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Thread: Understanding a BMS -- reasons for and against them

              
   
   
  1. #21
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    Quote Originally Posted by DaveAK View Post
    So what's the actual mechanism for bottom balancing? When one cell hits LVC, how do you balance the others down to this point?

    Also, in your explanation you seem to be talking about LVC and HVC as pack events, not cell level events. Am I understanding you correctly? That would be a fundamental design decison for a BMS. I'm not saying that it has to be cell level, but I am saying that a cell level BMS is a different beast to a pack level one. My preference would always be to have a cell level BMS over a pack level one. In a cell level scenario I'm not sure your explanation holds up, at least I'm not sure I understand it.
    Having both cell-level LVC & HVC is ideal. It's a tradeoff of cost vs necessity. If the cells are operating far enough away from the knees, the chances of them drifting are very low. Operate cells in the middle 70-80% of their range instead of 90-100%. Doing so also extends cell life significantly.

    In a bottom-balancing scenario, you could get by with pack-level HVC/LVC if you keep them away from the knees. It's a bit tougher with LFP chemistries since they're so flat in the middle. At LVC all the cells should hit the low point at the same time so it's # of cells * per-cell LVC (add a bit of buffer just in case). For charging, set the pack HVC right around the per-cell knee voltage. What happens for LFP is that all the cells will hang tightly together until the weak cell fills up. When it does, its voltage will start to shoot up, triggering pack HVC. For instance let's say the knee is @ 3.6v. 24 cells * 3.6v = 86.4v. As soon as the weak cell fills up its voltage will rise quickly.. 23*3.6v + 1*3.7v = 86.5v, above the HVC. There can be a problem with this approach if the cell capacities differ too much. It's not a fool-proof method (contrary to Jack's belief), but it works if the cells are decently matched. Obviously if you threw together a bunch of random cells, this method would start to fall apart. That's where a well-designed BMS with cell bypass would show its strength.

    To bottom-balance cells: Pre-balance them out of the vehicle. Drain each one to the selected LVC (doesn't have to be perfect, no more than 0.1v apart from each other). Carefully wire them all up in parallel, let sit for a while and they will equalize. LVC in the 2.7-2.9v range should be reasonable choices for TS LFP (unless someone has a better recommendation).
    If you're asking about periodic maintenance bottom balancing, that would need to be performed cell by cell in the string. Based on the experiences of bottom-balancers, re-balancing is rarely if ever needed.


    N&V: The age of a cell also affects its capacity, regardless of number of cycles on it. Best bet is to use cells manufactured around the same time and with a similar # of cycles on them. I think it might take a chemist to address your questions
    Last edited by chef; 12 March 2011 at 0128.

  2. #22
    Seņor Member podolefsky's Avatar
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    Quote Originally Posted by chef View Post
    Having both cell-level LVC & HVC is ideal. It's a tradeoff of cost vs necessity. If the cells are operating far enough away from the knees, the chances of them drifting are very low. Operate cells in the middle 70-80% of their range instead of 90-100%. Doing so also extends cell life significantly.
    This makes me think that as long as you stay below 80% DOD, then top-balancing lets you take advantage of the strong cells. If you bottom balance, you're not using that extra capacity, and unless the pack is way out of balance, you'll only lose cells at the very end of discharge.

    I like my BMS because it has nifty blinky lights that turn on/off while it's balancing. I have only anecdotal data (with sample size of N=1) as evidence that this is amusing. Actually, N=2, but how neat my wife thinks it is depends on the temperature in the garage.
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  3. #23
    Senior Member Coninsan's Avatar
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    Great thread, deffinetly worth a sticky.
    The words uttered here has deffinetly helped me understand pack dynamics.

    But I am wondering over the importance of top cell balancing if you buttom balance the pack propperly, given that the weak cells would still limit the pack capacity even if the whole pack was top balanced?
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  4. #24
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    Quote Originally Posted by Coninsan View Post
    But I am wondering over the importance of top cell balancing if you buttom balance the pack propperly, given that the weak cells would still limit the pack capacity even if the whole pack was top balanced?
    Here's my take on the subject. BMS, top balance, bottom??? The method can vary. The resulting protection is what is important. Things you need to do:

    1.) Insure no cell is overcharged. Maybe the most important because this can cause a fire.

    2.) Insure no cell is overly discharged. This will damage the cell. It can cause overheating and possible fire if the cell is driven negative, but in most cases, will just damage the cell.

    3.) Monitor temperature of the battery pack to detect problems and maintain suitable operating temperature range which will yield suitable cycle life.

    Beyond these three items, it is a choice for the battery owner and operator of what he (or she) wants to spend money on and perceived value in return. Is it more important to be able to fully utilize the entire energy capability of the battery pack or is it acceptable to only have a fraction of that available to avoid the cost of top balancing or even decreased cycle life? Is it acceptable to charge the battery pack to less than 100% to avoid cost when this will reduce performance meaning peak power and operating range? How autonomous must the system be? Can periodic service intervals be scheduled to manually balance cells, or even replace lesser performing cells?

    These are issues I see with Lithium battery systems. And the choice will differ depending on the intended application. For a competition bike, one choice, for the commuter, something else. I have been messing around with these batteries for a couple of years. Most of what I do is in the lab. I do have a few in service, with monitors, not management systems. But I tend to be very careful when I charge and use the things. I am still undecided as to what is the best method. There does seem to be a lot of choices.

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    Seņor Member podolefsky's Avatar
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    Quote Originally Posted by Coninsan View Post
    Great thread, deffinetly worth a sticky.
    The words uttered here has deffinetly helped me understand pack dynamics.

    But I am wondering over the importance of top cell balancing if you buttom balance the pack propperly, given that the weak cells would still limit the pack capacity even if the whole pack was top balanced?
    This is my understanding (of my own statement above). Someone correct me if I'm wrong.

    If you are thinking that total capacity is what you can get out of the pack at 100% DOD, then yes it will be limited by the weakest cell. But if you think of it as what you can get at 80%, then it's limited by how much total energy you can put into the pack. Only way this isn't true is if the weakest cell is so much weaker that it hits LVC before 80% DOD for the pack.

    Say you have 3 cells, just for the sake of simplifying. Two of them are 60Ah, and the weak one is more like 55Ah. You bottom balance them to 2.8V. Now you charge them until the weak cell hits 4V. At this point, the other two cells are at about 3.7V each (roughly 55/60*4V). Total pack is now at 11.4V and has 664 kWh.

    Now suppose instead you top balance them. Now you can get all three cells to 4V, which gives you 12V and 700 kWh. So by top balancing you have move voltage and more energy (kWh).

    Now you run both packs down. IF you go to 100% DOD, then the bottom balanced cells will all end up at 2.8V (all other things being equal). The top balanced pack will end up with the weak cell <2.8V, which is bad. But, if you only go to 80% DOD, then the cells will all be above 2.8V. I don't know exactly where they'll end up, but probably something like 3.1, 3.2, and 3.2. Basically, you're still in the safe zone, and you had more voltage to play with from the start.
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  7. #26
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    Quote Originally Posted by podolefsky View Post
    Now suppose instead you top balance them. Now you can get all three cells to 4V, which gives you 12V and 700 kWh. So by top balancing you have move voltage and more energy (kWh).

    Now you run both packs down. IF you go to 100% DOD, then the bottom balanced cells will all end up at 2.8V (all other things being equal). The top balanced pack will end up with the weak cell <2.8V, which is bad. But, if you only go to 80% DOD, then the cells will all be above 2.8V. I don't know exactly where they'll end up, but probably something like 3.1, 3.2, and 3.2. Basically, you're still in the safe zone, and you had more voltage to play with from the start.
    As noah stated, One advantage I see of top balancing is that you increase your pack voltage a little, which could decrease your current draw at upper SOC. This gives you more energy out of the same pack. But this doesnt matter that much in LiFePO4 because your cells drop from 3.6V to 3.2V super quick. So leaving strong cells at 3.5V isnt that important because all cells will drop quickly to 3.2V and in the end the weak cell will be the first to LVC. Then you have LiPo where the capacity lost between 4.2V and 4.1V is about 1-2% SOC, thus it makes sense to me that letting the weak cell hit HVC to stop charging is a good option.

    Also to counter noah, If one cells is 55Ah 80% DOD give you (55*.8) 44Ah out of this cell. That is only (44/60) 73% DOD on the 60Ah cells. So you charge the weak cell higher to 100%SOC and the strong cells to 93% SOC this way all cells end up at 20% SOC when LVC kicks in. No matter how high you charge the strong cells you will still only pull out the same number of Ah that the weak cell can deliver. So yes top balancing will raise your voltage and energy some, but at the addition of a more complicated BMS and extra charging time.


    I want to keeping pushing, does anyone know the science behind cells falling out of balance? Is it caused by a simple difference in cell capacity so each cell is going to be at a different SOC all the time? If that is the case then balancing may be less beneficial than i once thought

    Thanks for the great responses
    Last edited by Nuts & Volts; 12 March 2011 at 1040.
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    Quote Originally Posted by Nuts & Volts View Post
    Now I think a good question to answer now may be how/why do cells get out of balance?
    Factors I can think of; cells with varying cycle life, different Ri, heat level and different capacities. But still how do these factors "un" balance a pack?

    Any ideas to the root internal battery cause of the invention of a BMS?
    Yup, you got it. In a world of perfect manufacturing, you would see an equal voltage drop across all cells, would all charge at the same rate and would be "full" at the same time. However, most cells available to us hobbyist have variation which is not negligible, which is why the BMS was invented. Like magicsmoke said earlier, since we charge our batteries in series, the current passing through each battery is the same. (Assuming a 1P configuration.) A simple analogy is the water/bucket scenario. You got some buckets (batteries) which you want to fill with water but you have this strange hose that gives equal water flow to every bucket. If they're all the same size, it's fine.......but they're slightly different sizes. If you fill until the biggest bucket is full, every other bucket just got water on the floor. (This is a problem.) A BMS stops the flow of water and drains a little bit off buckets that are close to full and then allows water to flow again until they are all full. Individually charging is filling each bucket independent of the other buckets, which unlike series charging (previous example) doesn't make to you drain off any "water". I think it's important to realize BMS dump the water, they don't use it to fill low buckets. (A BMS that could do that would be quite complex.) Hope this helps.
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  9. #28
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    Quote Originally Posted by Nuts & Volts View Post
    does anyone know the science behind cells falling out of balance?
    Some would say they don't :-) I have been under the impression that manufacturing tolerance can result in small capacity and Ri variance with new cells. Even a very small difference can become significant over a 1000 cycles. Also that the nature of cell placement in battery packs can cause difference in heat and therefore difference in aging which will cause the cells to drift apart in capacity. Actually I have seen very little drift w/r/t cell voltage during repeated discharge and charge testing without any active balance mechanism. But this has only been to the extent of several dozen cycles at most.

    On the other hand, I have seen a battery with a BMS spend several hours balancing the cells at the end of every charge. A different type of cell in this case.

  10. #29
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    Quote Originally Posted by Tony Coiro View Post
    Yup, you got it. In a world of perfect manufacturing, you would see an equal voltage drop across all cells, would all charge at the same rate and would be "full" at the same time. However, most cells available to us hobbyist have variation which is not negligible, which is why the BMS was invented. Like magicsmoke said earlier, since we charge our batteries in series, the current passing through each battery is the same. (Assuming a 1P configuration.) A simple analogy is the water/bucket scenario. You got some buckets (batteries) which you want to fill with water but you have this strange hose that gives equal water flow to every bucket. If they're all the same size, it's fine.......but they're slightly different sizes. If you fill until the biggest bucket is full, every other bucket just got water on the floor. (This is a problem.) A BMS stops the flow of water and drains a little bit off buckets that are close to full and then allows water to flow again until they are all full. Individually charging is filling each bucket independent of the other buckets, which unlike series charging (previous example) doesn't make to you drain off any "water". I think it's important to realize BMS dump the water, they don't use it to fill low buckets. (A BMS that could do that would be quite complex.) Hope this helps.
    I love it thanks tony!

    So it is the difference in capacity and Ri that causes the voltage imbalance. The capacity difference of cells means that show cells will fill up quicker. Also Ri difference will affect charging too. A cell with a higher Ri will turn more current into heat and less stored energy. This will cause that high Ri cell to fill up slower than a lower Ri cell.

    So worst cause is a low capacity cell with low Ri (4ah, 1mohm) charging with a higher capacity with high Ri cell (5ah, 2mohm). These will be the most imbalanced cells. I love when things start get simpler. Knowledge is power!

    So we might not be able to understand the exact chemical properties that cause these changes in Ri and capacity, but just knowing the Ri and capacity differences per cell is very useful
    Last edited by Nuts & Volts; 12 March 2011 at 1133.
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  11. #30
    Seņor Member podolefsky's Avatar
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    Quote Originally Posted by Tony Coiro View Post
    Like magicsmoke said earlier, since we charge our batteries in series, the current passing through each battery is the same. (Assuming a 1P configuration.) A simple analogy is the water/bucket scenario. You got some buckets (batteries) which you want to fill with water but you have this strange hose that gives equal water flow to every bucket. If they're all the same size, it's fine.......but they're slightly different sizes. If you fill until the biggest bucket is full, every other bucket just got water on the floor. (This is a problem.) A BMS stops the flow of water and drains a little bit off buckets that are close to full and then allows water to flow again until they are all full. Individually charging is filling each bucket independent of the other buckets, which unlike series charging (previous example) doesn't make to you drain off any "water". I think it's important to realize BMS dump the water, they don't use it to fill low buckets. (A BMS that could do that would be quite complex.) Hope this helps.
    Filling buckets is a good analogy for filling batteries. Just have to remember that in this analogy, the water is *energy*, not current. You don't dump current into a battery, you dump energy. The current goes in one side and out the other.

    The analogy I like for current in a series circuit is a bicycle chain (or motorcycle chain). Imagine that the links are electrons, and the chain is threaded through all the batteries. The chain is moving at the same rate everywhere - that's how current works. The electron "chain" is moving at the same rate everywhere in the circuit. (Water is tricky because if you change the pipe diameter, the flow rate changes...unlike current where it's the same no matter what it is flowing through.)

    There are so called "charge shuttle" balancers that use capacitors to take energy out of full batteries and shuttle it to less full batteries.

    This paper might be useful: What to Balance and How
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