Refurbishing two 15S 4P batteries. Disassembly recommended? Rewire to 16S or 17S?

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  • _eunice_
    Junior Member
    • Mar 2024
    • 3
    #1

    Refurbishing two 15S 4P batteries. Disassembly recommended? Rewire to 16S or 17S?

    Hello all,

    first of all, a wonderful day to everyone who is reading this. I am sure this is a newbie question, as I am new to this hobby, but I am stuck and I could use some guidance from this community.

    I purchased two used, broken battery banks (48V 15S 4P 200Ah LiFePo4 each) and an inverter (10kW 48V) to start my hurray into getting independent from the grid. I live in Costa Rica in the jungle with lots of sun and lots of power outages The BMS of the packs are completely dead, so I ordered a Daly BMS and later learned that JK BMS are supposed to be better and I also ordered a 24S 200Amps JK BMS. The batteries are connected with what looks like laser welded aluminum strips, so disassembly would probably be done with a dremel and for reassembly I would have to buy a spot welder or something.

    Now to the questions I did not find an answer to:

    1) Do I need to disassemble the battery banks and capacity test each battery or can I keep the 4P groups and treat them as one battery? If disassembly is required, what is the fastest way to do this with 120 LiFePo4 cells with 50Ah each?

    2) Currently the cells are wired as two battery banks with 15S 4P. Is it worth it to rewire everything to 16S or even 17S? If I only charge the batteries to 3.45V my inverter should handle a 17S system, or is this naive and not recommended?

    3) Can I run the two banks as 8P with one JK BMS or should I continue running it as two separate 4P systems and use the Daly or buy another JK? Is there a recommended limit on how many cells you should wire in parallel?

    Thank you so much for any answers even if it's hints on literature, blogs, youtube channels on how I can teach myself to know what to do.

    Eunice
    Tags: None
    • scrambler
      Solar Fanatic
      • Mar 2019
      • 500
      #2
      Not an expert per say but I can chime in on a couple of things.

      1- You can start by testing the 4P master cell as one unit, see if they behave normally. If there is an indication something is off on a 4P unit, then you you have to disconnect the cells to test individually.
      test you can make:
      • Check temperature of cells as you charge the 4P pack
      • Test Internal resistance of the 4P pack and compare with theoretical values.
      • Test the capacity of the pack (full charge and discharge) using the Cells know voltage curves and see if it is consistent with the nominal
      2- Rewiring the pack all depends on the voltage you need for your inverter versus the maximum number of cells (capacity of the whole pack) that could be used in the chosen configuration.
      If you have a 48V inverter you should configure the pack for that voltage, and given your configuration, it seems that will allow you to use all the cells.

      3- I don't know of any reason against or for wiring the pack as 8P rather than two 4P put in parallel, other than checking what the BMS that would be used recommend. I would say if the BMS have no preference, whichever is easiest to implement.

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        • _eunice_
          Junior Member
          • Mar 2024
          • 3
          #3
          scrambler Thank you so much for your answer, it is of great help to me.

          Sadly I have next to no documentation about the cells, I only know that it's 15S4P 48V with 200Ah. But I think can measure internal resistance of all 4P packs and compare them against each other, I am sure I will see any defective groups.

          How would you suggest to test the capacity of a 4P pack?

          I bought one of those tenergy 5-in-1 capacity meters, but they can only discharge at 180mA, a 4P pack would take around 100 hours until it's discharged.

          No I am thinking about connecting a known resistor and using an oscilloscope to plot the voltage curve across the resistor. This seems like a very complicated way to do it as I would have to connect the oscilloscope to a computer and collect the data over time and then actually calculate the capacity from the curve. Is there an easier way to do it?

            Comment

            • scrambler
              Solar Fanatic
              • Mar 2019
              • 500
              #4
              There are charge / discharge devices that allow you to discharge at the high safe rate and that will tell you the Amp hours going in and going out. Although I dont know how high they go in discharge amps.
              Once you know the discharge curve of a specific cell, you can fully charge the master cell (4P), then discharge it to its lowest safe voltage, and see if the amp hour of the discharge are close to the number extrapolated from the single cell discharge curve.

              If you unwrap the pack, you should be able to see the Cell model number on them and then find its discharge curve.
              If you cant, you can use the discharge curve of a similar cell (one of the same chemistry and approximately the same Ah)

                Comment

                • scrambler
                  Solar Fanatic
                  • Mar 2019
                  • 500
                  #5
                  The device I am thinking of is a discharge monitoring device / circuit. You connect the battery to a load with the monitoring device in the circuit, and it will monitor amps and voltage and calculate the Amp hour.
                  You just need to connect a load to the battery that is strong enough so it discharges in a couple of hours. A single 4P master cell would be 3.2V x 200 Ah = 640 Wh
                  You do need to know what is the peak Amp discharge acceptable for the cell. That information is also key for your installation, as you will need to know what is the max Amp charge and discharge that will be acceptable for your pack.
                  Amp hour are only half of the story....

                    Comment

                    • _eunice_
                      Junior Member
                      • Mar 2024
                      • 3
                      #6
                      Thank you so much. I will try to find a suitable discharge monitoring device. WIth my oscilloscope I might be able to discharge and calculate the capacity for four batteries at once but I will have to babysit the entire process to disconnect the load once the voltage goes too low.

                      This is a photo of one of the two batteries: https://imgur.com/a/6sdqLcs , sorry for the external link but I failed to upload the image here.

                      Regarding the safe discharge Amps: I did buy the inverter that was set up for these batteries from the factory, it had the max discharge current set to 200A with two battery packs wired in parallel. Since longevity is more important to me than power I will probably go safe and set max. charge and discharge currents to 0.5C.
                      Last edited by _eunice_; 03-12-2024, 06:40 PM.

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