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Lithium-ion multiple batteries on parallel - same challenge as Lead Acid or not?

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  • #16
    Originally posted by Sunking View Post
    Wrong, dead wrong.
    I'm doing it right here, right now on my ESS bank. Your statements suggest that you haven't run a large bank with multiple paralleled, managed strings before.

    A BMS is the root cause of Lithium battery failures.
    It's far from the only root cause. A cell can have a manufacturing defect. A cell can be subject to too much heat or cold. A cell can be overcharged with no BMS present. And plenty of other things can cause a failure.

    No two strings charge or discharge at the same rate. You are on the wrong track semi-right idea. A 100 AH Cell will have roughly half the Resistance of a 50 AH cell. No 2-cells of the same capacity have equal resistance.
    I'm not sure what you're trying to say, but let me be very clear: if you place two cells of unequal capacity but identical chemistry in parallel and hold them at an equal charging voltage (which is what "parallel" means), they will each charge in proportion to their capacity and up to the same SOC and the same voltage.

    It is this principal that allows strings of series cells to be placed then in parallel and charged with one charger under one charge algorithm, safely and with no issues. As long as the strings have the same chemistry and thus voltage curve, their individual capacities are irrelevant for parallel charging.

    Unequal charging is not a problem for any battery type except lithium.
    I'm not sure what this means.

    Top Balance is a killer of lithium cells and the root cause of failures and fires. All courtesy of the BMS. That is why EV manufactures DO NOT TOP BALANCE or ever allow the customer to ever fully charge a lithium battery. They would be sued into bankruptcy if they did on warranty claims and loss of property and life.
    Balancing within a series string is a separate concern and is independent of whether charging several strings in parallel is possible (yes), possible with one charger (yes; contrary to what you wrote), or a foolish decision (no, in some cases; also contrary to what you wrote).

    However, for your own edification and perhaps as material for discussion in another thread, I encourage you to go look at what Tesla does with their balancing algorithm. It happens at the top of the charge envelope, not in the middle or at the bottom.

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    • #17
      Originally posted by nebster View Post
      I'm doing it right here, right now on my ESS bank. Your statements suggest that you haven't run a large bank with multiple paralleled, managed strings before.
      In just one tiny small job, I use more batteries than you will in a lifetime. I build very large scale projects. Ever design and built a 500,000 watt system before or use up to 48 volt 60,000 amp hour battery plants before? I know the answer is NO. I have over 100 times. I have also built 10 custom EV and a half dozen racing golf carts all using Lithium batteries. I do this for a living for 40 years.

      Originally posted by nebster View Post
      I'm not sure what this means.
      Yes I know, it is over your head and pay grade.
      MSEE, PE

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      • #18
        Originally posted by Sunking View Post
        In just one tiny small job, I use more batteries than you will in a lifetime. I build very large scale projects. Ever design and built a 500,000 watt system before or use up to 48 volt 60,000 amp hour battery plants before? I know the answer is NO. I have over 100 times. I have also built 10 custom EV and a half dozen racing golf carts all using Lithium batteries. I do this for a living for 40 years.
        Precisely none of which involve a lithium pack with multiple parallel, managed strings.

        Are you done ranting now?


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        • #19
          Originally posted by SunEagle View Post

          I agree if the BMS does it's thing. But even then I have seen LiPo's get very hot and ignite while they are being charged. That is why I use a thermal bag for my RC batteries. You just never can tell when one cell decides to commit suicide.
          Is this an RC forum? I don't think anybody in their right mind would put LiPos in a vehicle or even for stationary storage. Most of the horror stories about Lithium batteries are LiPo stories. The few Tesla fires have all been because of severe trauma to the pack. There may be a case of spontaneous combustion of a Tesla pack but I have not heard one.
          Last edited by Ampster; 11-20-2018, 02:00 AM.

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          • #20
            Originally posted by Ampster View Post

            Is this an RC forum? I don't think anybody in their right mind would put LiPos in a vehicle or even for stationary storage. Most of the horror stories about Lithium batteries are LiPo stories. The few Tesla fires have all been because of severe trauma to the pack. There may be a case of spontaneous combustion of a Tesla pack but I have not heard one.
            The LiPo example was to show that even with a high grade BMS charger those batteries can over heat and go off on you. There are other types of Li batteries that are being used very successfully in EV's and Storage systems but they also have expensive and very smart BMS's to keep them happy and cool.

            IMO just about any Lithium chemistry battery has the potential to over heat and ignite. Unless you have the knowledge and equipment anyone using a Li battery in a DIY plan is playing with fire. Just be careful and never get complacent.

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            • #21
              Originally posted by nebster View Post
              No, you can't have a "BMS per cell" in your scenario. Once three cells are paralleled, they are electrically one. You can put a monitor on each 3p.
              Agree It would be a BMS per 3cells

              Originally posted by nebster View Post

              First of all, at issue is what degree of protection you want, and you could, of course, choose to have less protection. It would work fine until it maybe didn't, or until it turned into a blowtorch. Blowtorches are more likely with some lithium chemistries and almost impossible to achieve with others.

              The Tesla modules are 74p, and then 6s. Then 16 of those modules are placed in series to make 74p6s16s. Tesla uses a monitor on each of those 96 74p blocks. If you just buy a Tesla module, you would want to implement a similar monitor on your 6 blocks.
              ......
              Does that help? .........
              Yes it does. Struggle a bit with the nomenclature though - is my modules 7s3p or 3p7s or is it the same? When connecting 10 of these in parallel would that be 10p7s3p?

              The A123 cells in the modules is apparently of a safer design that that of a Tesla module.

              Originally posted by Sunking View Post
              You are correct that a BMS is at the cell level, but not understanding how to do it. First thing is you cannot define a BMS or what it is. It can be a voltmeter or a Vampire Bleeder Board. A BMS needs to be integrated into the charger.

              OK say we have 4S2P as pictured above. Conventional method would require 8 Cell Boards connected with 4-cells per charger. Each string will not charge at the same rate. One would be charged before the other thus requiring a second charger or control which is expensive and a poor solution that does not work well. The Alternate method is what Lithium batteries use paralleled at the cell level, not string level. Regardless if it is 4S2P as shown, or 4S99P only requires 4 cell boards and one charger. Otherwise you would need 396 cell boards and 99 chargers if you used Conventional. Good luck with that DYI.
              Sunking - your statment of individual chargers is difficult to understand, individual cell monitoring yes, but chargers?

              My assumption is that monitoring all of the 70 cells in a 10p7s3p set of cells with a proper BMS solution that does control both charging and load and that is capable of disconnecting both is a safe solution. The Batrium solution I am considering does have integration with both my Victron Multiplus Inverter and my Victron MPPT and it will trigger a disconnect if cell temp or voltage is out of range.

              Screenshot 2018-11-20 at 22.06.41.png






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              Last edited by Roil; 11-20-2018, 05:15 PM.
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              • #22
                Originally posted by Roil View Post
                Yes it does. Struggle a bit with the nomenclature though - is my modules 7s3p or 3p7s or is it the same? When connecting 10 of these in parallel would that be 10p7s3p?
                There's unfortunately no standard ordering, at least on the web. I choose left-to-right, smallest-to-largest ordering, because in real life everything is built from small things up to big things. In any event, 7s3p and 3p7s are most certainly not the same if used in the same sentence!

                For your proposal, I would call your pack "3p7s10p". If you like to describe the topology from big to small, your way works, too.

                The A123 cells in the modules is apparently of a safer design that that of a Tesla module.
                They are LiFePO4, which is a relatively safe lithium chemistry compared to some of the blowtorches.

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                • #23
                  Originally posted by Ampster View Post
                  Most of the horror stories about Lithium batteries are LiPo stories.
                  Wrong again. The bad boys of lithium batteries are Lithium Cobalt, the exact ones Telsa uses, laptops and cell phones. RC planes do not make the news.. Yes LiPo's are dangerous, but RC pilots are fully aware of the dangers and take necessary measures and know what they are dealing with. Consumers are clueless of the dangers.

                  MSEE, PE

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                  • #24
                    Originally posted by Sunking View Post
                    .............. That is why EV manufactures DO NOT TOP BALANCE or ever allow the customer to ever fully charge a lithium battery. They would be sued into bankruptcy if they did on warranty claims and loss of property and life.
                    /
                    When you say top balance do you mean when the BMS shunts start working or are you talking about putting all cells in parallel and charging that group to something close to max voltage?

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                    • #25
                      Originally posted by Ampster View Post
                      When you say top balance do you mean when the BMS shunts start working or are you talking about putting all cells in parallel and charging that group to something close to max voltage?
                      It is the same thing. A consumer grade BMS cannot balance an unbalaced battery pack. It can take weeks or months to do that if you tried and would destroy the weaker cells.

                      MSEE, PE

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                      • #26
                        Originally posted by Sunking View Post
                        ..... A consumer grade BMS cannot balance an unbalaced battery pack. ....
                        My question was related to an EV not a consumer BMS. I have always thought the BMSs on my EVs ran the balancing currents near the top of the charge curve. Therefore I assumed they were top balancing. Is that not the case?

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                        • #27
                          Originally posted by Ampster View Post

                          My question was related to an EV not a consumer BMS. I have always thought the BMSs on my EVs ran the balancing currents near the top of the charge curve. Therefore I assumed they were top balancing. Is that not the case?
                          No all commercial EV's Middle Balance. No EV manufacture would allow the consumer to ever fully charge the battery. If they did, they would go bankrupt with warranty claims. Depending on the EV auto manufacture, they only allow the battery to be charged up to 80 to 90% capacity, and only discharge to 10 to 20% capacity.

                          EV manufactures do not use Balance Vampire Boards. Only consumer grade BMS use Balance boards. Near the TOP is Not TOP BALANCE, it is Middle Balance. For me on every EV, Golf Cart, and Solar System, I DO NOT USE ANY BMS. Custom EV Conversion companies learned a long time ago the BMS is the number 1 killer of Lithium batteries. We mimic what commercial EV manufactures do by using Bottom Balance, and only charging to 90 to 95% capacity, and cut-off at 10%. This doubles to quadruples cycle life and eliminates the possibility of over discharging a battery. If you Top Balance sets you up for the perfect storm of over discharged cells. Super easy to do and requires no add-on equipment like an expensive BMS or special charger.

                          MSEE, PE

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                          • #28
                            Originally posted by Sunking View Post
                            .......No all commercial EV's Middle Balance. ......
                            ..... Near the TOP is Not TOP BALANCE, it is Middle Balance. ........
                            Thank you. You confirmed my understanding that the balancing on my Tesla starts somewhere between 80 and 90% of what they show on the screen as 100%. In actuality that 100% is probably only 90% of full capacity. You can call that middle balancing if you like. I think it is more clear to say it is near the top. To me the middle is closer to 50% but that difference may be semantics and not worth disagreeing over.

                            This entire discussion also gives me confidence that I will get several hundred thousand miles on that pack.
                            Last edited by Ampster; 11-21-2018, 11:39 PM.

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                            • #29
                              These transformers you speak of certainly accomplish more than any transformer I've ever seen. Please give an example make and model of such a device.

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                              • #30
                                Originally posted by AlOlmstead
                                ....

                                THIRD, since most people (customers) do not understand DC, and are unwilllng to lean even for efficiency's sake, convert to AC at the earliest physical opportunity. Given the minimal configuration described above, input voltage will range between 37v and 44v (the upper value allows for 10% rise in voltage during cold weather, with corresponding drop in amperage), This input should [B]power a (usually automotive or marine) transformer with an input range about 15v to 50v and a constant output of about 12.2v.[/B] That output is perfect for just about every modified or pure sine wave inverter.....
                                Wow. a DC transformer with 15-50v input and 12.2v output. care to share any sources or specs for these wonderful creatures ?
                                ( I added the [B]bold [/B]font to your quoted post )

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