different chemistries, But the plan is to add these two, to bank in a mobile manner, if fridge is needed to run, and prevent inverter to charge batteries, during the operation.
    Of course, I am open to advises which will ease the load from compressor of fridge.

    Regards.

  • #2
    Smart money is to use two 6 volt AGM batteries and cal it done. Assuming you use an appropriate sized AGM battery, thay can handle large starting currents.

    Also you are stuck inside a 12 volt box, get out of it. Large loads demand 24 and 48 volt battery systems. Last thing you want is parallel batteries. If you need 400 AH, then use 400 AH batteries. They will be either 4 or 6 volt batteries.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Smart money is to use two 6 volt AGM batteries and cal it done. Assuming you use an appropriate sized AGM battery, thay can handle large starting currents.

      Also you are stuck inside a 12 volt box, get out of it. Large loads demand 24 and 48 volt battery systems. Last thing you want is parallel batteries. If you need 400 AH, then use 400 AH batteries. They will be either 4 or 6 volt batteries.
      Thanks Sunking, as I said, I was planning to use 12V 100ah ones as 2S4P, so 24V 400ah, since my inverter choice is already 24V, (cannot go for 48, too expensive for the purpose).

      Then can you please say which one will be the wise move, for my once or twice a month, short period (1-2 hours max) usage (batteries and inverter will stay inside home);

      -Use two of "as high AH as possible" 12V AGM batteries. And dont go beyond that by parallel.
      or,
      -Use four of high ah AGM 6V batteries, again, stay there.

      This was actually a question in my mind, too. Could not decide whether 6V packs are good idea. Is there any special advantage to them?

      Comment


      • #4
        Originally posted by Ontheway View Post
        Thanks Sunking, as I said, I was planning to use 12V 100ah ones as 2S4P, so 24V 400ah, since my inverter choice is already 24V, (cannot go for 48, too expensive for the purpose).
        You are stuck inside a 12 Volt Box, get out of it. Does not matter what voltage you are using.

        You never want to use parallel batteries unless absolutely necessary. Unless you are needing more than 4000 AH it is never necessary to parallel batteries. There is nothing wrong with using 12 volt batteries if they fit the application even if using a 48 volt system. If all you need is say 100 AH in either 12, 24, or 48 volts then you can use 12 volt 100 AH batteries.

        But if you want 400 AH you are not going to find any 12 volt 400 AH batteries. Even if you could you would not want to use it because it would take a lift to move it around weighing in at 315 pounds. But very easy to find 400 AH batteries in 4, 6, and 8 volt batteries. A 4 volt 400 AH battery weighs around 105 pounds, 6 volt around 155 pounds, and 8 volt around 210 pounds. In 2 volt or single cell battery start at 800 AH and go up as high as 4000 to 6000 AH. There is a direct correlation of voltage, amp hours, and weight. A Pb battery weighs about .065 pounds per 1 wh of capacity. Battery Watt Hour Capacity = Battery Voltage x Amp Hour. So a 12 volt 400 AH battery would have to weigh in around [12 volts x 400 AH] x .065 pounds = 312 pounds.

        If you are 24 volt system your battery voltage that can be used are 24, 12, 8, 6, 4, and 2 volts. You want 400 AH batteries buy 400 AH batteries. You wil not find 24 or 12 volt batteries in 400 AH.

        So if you want 400 AH, buy 400 AH batteries, but they will not be in a 12 volt box you have trapped yourself in. You gotta think outside the box.
        MSEE, PE

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        • #5
          I got the idea, thank you for summarizing the situation with batteries.

          Now, in order to keep costs down a bit I can start with four of 6v 100s or 150s. Then some time later I can create another set build with 260s and manually change from one set to another, depending of the power need or duration of power failure.
          A quick question, is there any equipment to switch from one bank to another, either manually or automatically ? Too weird to ask?

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          • #6
            Originally posted by Ontheway View Post
            A quick question, is there any equipment to switch from one bank to another, either manually or automatically ? Too weird to ask?
            Switch?

            You don't need any switch for Dual Conversion UPS. Rectifiers > Battery > Inverter >Fridge.

            The system should be designed backwards. First you determine how many watts the fridge uses, say 1000 watts. That immediately tells you the minimum size battery required. You can discharge a AGM battery at C/4 rate. So if you are running 24 volt battery with a 85% efficient Inverter the fridge is going to draw [1000 watts / 24 volts] / .85 = 49 amps. Round that up to 50 Amps.

            So than makes the minimum battery size 200 AH at C/4 discharge rate. That will give you about 2.5 hours compressor run time to 80% DOD. Depending on the on/off cycle of the fridge of say 25% about 10 hours.

            The rectifiers have to be sized large enough to run the fridge, plus recharge the batteries is some amount of time. For AGM you can charge fairly slowly but I would not go below a 12 hour charge of C/12. C/12 on a 200 AH battery is 200 AH / 12 H = 16.667 amps. So the minimum size rectifier required is 50 amps + 16.667 amps = 67 amps. So mow you get to go shopping for a 24 volt 75 to 100 amp rectifier. Good luck finding one that uses 120 volts.

            So no wget your real numbers and crunch them down. First number you need is how many watts the fridge is pulling. If the fridge pulls say 750 watts with defrost cycle, don't forget defrost cycle because that uses the most power running the electric heater inside, you are going to need a 1500 watt Inverter.

            Up to you to do your own work now. .
            MSEE, PE

            Comment


            • #7
              Originally posted by Sunking View Post
              Switch?

              You don't need any switch for Dual Conversion UPS. Rectifiers > Battery > Inverter >Fridge.

              The system should be designed backwards. First you determine how many watts the fridge uses, say 1000 watts. That immediately tells you the minimum size battery required. You can discharge a AGM battery at C/4 rate. So if you are running 24 volt battery with a 85% efficient Inverter the fridge is going to draw [1000 watts / 24 volts] / .85 = 49 amps. Round that up to 50 Amps.

              So than makes the minimum battery size 200 AH at C/4 discharge rate. That will give you about 2.5 hours compressor run time to 80% DOD. Depending on the on/off cycle of the fridge of say 25% about 10 hours.

              The rectifiers have to be sized large enough to run the fridge, plus recharge the batteries is some amount of time. For AGM you can charge fairly slowly but I would not go below a 12 hour charge of C/12. C/12 on a 200 AH battery is 200 AH / 12 H = 16.667 amps. So the minimum size rectifier required is 50 amps + 16.667 amps = 67 amps. So mow you get to go shopping for a 24 volt 75 to 100 amp rectifier. Good luck finding one that uses 120 volts.

              So no wget your real numbers and crunch them down. First number you need is how many watts the fridge is pulling. If the fridge pulls say 750 watts with defrost cycle, don't forget defrost cycle because that uses the most power running the electric heater inside, you are going to need a 1500 watt Inverter.

              Up to you to do your own work now. .
              ı really dont like causing people spend effort due to my ignorance on a subject, but in this case I am clueless, please forgive me for asking so many questions..

              By "switch", I meant switching from one battery bank (ie, the active bank connected to inverter and discharged to shutoff level) to another bank sitting next to it, freshly charged and ready for action. I want to physically disconnect first battery bank and connect second one, either by turning inverter off, disc., then connect new one, or with a gadget that will do this job for me, if such a thing ever exists.

              In order to provide some more info on off-grid inverter/solar charger system I am planning to buy, here are some links;

              http://www.effekta.com.de/AX-Series

              http://www.effekta.com.de/WebRoot/St...l_20140623.pdf

              I will buy 3000VA version of them (2400 w, 4800 peak), the version uses pwm charger. I will probably never use solar charging feature, unit will remain connected to mains, for charging the battery bank. Finally, I am living in a 220V/50hz region of the world.

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              • #8
                Ok I found something that makes what I am asking, switching between two battery banks.

                http://www.newmartelecom.com/Automat...-Selector.html

                Comment


                • #9
                  Originally posted by Ontheway View Post
                  Ok I found something that makes what I am asking, switching between two battery banks.

                  http://www.newmartelecom.com/Automat...-Selector.html
                  it's much more elegant to use the proper size batteries, then to muck about using too small of batteries and trying to remember to switch them back and forth for recharging. And less expensive, only half the interconnect cables needed.
                  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


                  • #10
                    Originally posted by Mike90250 View Post
                    it's much more elegant to use the proper size batteries, then to muck about using too small of batteries and trying to remember to switch them back and forth for recharging. And less expensive, only half the interconnect cables needed.
                    Thanks Mike, Since initial costs are holding me, I actually started thinking on starting with a small set of batteries to gain experience on stuff, then in the near future, replace them with a bigger set, and use the former ones with another properly sized inverter, in somewhere else.

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