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Wiring a battery bank for Solar Panels OFF GRID

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  • #16
    Originally posted by FloridaSun View Post
    6volt?? I thought you had "10 12v 110ah batteries" as your first post says.
    I was trying to get his mind around what type of batteries he should get once his first set of 12 volt ones bites the dust. They have been in use for a while and with his estimated increase of usage they may not last very long.

    Better to get the next set at a voltage and size that works for him then to just replace the old ones with the same type.

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    • #17
      Originally posted by SunEagle View Post
      I was trying to get his mind around what type of batteries he should get once his first set of 12 volt ones bites the dust.
      That will be very soone when he learns he made a huge mistake and has to replace them within a year. Loosing money and failure is a great teacher and motivator to learn.
      MSEE, PE

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      • #18
        Originally posted by SunEagle View Post
        I was trying to get his mind around what type of batteries he should get once his first set of 12 volt ones bites the dust. They have been in use for a while and with his estimated increase of usage they may not last very long.

        Better to get the next set at a voltage and size that works for him then to just replace the old ones with the same type.
        Agreed

        It's also good to keep going over this stuff for the non registered readers on this forum.... of which there are 106 presently compared to the 6 members online now.

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        • #19
          Hi FloridaSun and SunEagle, thanks for your advice this is great.

          It is all still a bit black magic to me, so i have looked on line after your advice on 6 volt batteries, and found 400 ish AH batteries, twice the price of 110AH 12volt batteries, thus giving me twice the AH for the same $/£.

          SunEagle i understand the cell size in volts ie 2 x 6 = 12 thus getting to 48. Bit confused about the AH. How does that relate to series wiring in that does that follow the same formula? ie 2 batteries 800 ish, so would that be in a 48 volt system, 3200 AH?

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          • #20
            Sunking,

            Absolutely, which is why i dont want to do it a second time, and after reading your detailed explanation on inverter size the only fire i want is in the stove!!

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            • #21
              Originally posted by Thomastheo1 View Post
              Hi FloridaSun and SunEagle, thanks for your advice this is great.

              It is all still a bit black magic to me, so i have looked on line after your advice on 6 volt batteries, and found 400 ish AH batteries, twice the price of 110AH 12volt batteries, thus giving me twice the AH for the same $/£.

              SunEagle i understand the cell size in volts ie 2 x 6 = 12 thus getting to 48. Bit confused about the AH. How does that relate to series wiring in that does that follow the same formula? ie 2 batteries 800 ish, so would that be in a 48 volt system, 3200 AH?
              When you wire the batteries in "series" you add up the voltages but the AH rating of the system is the same as one battery.

              So for a 48 volt system using a 8 x 6 volt 400Ah battery will get you a battery system of 48 volts at 400Ah = 19200 watt hour system.

              When you wire the batteries in "parallel" you add up the AH rating but the voltage of the system is the same as one battery.

              So for a 12 volt system using 10 x 12 volt 110Ah battery will get you a battery system of 12 volts at 1100Ah = 13200 watt hour system.

              The big issue is that with that many batteries wired in parallel you will end up with them being charged and discharged unequally which leads to failure.

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              • #22
                Here is the problem, you are paralleling batteries which is bad news. At most no more than 2 parallel strings.

                Design rule # 1. Use only 1 single string of batteries.
                2. If you must parallel batteries, no more than 2 strings in parallel.
                3. There is no reason you have to parallel batteries unless you need more than 4000 AH.

                Here is the deal. Let's say you need 24 volts @ 400 AH. What you are likely looking for is 6 volt 400 AH batteries for a total of 4 batteries if you can handle the weight. If not then look for 4 volt 400 AH batteries and buy 6 of them. The way you are thinking is to buy 12 volt 100 AH batteries which would require 4 parallel strings and 8 batteries.

                The catch is weight. For each Kwh of battery capacity you are looking at about 60 lbs/Kwh. With that in mind all the following battery sizes weigh the same and have the exact same amount of capacity at 2 Kwh weigh in at roughly 120 pounds.

                12 volt 170 AH
                6 volt @ 340 AH
                4 volt @ 500 AH
                2 volt @ 1000 AH

                All those batteries just listed have the exact same amount of energy and weigh roughly the same. So lets say you need a 48 volt 1000 AH battery. What do you do? Well you are looking to make it a single string so you go shopping for 1000 AH batteries and all you can find is 2 volt cells. Why is that? Well a 2 volt 1000 AH cell weighs roughly 120 pounds about what you can lift and handle. So you would need 24 of them to make 48 volts. But why can you not find a 12 volt 1000 AH battery. Real simple it would weigh 750 pounds.

                Now with that said the largest cells you can find is 2 volts @ 4000 AH. That puppy weighs in at around 600 pounds each. In a submarine we have around 220 of them wired in series. Exact number is classified.
                MSEE, PE

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                • #23
                  Hi again thanks for your replies, Sunking and Floridasun. What I am now trying to achieve is a system for the house when it is built !
                  So the system I have, I now know wont be the one I will be using.
                  Although I havent worked out total watts or watt hour, I have have an idea it is going to be larger then I have at the moment ( 13200w )
                  Agreeing that all equipment can be replaced and starting from new, how would you go forward with choice of battery volt/ah and wire system.

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                  • #24
                    Originally posted by SunEagle View Post
                    When you wire the batteries in "series" you add up the voltages but the AH rating of the system is the same as one battery.

                    So for a 48 volt system using a 8 x 6 volt 400Ah battery will get you a battery system of 48 volts at 400Ah = 19200 watt hour system.

                    When you wire the batteries in "parallel" you add up the AH rating but the voltage of the system is the same as one battery.

                    So for a 12 volt system using 10 x 12 volt 110Ah battery will get you a battery system of 12 volts at 1100Ah = 13200 watt hour system.

                    The big issue is that with that many batteries wired in parallel you will end up with them being charged and discharged unequally which leads to failure.
                    Thanks for this Suneagle it helps to understand the calculation.

                    Comment


                    • #25
                      Originally posted by Thomastheo1 View Post
                      What I am now trying to achieve is a system for the house when it is built !
                      Although I havent worked out total watts or watt hour, I have have an idea it is going to be larger then I have at the moment ( 13200w )
                      Agreeing that all equipment can be replaced and starting from new, how would you go forward with choice of battery volt/ah and wire system.
                      That's like asking someone how big a fuel tank one would need to drive from one point to another without telling distances involved, weight to transport... or how fast you need to get there and where the gas stations are.
                      The only thing for certain is a 48 volt system. Too many variables for other definite answers. Battery type (AGM or FLA) would depend on insolation, location. Battery bank size would depend on daily kWh used of course, wiring system size depends on volts, amps drawn and distances involved.

                      Many things could be planned for less electrical use such as passive heating/cooling, super insulation, rainwater catchment to reduce pumping costs, solar water heating, sky lights for reduced daytime lighting, low water consumption, etc.

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