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How does a Battery management board work?

Discussion in 'Power Electronics' started by AprilSteel, Jul 24, 2018.

  1. AprilSteel

    AprilSteel

    111
    9
    Feb 2, 2017
    I have looked at a few of these BMS boards and they seem to work on controlling the negative line . Cutting off current or connecting it via a bank of MOSFETs.
    Straight forward stuff I guess when the gate is set with a certain voltage ,current either flows or doesn't through them depending on PNP or NPN .

    So I'm looking at the one below and trying to figure out what the circuit would be . Does anybody have a clue how they work to sense the individual voltages and switch on or off the whole bank?

    What would the main current flow look like? Could anybody venture a circuit? Circuit diagrams-l1600.jpg
     
  2. 73's de Edd

    73's de Edd

    2,744
    1,133
    Aug 21, 2015
    AprilSteel . . . . .

    On this board depicted, it shows the taking of individual cell voltage samples in from the lines at the left and going into the U 23 thru 29 buffer stages and then visually having LED indications and also passing samplings on into two vertical clusters of series arranged " 017" optical isolators.
    One set is oriented to undervoltage threshold function and the other toward overvoltage threshold with daisy chained photocell secondarys of the two sets then feeding into the 14 pin flat pack IC. That IC then feeds the far right POWER FET sets to have them connect or disconnect the negative battery connection in accordance to sampled battery voltages situations thresholds .
    ASIDE . . . .
    Not having been additionally given the board number nor any info about it.
    With those hefty 51 ohm frontal flat pack metal film units and if U23-29 additionally involved an internal FET, there could be an additional capability of bleeding down cells to balance out their charge state upon them.
    Not seeing the other board sides foil interconnects . . . unable to ascertain.

    What's with the April Steele moniker . . . . .GIF fully actuates on profile page only.


    73's de Edd
    .....
     
    Last edited: Jul 24, 2018
    AprilSteel likes this.
  3. AprilSteel

    AprilSteel

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    Feb 2, 2017
    RearFoils-l1600.jpg Thank you Kind Sir.
    Ed how would the voltage limits be set? Is the IC programmable or is it set by resistors?

    Not sure either on the profile Pic. But I guess thats just for the people who want to see my eyes flash ! Its just how it came out when I put it in the Profile
    Here are a couple of listings- They are USD $8 from China no postage, $20 Amazon

    https://www.ebay.com/itm/201736521962?ssPageName=STRK:MEBIDX:IT&fromMakeTrack=true
    https://www.amazon.com/Lithium-Battery-Protection-Balancing-Function/dp/B073GZ635J
    http://vi.vipr.ebaydesc.com/ws/eBay...descgauge=1&cspheader=1&oneClk=1&secureDesc=0
     
    Last edited: Jul 24, 2018
  4. twister

    twister

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    6
    Feb 12, 2012
    Why do the voltages of the batteries have to be the same?
     
  5. AprilSteel

    AprilSteel

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    Feb 2, 2017
    I see that a lot of these battery chips are predetermined for set voltages . Can't read this chips value above and don't have a board to look at yet. 8205A-B Battery Protection Chip.png
     
  6. AprilSteel

    AprilSteel

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    Feb 2, 2017
    The battery bank is made up of 8 off 3.2 V cells in series giving 24 Volts odd .These are LiFePo4 cells which spend most of their time at 3.2Volts . Full charge is depicted by the cell reaching a cut off point of say 3.65 Volts Then that cell is known to be full. Each cell is brought to that point and then the pack is said to be balanced . Each cell is at its full capacity so then the pack is at 100% capacity.

    Is that what you asked? If not please be more specific.
     
    Last edited: Jul 28, 2018
  7. twister

    twister

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    Feb 12, 2012
    That explains a lot. Will the batteries explode or catch fire if they become unbalanced?
     
  8. AprilSteel

    AprilSteel

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    Feb 2, 2017
  9. AprilSteel

    AprilSteel

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    Feb 2, 2017
    OK Edd seems to have abandoned ship so here is what i have unearthed so far.
    Many of the circuits use a chip which is set to the required top and bottom voltages .
    They have as many feed ins as they need for the cells .
    The Negative line is controlled by two mosfets back to back so if either one is off no current passes through the battery pack. The general circuit is as below and the chart above and here shows how the chips are picked for their values.
    BMS circuit.jpg
     
  10. AprilSteel

    AprilSteel

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    Feb 2, 2017
    Here is a better view of the generic type of circuit used .
    BATT would be the charger input I suppose

    8205ABatteryprotectionCircuit9PnxX.png
     
  11. twister

    twister

    161
    6
    Feb 12, 2012
    You show external mosfets in your simplified circuit. So are the mosfets in the IC that do the same thing? And maybe there is a mosfet at the negative of each battery?
    There is also a resistor at the negative of the whole battery pack that would let the batteries charge with no control that I can see from the IC, in the schematic.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,220
    2,695
    Jan 21, 2010
    There are also MOSFETs shown in the more complex circuit shown earlier.

    I'm not sure what you mean.

    No it won't. This provides power to the IC.
     
  13. AprilSteel

    AprilSteel

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    Feb 2, 2017
    Well it gets a bit more confusing each time I get some detail.
    The below board called a PCM board is on each cell and it allows the cell to leak voltage (current) out of it to stop it overcharging it says.


    If a charger is putting 30 Volts 5 amps into the battery pack (8 cell Series 24 Volt) and one cell goes over the 3.6V set by this then it will start discharging current ,which I assume means it shunts 1.7 Amps of current on to the next cell , what good would that do ? It has 5 amps that must pass through it for the rest of the cells to charge ,1.7 shunting across so 3.3 Amps must continue in so the cell voltage would continue to rise?


    Specifications

    1PCM-B01S20-417(A-1) is a smart balance module for individual LFP cell with capacity >= 10.0Ah to improve battery's cycle life!

    Working principal:

    Each 3.2V LFP cell has a small difference in internal resistivity ( so called impedance ) due to fabrication tolerance. Especially for cell's capacity >=10.0Ah and in application require very high discharge current ( >5C ) which will cause unbalanced voltage in each cell during charging. When building a multi-cells battery pack (in series or in parallel), unbalanced voltage will reduce battery pack‘s energy power and life cycle significantly.


    The balance module will keep each LFP cell’s voltage at 3.60V peak during charging by draining excessive voltage with variable discharge current (0.65A/ 0.9A/ 1.44A /1.7A). It will wait for other cells within the pack to reach same voltage level to ensure all cells within the pack are balanced.


    Adding the balance module PCM-B01S20-417(A-1) to each LFP cell before making a battery pack will maintain battery power, increase cycle life and save money. Using this balance module means you don't need a pcm (protection board with balance function) but can use a pcb (no balance function) instead.

    PCM-BMS-Balance-Module-for-3-2V.jpg_350x350.jpg
     
    Last edited: Aug 4, 2018
  14. Functional Artist

    Functional Artist

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    Jul 21, 2018
    What type of cells are you referring to & how many do you plan on using?

    Working with Lithium batteries & choosing "proper" components is confusing because there are (2) main but, very different types (chemistries) with different "safe" usable voltage ranges.
    ...some are 3.2V nominal (2.8V - 3.7V) Safe(er) usable range is ~3.0V - 3.5V
    ...& some are 3.7V nominal (3.2V - 4.2V) Safe(er) usable range is ~3.0V - 4.1V

    To make it more confusing, you have to factor in how many cells (& of which type) the battery pack has in series

    Like, average 18650 cells are 3.2V nominal
    so, at
    ...12S = 38.4V
    …13S = 41.6V

    the other type 3.7V
    ...12S = 44.4V
    ...13S = 48.1V
     
  15. Functional Artist

    Functional Artist

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    Jul 21, 2018
    Oops, I (now) see "8 of 3.2V cells"
    ...my point is, be sure the component(s) your looking at are for the correct voltage range
    …& # of cells
     
  16. AprilSteel

    AprilSteel

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    Feb 2, 2017
    Yes ..so ?
    See the heading LiFePo4 look it up
    https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery

    Now I can't go back and edit the last post so I have to make an adjustment here .

    I now understand the last board I posted a picture of is simply a shunt board that sits across a cell's negative and positive terminals and when the voltage rises over a set point , in this case 3.6 Volts it starts bleeding current through that bank of resistors as heat . So it is shorting the cell . That is all it does . It does not affect any other cells and has no other contacts.
     
    Last edited: Aug 5, 2018
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,220
    2,695
    Jan 21, 2010
    Well, not actually "shorting".

    It is fairly common that a BMS will level the cells during the charge cycle by dissipating excess charge on the lowest capacity cells as heat.

    Some boards just use a MOSFET for this task.

    It is possible to do this equalization just when the cell reaches its maximum voltage, but doing it during the charge process reduces the power handling capacity required. If you're going to do it yourself you can do it the simple way.

    The other half of the process (detecting end of discharge) is a little less trivial.

    Are you contemplating making one yourself now?
     
    AprilSteel likes this.
  18. AprilSteel

    AprilSteel

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    9
    Feb 2, 2017
    Hi Steve . No, I don't think it is worth making one . there are plenty out there and they are cheap . The problem is knowing what you want when you order it . Because of the various chemistries there are many choices for the control chip cut-off voltages where that is the method used .

    Then its a choice dependent on the battery pack voltage used 1S 2S 10S etc.

    No, I am just trying to get my head around what choices there are thanks .

    PS my particular interest is controlling an 8S LiFePo4 pack at 3.55V cut off charge and 2.5V cut-off discharge. I have 2 of 4S boards and have been unwilling to try to connect them in series so far but have some spares coming so I might try that . My electronics knowledge is not good enough to work out if it is safe with the boards I have and I have not found a source of an 8S board with close to those values. I also have a 10S board coming that might work by wiring across to battery Neg from the 8th sense point or perhaps just not connecting 9 and 10.
     
    Last edited: Aug 5, 2018
  19. alsmith

    alsmith

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    1
    Aug 18, 2015
    slight aside here- if the battery pack isn't in use for long periods then disconnect the BMS from the battery- the first cell powers the BMS in most (all I've used) units so it will discharge that one cell unbalancing the pack, sometimes very badly. It's possible (if left in storage long enough) to kill cell 1 while the others remain ok.
    This is experience from ebike battery packs, and this has also been a timely reminder to maintenance charge two 36V 15Ah packs.
     
  20. AprilSteel

    AprilSteel

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    9
    Feb 2, 2017
    alsmith..My first impression of that is I doubt it very much . Certainly not for the boards we are discussing here. If you investigate the datasheets of the control chips discussed above you will see they are powered by the full battery pack . For a 4 or 5 series pack the max input volltage was 28Volts when I checked. For the resistor pack above it is powered by each cell and each cell would therefore be in jeopardy if left in storage with it connected possibly but again it is only powered up when the voltage exceeds a defined limit . It does not operate to affect the discharged cell and I can't see a part of that circuit that would consume power otherwise.
     
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