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Thread: O.T. DIY battery pack BMS problem

  1. #21
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    Connect a charger and measure.
    Helder Ferreira
    Setúbal, Portugal

  2. #22
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    Quote Originally Posted by Mike Amick View Post
    Something is bugging me. Why would this bms have circuitry going to all 13 strings if it is not
    balancing. If all it is doing is monitoring total charge and discharge it only needs to connect to
    the + and - .

    Was thinking about it in the shower .. lol.
    It's surely sensing individual cell voltages so it knows when to shut off the charging when one of them reaches a specified voltage

    Ed
    For just a little more, you can do it yourself!

  3. #23
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    It also probably shuts off if any cell dips below the minimum value of about 3 volts, as well as the ability to shut down on overcurrent. The only thing to be gained with a balancing BMS is the ability to shunt charge on weaker cells so that the stronger ones can achieve full charge. But then during discharge the weaker cells will deplete before those that still have adequate charge. There have been endless arguments for "bottom balancing" versus "top balancing", but the final analysis shows that the pack is only as strong as the weakest series element.

    There are some BMS designs that redistribute charge from stronger cells to weaker cells, but if the series packs are reasonably close in capacity, there is little to be gained.

    It would be good to have some indication of SOC and warning when a cell is nearly depleted, and I didn't see that on the BMS board that was used. Some people use a simple balance monitor that compares the top half of the pack with the bottom half, but it is not very accurate, and won't work for a 13S pack. Well, it might be configured to work.



    Showing the circuit in operation for a 24V pack:



    A more sensitive PIC-based circuit:

    https://www.youtube.com/watch?v=xbS5d47A01U


  4. #24
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    Quote Originally Posted by Mike Amick View Post
    Actually the cells are somewhat matched. I drain tested every cell, and these are the cream of the crop. But I do
    understand Lakeside, that good doesn't mean matched. But doesn't the discharge control only let it drop down to
    about 40v from the fully charged 52v ?

    Besides it is what it is. I paid a dollar a pound for these and like I said every battery in this pack checked out
    to 2800 ma hr or higher which is pretty respectable.

    I appreciate Paul doing that analysis and just hope a balancing bms will give me some life.
    The discharge at cell level control has to stop ANY of the cells from going below 2.7 or so... (actual value depend on the cell). If they go much lower they are permanently damaged. If they start off balanced and all age the same amount, no real problem, but they will diverge over time. The best BMS monitor each cell for both charge and discharge levels. For small packs I just use cells that have BMS built in.

  5. #25
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    Looking at the size of the components on the balance side of the circuit, all that they can do is to alter the apparent resistance of the series circuit. On that circuit in particular, the maximum charging current is around 2A so, to balance, a few mA load on a particular cell group is enough to achieve this.
    Helder Ferreira
    Setúbal, Portugal

  6. #26
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    Quote Originally Posted by Noitoen View Post
    Looking at the size of the components on the balance side of the circuit, all that they can do is to alter the apparent resistance of the series circuit. On that circuit in particular, the maximum charging current is around 2A so, to balance, a few mA load on a particular cell group is enough to achieve this.
    Hi Helder,

    are you saying that it does do balancing ?

    I wrote the company and their only reply was ... p+ is discharge positive P_ is charging negative
    John Titor, when are you.

  7. #27
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    Again, looking at the board, there are 4 what look like fet's on the left side and 1+ 3 empty spaces on the right. Judging on the company's response and the site, the 4 has the ability to switch off the 20a load and the lone one can switch off the charge current.
    I would connect a charger and measure the voltage on each of the series strings. I would bet that you can see the same kind of balanced value. That value must, at least be below 4,2v which I think is the maximum voltage permissable for those cells.
    Helder Ferreira
    Setúbal, Portugal

  8. #28
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    Would it work out of circuit(battery) I've already unsoldered it. It's becoming a moot point, with the newer
    bms coming any day. But if I can test it laying on the bench, I would be curious, I just don't know what it
    would do not seeing any voltage on any of the strings.
    John Titor, when are you.

  9. #29
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    You could just solder little pieces of wire to get the readings
    Helder Ferreira
    Setúbal, Portugal

  10. #30
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    Fwiw ..
    Lithium cells should be balanced for long life and good performance.

    In the early days long ago pre-cell-production what tesla did was measure all purchased cells, and sort/group them.
    This resulted in great performance.

    Sandy Munro refers to this in a youtube video, where he quotes 1 mamp differences.

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