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

              
   
   
  1. #1
    Not to be taken seriously DaveAK's Avatar
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    Understanding a BMS -- reasons for and against them

    This is Kyle editing Dave's post

    This thread is for discussing the reasons why BMS systems are used and reasons why people may not want to use a BMS. This is related more towards idea of keeping batteries happy while getting the most out of them. I would hope to stay away from practical use of BMS (ie how they work, the circuits), but to stick more to the science and theory of monitoring and balancing your batteries. This is both during discharge and charge.

    Try to keep opinions out of the arguement and based statements of the science of them. (so dont tell me how many cars have burned from a BMS, cuz it doesnt change intended purpose of the BMS)

    Thread originated from posts in "Dave's LiPo Thread" but the relevant paragraphs/posts have been copied below.
    Thanks
    Kyle



    This is Dave:
    What do you guys need in a BMS? None of it should be that difficult to achieve with a simple microcontroller.

    Basic:

    LVC/HVC - simple ADC per cell measurement with set points based on chemistry. Can easily add LVW/HVW (warning) points if required.

    Intermediate:

    Temperature compensation for LVC. Based on ambient or cell temperature.
    Cell temperature monitoring for cell safety.
    Balancing - a little trickier if you want to design something for a broad range of pack setups, but still plenty of stuff out there.

    Full:

    Coulumb counting to act as a fuel gauge.
    Charger integration.
    ?????? - What else have I missed?

    All of this can be data logged and analyzed if required. A number of output choices from LCD displays to tripping of relays. My plan is to eventually incorporate all of this in my bike, together with GPS data so I can plot the energy requirements of a whole route, against elevation changes and speed/acceleration. It's just a question of time to put it all together, so I'll do it a bit at a time.

    Each step can pretty much be done independent of others, so if for example you just want LVC/HVC, then that's all you need to implement.
    Last edited by Nuts & Volts; 11 March 2011 at 1805.

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    Senior Member Coninsan's Avatar
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    I would just want LVC/HVC that ccould be wired to the Controller/Charger and a status LED that could be placed remotely on the dash.
    Luxury items would by LVA/HVA (Alarm) and cell balancing.

    Which lead my to a question actually..
    Let say that your charging your pack up without a balancing feature. One cell triggers the HVC at 4.2V but one or more cells are still at 3.9 and 4V, will these cells balance out on their own to a common 4.1V or would the voltage untill discharged?
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    Moderator Nuts & Volts's Avatar
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    At a minimum:
    I will need LVW so i know to slow or stop. I can make the decisions. Controller can have a LVC.
    I need HVC to stop charging
    A few (5-10) temp sensors to cut off charging when its cold or if too hot. and just to monitor.

    I would like balancing at HVC. Or could run a smaller circuit to balance part of the pack at a time.
    I have heard that Lipo mainly loss balance at low SOC (3.6V and lower, unloaded I believe). If this is true I may only need to balance a few packs with an RC charger or single cells chargers.

    Coulomb counting should be done with my controller so ill just data log that measurement.

    Quote Originally Posted by Coninsan View Post
    I would just want LVC/HVC that ccould be wired to the Controller/Charger and a status LED that could be placed remotely on the dash.
    Luxury items would by LVA/HVA (Alarm) and cell balancing.

    Which lead my to a question actually..
    Let say that your charging your pack up without a balancing feature. One cell triggers the HVC at 4.2V but one or more cells are still at 3.9 and 4V, will these cells balance out on their own to a common 4.1V or would the voltage untill discharged?
    Nope those cells will stay at a lower voltage at all times. Those lower V cells will also contain less capacity, this is why balancing is so important
    Last edited by Nuts & Volts; 11 March 2011 at 1224.
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    Senior Member Coninsan's Avatar
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    Hmm that's a pickle, any good suggestions as to how to charge and balance a LiPo pack then?
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    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by Coninsan View Post
    Hmm that's a pickle, any good suggestions as to how to charge and balance a LiPo pack then?
    Same as any pack, HVC triggers either a bleed resistor circuit or a more complicated capacitor/inductor energy distribution circuit. This keeps the top cell from taking any more charge while allow the other, lower cells to continue charging.

    You could do this individually with single cell chargers or an RC charger. RC chargers usually have a balancing feature but are limit to between 6s and 14s packs.
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    You're limited by the weakest cell and LiPo has a tendency to catch fire when discharged too low or charged too high. My recommendation is to bottom balance and end the charge when the first cell hits HVC. This will reduce the chance of any single cell draining lower than the other cells. You could spend money on top balancers but the weak cell will still hit LVC first and you pretty much have to stop discharge with LiPo. The next gen BMS's have bypass circuits that allow the pack to continue operating without draining the low cell further. I'm guessing they're pricey buggers though.

    If you have a cell at much lower capacity than the others, it'd be better to replace it.

  7. #7
    Not to be taken seriously DaveAK's Avatar
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    I've never understood what's meant by "bottom balance". Can you elaborate?

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    Moderator Nuts & Volts's Avatar
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    I think what he means is to not actually charge all cells to 4.2V. Instead charge until one cell hits HVC, this cell is the weak cell. I think this assumes then that all cells will then discharge the same capacity (Ah) and end at LVC at the same time.

    Theoretically this basically operates each battery between different start and end SOCs. Battery 1 goes from 100% done to 30%, while Battery 2 goes from 95% to 25%. So LVC (say 3.5V) hits at 30%SOC for cell 1, but LVC hits at 25%SOC for cell 2. This balances your cells based on capacity. Cell 1 is the weak cell because it limits the max SOC of the other cells.

    Top balancing, balances the cells based on voltage/SOC. So if you have cells that vary in capacity (actual Ah may vary based on quality control) then top balancing fills every cell to its max capacity.

    Either way you are limited to the capacity available in the weakest cell. But the bottom balancings doesnt waste time or energy, putting energy into the stronger cells. Please correct me if I am wrong. I may have just convinced myself that balancing is unnecessary if my statements are correct??

    Dave, do you care if I move this BMS talk to a new thread?
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  9. #9
    Not to be taken seriously DaveAK's Avatar
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    Quote Originally Posted by Nuts & Volts View Post
    I think what he means is to not actually charge all cells to 4.2V. Instead charge until one cell hits HVC, this cell is the weak cell. I think this assumes then that all cells will then discharge the same capacity (Ah) and end at LVC at the same time.

    Theoretically this basically operates each battery between different start and end SOCs. Battery 1 goes from 100% done to 30%, while Battery 2 goes from 95% to 25%. So LVC (say 3.5V) hits at 30%SOC for cell 1, but LVC hits at 25%SOC for cell 2. This balances your cells based on capacity. Cell 1 is the weak cell because it limits the max SOC of the other cells.

    Top balancing, balances the cells based on voltage/SOC. So if you have cells that vary in capacity (actual Ah may vary based on quality control) then top balancing fills every cell to its max capacity.

    Either way you are limited to the capacity available in the weakest cell. But the bottom balancings doesnt waste time or energy, putting energy into the stronger cells. Please correct me if I am wrong. I may have just convinced myself that balancing is unnecessary if my statements are correct??

    Dave, do you care if I move this BMS talk to a new thread?
    I think maybe this is the argument against balancing, i.e. let the cells bottom balance. I've nothing invested in that particular argument but was always curious as to the methodology people were refering to. More than happy for you to move this to a new thread as I'm very interested in al lthe various thoughts on what a good BMS should be.

  10. #10
    Moderator Nuts & Volts's Avatar
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    Additional thoughts to my statements in post #8.

    Inaccurate statement so i removed it , see next couple of posts


    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?? I'm trying to work it out, but think i may be more than a circuit analysis and i may be chemically related?
    Last edited by Nuts & Volts; 11 March 2011 at 2045.
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