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Using 2 strings of FLA batteries in parallel versus 1 string 48V 710 Ah bank

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  • #16
    Originally posted by Mike90250 View Post
    Imagine that their example of a 12V cell, is your 48V string. (4, 12V in series or 8, 6v in series)
    Then tie each end of the strings - to -, + to +. Then connect your power connections on the diagonal so the + goes to 1 terminal, and a jumper to the other +, the - lead goes to the - terminal for the + Jumper.
    OK, I see. So when people here say remember to connect on the diagonal, this is what they mean. Thanks.

    I am leaning towards a single string, but I wanted to clarify what you meant.

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    • #17
      Originally posted by Living Large View Post
      I am posting this as its own thread, though it came up in another I posted here. It is a stand-alone topic.

      I need about a 710 Ah 48V battery bank. I located a 375 Ah 6V battery, and was planning to create a 750 Ah bank with 2 strings of 8 batteries in parallel. Sixteen for $5600. I had seen advice to never use more than 3 strings in parallel.


      This leads to a question. Are the disadvantages of two parallel strings so great as to go to a single string? Should I keep looking for 2V cells that are more economical? A gut feeling, not based on fact, is that unless I could get in for a 50% premium (~$8000), it wouldn't be worth it. On the other hand, these 2V cells may last twice as long. Maybe I should be trying to find 24 2V cells for $11,000?

      Perhaps someone could address the issues I might face at some point with two strings instead of one.

      .
      I am not a professional, I still learning about the battery in regarding charge and discharge and so on, but let me tell one thing that I have encounter lately.

      During the regular charge and discharge, even through one might work a bit harder than the other but that does not have a major problem (to me) as the stronger string goes down the other weak one will have to come up to help. unless you just doing the shallow charge and discharge less that 10%.

      How ever there is a big problem I notice last week when I try to charge the banks with the generator and the grid afterward, The rain was falling for two days, I charge the battery bank with the generator to absorption and turn off the generator and start charge the battery with the grid, because my battery always run less than full SOC, I decide to try absorb a bit longer and to equalize the battery bank.

      I have a thermal meter connect to the battery bank, how ever it is on the larger bank which I do equalize more often so I can monitor temperature. When I check the SG on the batteries I notice the other bank was boiling. I shift the battery thermal meter the smaller bank, to my horror the temperature on that bank went to 120 degree F. I have to turn off that string immediately.

      What happen is that the weaker string get equalized before the other one and the internal resistance got lower than other one so it pull more current to it and generate a lot of heat. lucky thing I have two different disconnect on the two strings so I can turn off one if I wish, That is what save the battery bank from being toasted.

      I hope this will help you.

      Comment


      • #18
        Originally posted by paulcheung View Post
        I am not a professional, I still learning about the battery in regarding charge and discharge and so on, but let me tell one thing that I have encounter lately.

        During the regular charge and discharge, even through one might work a bit harder than the other but that does not have a major problem (to me) as the stronger string goes down the other weak one will have to come up to help. unless you just doing the shallow charge and discharge less that 10%.

        How ever there is a big problem I notice last week when I try to charge the banks with the generator and the grid afterward, The rain was falling for two days, I charge the battery bank with the generator to absorption and turn off the generator and start charge the battery with the grid, because my battery always run less than full SOC, I decide to try absorb a bit longer and to equalize the battery bank.

        I have a thermal meter connect to the battery bank, how ever it is on the larger bank which I do equalize more often so I can monitor temperature. When I check the SG on the batteries I notice the other bank was boiling. I shift the battery thermal meter the smaller bank, to my horror the temperature on that bank went to 120 degree F. I have to turn off that string immediately.

        What happen is that the weaker string get equalized before the other one and the internal resistance got lower than other one so it pull more current to it and generate a lot of heat. lucky thing I have two different disconnect on the two strings so I can turn off one if I wish, That is what save the battery bank from being toasted.

        I hope this will help you.
        Yes, this does help. Thank you. For what it is worth, I have decided to go with a single string if I actually go through with this insanity. But I am interested in learning more and people with experience is a great way to do so.

        This seems like a good example of how two strings that have different characteristics and/or states can present issues beyond the routine maintenance concerns.

        You refer to a smaller bank and a larger bank, and it sounds like they are in parallel. Is that correct? Are they of different Ah capacities?

        Comment


        • #19
          Originally posted by Living Large View Post
          Yes, this does help. Thank you. For what it is worth, I have decided to go with a single string if I actually go through with this insanity. But I am interested in learning more and people with experience is a great way to do so.

          This seems like a good example of how two strings that have different characteristics and/or states can present issues beyond the routine maintenance concerns.

          You refer to a smaller bank and a larger bank, and it sounds like they are in parallel. Is that correct? Are they of different Ah capacities?
          Yes one bank is Rolls S-460 350AH and the other one is Rolls S-530 which is 400AH. I use two disconnect to the inverter so I can monitor the current each string push or pull, normally they work together, the problem arise when the high voltage reach them when I do equalizing charge.

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