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

              
   
   
  1. #11
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    Only thing I am not sure is if a cell at a lower voltage will actually have more current drawn from it in a series string??
    No, in a series string the current is common to all cells regardless of their voltage (or anything else for that matter)

    Rob

  2. #12
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by magicsmoke View Post
    No, in a series string the current is common to all cells regardless of their voltage (or anything else for that matter)

    Rob
    Yea the more researching i did i realized my above statements are backwards so i am editing it. So a 3V battery with an Ri of 2mohm and a 5V battery with an Ri of 3mohm in series will have the current being sourced from them?
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  3. #13
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    So a 3V battery with an Ri of 2mohm and a 5V battery with an Ri of 3mohm in series will have the current being sourced from them?
    Yep, any cells of any voltage either similar or different, with any Ri, either similar or different, will all pass the same current if connected in series.

  4. #14
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    I don't know how many of you are familiar with Jack Rickard, he has a weekly show called EVTV on You Tube.
    http://www.youtube.com/user/marionrickard?feature=chclk

    He's a very knowledgeable and a somewhat entertaining guy, and would recommend anyone who's into EV's to check him out.

    Jack is rather opinionated and has stirred up a huge debate over the value of a BMS, claiming that a BMS is not necessary. He insists that bottom balancing your pack will eliminate the need for a BMS.

    To me, this debate is an argument over the glass being half full or half empty. Either way, your pack is limited by the capacity of the weakest cell. The only advantage of bottom balancing is that your entire pack will reach LVC at the same time (theoretically), therefore a simple pack voltmeter would be accurate enough to prevent you from over discharging a cell, again theoretically. It's seems to be working for him so far.
    He also relies on the charger for HVC, but only charges his TS cells to 3.8V if I remember correctly, giving him some headroom for error.

    So, I would say, if you aren't considering using a BMS, which I don't recommend, then bottom balancing and undercharging is necessary to provide enough room for error.

    If you do use a BMS, then top balancing will give you the advantage of getting maximum capacity from your pack, provided all of your cells are close in capacity, in addition to LVC and HVC.
    Considering how much money is devoted to our battery packs, it would be foolish not to be using a BMS.

  5. #15
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by magicsmoke View Post
    Yep, any cells of any voltage either similar or different, with any Ri, either similar or different, will all pass the same current if connected in series.
    So then maximum current is limited by the cell with the highest Ri and maximum energy is limited by the battery with the least capacity.
    In essence in an EV, max power is limited by the cell with the highest Ri and range is limited by the least capacity battery. If motor and controller are infinitely powerful

    I think it can be somewhat agreed upon that a HV and LV cutoff point is needed for safety purposes. This in my mind is a level "one" BMS. But I am not 100% convinced that balancing is need if LVC and HVC are properly implemented. I see balancing as a way to correct for quality control in battery manufacturing and to maximize energy throughput. I agree that the best way build a pack is to match Ri values at the cell level. Then you can try to match capacity, but this is relative.

    I am leaning towards having a BMS do LVC, HVC, temperature monitoring and Coulomb counting (Ah counting). Then every 10 or 20 cycles top or bottom balancing your pack. This way the little bit of differing voltages due to Ri differences will have limited impact on energy throughput on each cycle of your pack. Problem with this is that it may be harder to implement. A single cell charger would due the trick thou, bulk charge to 0.1V of full charge on the highest cell and then top off all cells.

    Other thoughts.
    Whats under my tank may shock you!!! R6 Build, Blog/, [/URL] OSU Current webpage

  6. #16
    Senior Member Skeezmour's Avatar
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    Great thread and I really appreciate the tone that this is been responded to so far. Should be good help for those entering the Elmoto world.


    Since we are on the other side of the issue as far as Jack is concerned I will just say as a guy who likes data it is easier to deal with either method when you have reliable sources to see what is happening to a pack on a cell level. Temp sensors on bus bars can help show not only cell temperatures but loose bolts/wires/corrosion ect. One thing we 100% agree with Jack on is the potential danger of wiring harness. Please do not skimp on type of wire/connectors and safety precautions if your doing any kind of harness in the high voltage pack.


    If anyone is interested in more specifics from us please just send a PM and I'll happily chat with you about it. I may post a separate thread if there is enough interest in me doing so. Just prefer to avoid an outright flame war.


    Gene

  7. #17
    Not to be taken seriously DaveAK's Avatar
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    @cycleguy: I think discussion about Jack Rickard is likely to be as lively as that of Chip Yates.

    @N&V: Bear in mind that with a healthy pack your best and worst cells shouldn't be that far apart. Also, in my mind balancing shouldn't really be required on a daily basis, again assuming a healthy pack, but is a safeguard against a cell creeping out of range over a period of time. My thought is that if you're going to build something to balance your pack it might as well do it every charge as not. Once it's built and installed I see no penalty for using it every time, even though I don't think it's necessary.

    I like Ed's method of cell balancing - individual charging of cells. Can't go wrong with that as far as I can see. I'm planning to investigate that as my primary option, and you're stealing my idea - bulk charge and then top off individually. You can even implement a conditioning charge stage if you've been a little less than careful with your cells.
    Last edited by DaveAK; 11 March 2011 at 2255.

  8. #18
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    Quote Originally Posted by cycleguy View Post
    I don't know how many of you are familiar with Jack Rickard, he has a weekly show called EVTV on You Tube.
    http://www.youtube.com/user/marionrickard?feature=chclk

    He's a very knowledgeable and a somewhat entertaining guy, and would recommend anyone who's into EV's to check him out.

    Jack is rather opinionated and has stirred up a huge debate over the value of a BMS, claiming that a BMS is not necessary. He insists that bottom balancing your pack will eliminate the need for a BMS.
    Jack has invested alot of time experimenting with TS cells and has shared his findings. I give him credit for sacrificing cells so that we don't have to. Trouble is his understanding is correct some of the time and wrong at least as much. You couldn't tell by the way he presents himself; he's always right and if you dare disagree then be prepared for an expletive-laden lashing. The BMS flame wars are one example. Jack is violently allergic to any BMS in practice or theory. On the other hand, his position on bottom balancing has merit.

    Personally I feel that BMS's should follow the doctor's mantra: Do no harm. Unfortunately there are BMS's that do more harm than good whether due to bad design or misunderstanding of a particular chemistry. It's a minefield out there.

    Quote Originally Posted by DaveAK View Post
    I've never understood what's meant by "bottom balance". Can you elaborate?
    Bottom balancing means to drain each cell to the LVC and balance them at that level. To understand why this is beneficial, consider what happens with top balancing. As an example let's say all the cells are at 100% capacity except for one which holds 80% relative to the others. At full charge all the cells are topped off (same voltage), then the pack is drained to the point where the weak cell is depleted. The rest of the cells still have 20% capacity remaining, so the pack voltage looks ok. Hopefully the BMS's LVC alarm goes off and limp mode is engaged either manually or automatically. If you continue limping too long, that weak cell will get driven way low, possibly to 0v and then reverse polarity (major cell damage).

    In the bottom balancing scenario, the cells are charged to 80% capacity except for the weak one which gets 100%. The 80% in this case is relative to the weak cell's total capacity which is less than the other cells (ex: 32Ah/32Ah = 100% for the weak cell, whereas 32Ah/40Ah is 80% for the stronger cells). It may be confusing but bear with me. As energy is drawn from the pack and the pack approaches depletion, the cells hit the knee at the same time. This causes the pack voltage to drop proportionately. The state of the cells is simply the pack voltage divided by # of cells. It's easier to predict when the pack will run dry. If you keep pushing the pack, the voltage plummets and the vehicle will pretty much refuse to budge because the cells can't deliver enough current. The pack self-limits before cell damage occurs. No single cell is the weak link at this point. They're all at the LVC.
    (An animated graph would make this much clearer... sadly I have no mad skillz in that dept)

    By contrast in a top-balanced scenario, the pack voltage could drop a few volts and seem ok. At the low end, one cell could drop to 2.0v while the others are happily at 3.0v. That means for a 24 cell pack, the pack has only dropped from 72v to 71v. Everything looks ok but the weak cell is getting driven into the ground.

    Top-balancing makes sense if the BMS does cell bypass. At the low end, a cell at LVC (say 2.5v) is essentially removed from the pack. Pack voltage drops by 2.5v which isn't a big deal. The rest of the cells can continue on until enough of them drop out and the voltage approaches pack LVC. Without cell bypass, top balancing is a wasted effort for the most part. You're spending good money on a top-balancing BMS but are still limited by the weakest cell. If you have a weak cell from the get-go, it's better to replace it. No amount of balancing will get more capacity out of the pack

    The only other scenario where top balancing is beneficial is on a new pack with poorly balanced cells. Assuming that the cells are all at the same capacity, the BMS will eventually bring each cell up to the same capacity. But this break-in should be a one-time deal. You could do this yourself by charging each cell to a similar voltage (preferably around one of the knees for LFP) and parallel them all. LiPo has a more predictable voltage curve so they can be balanced at most any voltage.

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  10. #19
    Not to be taken seriously DaveAK's Avatar
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    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.

  11. #20
    Moderator Nuts & Volts's Avatar
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    Interesting information guys. Thanks for the clarifications chef.

    A good process for an experienced EVer may be cycle cells to find any weak cells with high Ri or low capacity. Discharge all cells to same resting voltage once. Build your pack. Wiring in a LVC and a HVC. Monitor temp a few places in your pack. Ride.

    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.
    Dave I believe you bottom balance once or maybe twice and each cycle you charge 'til 1 cell hits HVC and stop. A cell level BMS is still needed. Or if you know the weak cell you put a HVC on that only. This could get risky.

    Since all cells in series receive the same current a you will be putting the same energy into all cells. So as you discharge the cells should reach LVC at the same time (see my differing SOC example in earlier post) because that same amount of energy is leaving each cell. In either balancing method you are trying to get all cells to either hit HVC or LVC at the same time which would then make balancing unnecessary. But cells do get out of balanced, which leads me...


    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?
    Last edited by Nuts & Volts; 12 March 2011 at 0016.
    Whats under my tank may shock you!!! R6 Build, Blog/, [/URL] OSU Current webpage

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