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    Basic questions on new pack build

    So far I have tons of used LG 2600 mAh batteries and 3 Opus BT-C3100 v2.2 battery testers. I think im going with a Tesla style pack using bus bars and fuses, I want to make the pack safe as possible. As for the ebike kit im thinking of a 24 inch 36V 250W front bldc hub motor - electric conversion kit. The kit is for my daughters Sun trike. The kit should pull about 15 amps so im looking to make a 10s 6p pack.

    Now here is my questions. BMS? what size do I look for, 20 or 15 amp? Would it matter if the BMS was a 30 amp?
    Where is the best place to buy a good BMS?
    How close should the mAh be on each cell, within a 100 or 200 mAh?
    Convert your 24inch original bike to e-bike with our 24 inch 36V 250W front bldc hub motor electric bike conversion kit – Leaf Bike

    #2
    I think I found a BMS and I see no reason I cant use a 20 amp unit. Now how close do the cells mAh need to be to each other?
    Cheap board board, Buy Quality board bms directly from China board battery Suppliers: 1 Pcs 10S Protection Board 36V 20A 10S Lithium Li-ion LiFePO4 Battery Battery BMS Protection Board with Balancing breadboard

    Comment


      #3
      Ideally, it would be good to make sure that your cells and BMS are capable of outputting more than the motor will draw with a good margin for safety Over time, the output of the cells may decline, but your motor will still be drawing the same current.

      If it were me, I'd probably over-build the pack, because the only real downside is a bit of cost and/or weight. The upside is better performance and less risk of overtaxing any part of the system (which could lead to catastrophic failure at some point).

      Comment


        #4
        Originally posted by thebootfitter View Post
        Ideally, it would be good to make sure that your cells and BMS are capable of outputting more than the motor will draw with a good margin for safety Over time, the output of the cells may decline, but your motor will still be drawing the same current.

        If it were me, I'd probably over-build the pack, because the only real downside is a bit of cost and/or weight. The upside is better performance and less risk of overtaxing any part of the system (which could lead to catastrophic failure at some point).
        Thank you, but im a bit confused on the amp draw of a cell. Does each cell need to put out the 20 amps or is that a combined total of all the cells in that parallel row?

        PS I did go with a larger BMS

        Comment


          #5
          Originally posted by rhconcepts View Post

          Thank you, but im a bit confused on the amp draw of a cell. Does each cell need to put out the 20 amps or is that a combined total of all the cells in that parallel row?

          PS I did go with a larger BMS
          Let's use a simple battery pack as an example, a 20s9p pack. This is 20 cells in series, 9 times in parallel.

          Let's look at one row, which is a 20s1p row: this row is 20 cells in series. The nominal voltage for 20 cells in series is almost always 72V.

          Let's say each of these cells is a 2000 mAh (or 2 Ah) cell. One row of 20 cells in series can put out 2 A (at 72V) for 1 hour, or 4 A for half an hour, or 1 A for 2 hours. Whatever product of current (in A) and time (in hours) yields a value of 2 (in Amp hours).

          Following this example, 9 rows can put out 9 times 2 A (at 72V) for one hour, or 18 A (at 72V) for 1 hour or 36 A for half an hour, or 9 A for 2 hours. Whatever product of current (in A) and time (in hours) yields a value of 18 Ah.
          Last edited by commuter ebikes; 04-24-2018, 08:08 PM.

          Comment


            #6
            Commuter ebikes offers a good summary for calculating the total amp hours of a pack. If I understand your question correctly, though, it seems that you're more interested in the amperage draw. Not sure what the correct terms are... Hopefully someone more knowledgeable than me can clarify the terms and confirm that I am not misleading you in any way.

            Each cell will typically be sold with a value or rating for max amp output. That is the peak amperage any individual cell can output at any given moment in time. When running near the peak amperage, cells tend to heat up and won't last as many recharge cycles. Thus, ideally, you want to plan your system so that each cell can operate most of the time well under the peak amperage. Then, when the cells degrade over time, you are still operating within the limits of the cells while still getting full power out of your system.

            Let's say you are using cells with a peak output of 10A. And your pack uses parallel groups of four cells. (Commuter ebike's example above shows the math for determining total amp hours of the pack, which might be, say, 8 Ah if you used that example of each cell having 2000 mAh of capacity.) However, we want to determine what is the peak amp output while running your system. That would be the peak output of each cell x the number of cells in parallel groups, so in this case, you'd have a peak output of 40A. If you have a BMS that can handle 40A (or even more), it will likely be a little more robust and should be better able to handle 20A than a 20A BMS.

            Now if your controller can only pull a max of 20A, and your battery pack and BMS can output 40A when the batteries are new and healthy, you should never even come close to the max amps per cell, thus keeping your pack healthier overall and hopefully able to last longer. That's what I was suggesting you consider in my earlier post.

            Comment


              #7
              There is also the "C rating" for the cells that you are using. Once, I bought a 20Ah battery (with a 100A BMS). The manufacturer claimed that the C ratings for the cells in the pack were 3 for continuous and 5 for burst currents. As such, the manufacturer claimed that I could run 3 times 20A for 60A continuous and 5 times 20A for 100A burst currents.

              A more realistic C value for an 18650 cell (depending on the cell) may be 2 for continuous and 4 for burst. You find the C rating of the cell by referring to the manufacturer's (or dealer's) documentation. Hopefully the dealer does not exaggerate.

              Of course, it is not so cut and dry. A burst may be loosely defined as 1-2 seconds, but who is timing their bursts? Also, when the pack is fully charged, the battery can provide both more continuous and burst currents. In the example in the first paragraph of this post, the battery could provide 60A continuous when fully charged, but not anything close to that when the battery is nearly discharged.

              As for the battery referred to above, I still have this battery three years later. It can provide 30A continuous until the battery is 55% discharged. It can provide 70A bursts when fully charged, but this declines sharply as the battery discharges; when it is 55% discharged, any burst over 30A will trip the circuit breaker. Once this battery is 60% discharged, it is of no use to me until I charge it because I won't ride the bike unless I can draw 30A continuous.
              Last edited by commuter ebikes; 04-25-2018, 08:59 PM.

              Comment


                #8
                Originally posted by thebootfitter View Post
                Commuter ebikes offers a good summary for calculating the total amp hours of a pack. If I understand your question correctly, though, it seems that you're more interested in the amperage draw. Not sure what the correct terms are... Hopefully someone more knowledgeable than me can clarify the terms and confirm that I am not misleading you in any way.

                Each cell will typically be sold with a value or rating for max amp output. That is the peak amperage any individual cell can output at any given moment in time. When running near the peak amperage, cells tend to heat up and won't last as many recharge cycles. Thus, ideally, you want to plan your system so that each cell can operate most of the time well under the peak amperage. Then, when the cells degrade over time, you are still operating within the limits of the cells while still getting full power out of your system.

                Let's say you are using cells with a peak output of 10A. And your pack uses parallel groups of four cells. (Commuter ebike's example above shows the math for determining total amp hours of the pack, which might be, say, 8 Ah if you used that example of each cell having 2000 mAh of capacity.) However, we want to determine what is the peak amp output while running your system. That would be the peak output of each cell x the number of cells in parallel groups, so in this case, you'd have a peak output of 40A. If you have a BMS that can handle 40A (or even more), it will likely be a little more robust and should be better able to handle 20A than a 20A BMS.

                Now if your controller can only pull a max of 20A, and your battery pack and BMS can output 40A when the batteries are new and healthy, you should never even come close to the max amps per cell, thus keeping your pack healthier overall and hopefully able to last longer. That's what I was suggesting you consider in my earlier post.
                Thanks guys, thebootfitter that's what I was looking for. I seen videos of people using cells that put out 4-5 amps, but a friend told me the amp draw of the cells does not increase when more cells are added.

                And thanks commuter ebikes for the info, If I remember right my cells showed c2 when I looked them up.

                I know this was a basic question but my friend through me off. And I could not find any info on if each cell needed to put out what the motor required or could that be taken from a group of cells in parallel.

                Comment


                  #9
                  Does it matter if the cells are charged while building the pack? I would hate to discharge all the cells but I can do it.

                  Comment


                    #10
                    I hope someone chimes in by tomorrow. I have 80 cells ready for spot welding and they are all charged. If they need to be discharged that will add a few days onto the build :(

                    Comment


                    • thebootfitter
                      thebootfitter commented
                      Editing a comment
                      Do you have a copy of Micah Toll's DIY Lithium Battery Pack book? If not... pick up a kindle copy for $10. Totally worth it. He's not necessarily the final authority on all things about lithium battery packs, but I think he offers pretty good reliable information. If you have a question, chances are the answer is in there.

                    #11
                    Originally posted by rhconcepts View Post
                    I hope someone chimes in by tomorrow. I have 80 cells ready for spot welding and they are all charged. If they need to be discharged that will add a few days onto the build :(
                    Thanks you. I learned from his videos and my battery is finished :) I just got back from a test ride and it was fun.

                    Comment

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