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Thread: O.T. DIY battery pack BMS problem

  1. #1
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    Default O.T. DIY battery pack BMS problem

    Ok, got the welder modified, works fine.

    Built my 48v battery pack.

    This is my first battery pack with a battery maint system so I am not
    sure of what the characteristics should be.

    When I measure voltage from the opposite ends of the battery pack excluding
    the BMS, I get the predictable 52v.

    But when I measure the output of the BMS, I only get 34v

    Ive heard talk about programming the BMS and not sure what they mean
    by that. Maybe I have to throw the charger on it, and that tells the BMS whats
    going on. There were no instructions for this thing, and I am assuming that I
    hook the charger up to the output contacts.(Those 2 holes on right side of pcb)

    Anyways .. looking for some advice.



    John Titor, when are you.

  2. #2
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    I know that some BMS's handle charge and discharge. In that they cutout the battery from the load/charger. Others just manage the charge and the load is connected directly to the battery terminals. This way, the motor controller is responsible for the low battery cutout. Looks like your case it just handles the charge and the reading you get at those terminals does not have to do with the battery's voltage.
    Helder Ferreira
    Setúbal, Portugal

  3. #3
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    You appear to have 7 strings of 12 cells in parallel. Assuming that each string is 4.8 volts, then 7 of them in series gives you 33.6 volts.

    I'm not sure how you are getting "from opposite ends of the battery pack" when the strings are in parallel. It would seem that you need 11 strings in series to get 52 volts.

    Dan
    At the end of the project, there is a profound difference between spare parts and extra parts.

  4. #4
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    Quote Originally Posted by Noitoen View Post
    I know that some BMS's handle charge and discharge. In that they cutout the battery from the load/charger. Others just manage the charge and the load is connected directly to the battery terminals. This way, the motor controller is responsible for the low battery cutout. Looks like your case it just handles the charge and the reading you get at those terminals does not have to do with the battery's voltage.
    That really makes sense, but I think/hope you are wrong. I would think what you are describing is a
    charge controller, while this is a true bms. This also limits the discharge to keep the batteries healthy.
    But I am definitely going to recheck that.


    Quote Originally Posted by danlb View Post
    You appear to have 7 strings of 12 cells in parallel. Assuming that each string is 4.8 volts, then 7 of them in series gives you 33.6 volts.

    I'm not sure how you are getting "from opposite ends of the battery pack" when the strings are in parallel. It would seem that you need 11 strings in series to get 52 volts.

    Dan

    Actually, this configuration is called a 13s 6p. There are 13 strings of 6 parallel.

    I can put my meter on the ground and in turn, touch each string, nicely progressing up to the 52v in
    4v increments.

    Someone on an E-bike forum answered me and said that it might just need to be cycled. I get my
    charge in a couple days and will try that.

    The only thing I am worried about is that for a couple of the pcb contacts, I used the spot welder and
    and I hope I didn't fry anything doing that.

    Again, here is the link to the bms, and now Noitoen has me worried, lol, it does say bmscharger.

    https://www.aliexpress.com/item/13S-...1f016857xVnICA
    John Titor, when are you.

  5. #5
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    13s 6p is what I see too.
    Helder Ferreira
    Setúbal, Portugal

  6. #6
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    Can be a bms charger and if so, you have to supply a suitable voltage/current to those bms terminals.
    Helder Ferreira
    Setúbal, Portugal

  7. #7
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    I don't recon the battery will last long without the cells being matched.

  8. #8
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    Lithium chargers typically start with a constant current mode where the charger adjusts it's voltage to whatever is required to get to the target current.
    For just a little more, you can do it yourself!

  9. #9
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    Quote Originally Posted by old mart View Post
    I don't recon the battery will last long without the cells being matched.
    The bms should distribute the charging voltage equally between the cells in series. Any difference in cell internal resistance will be corrected this way
    Helder Ferreira
    Setúbal, Portugal

  10. #10
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    With series lithium cells, all will of course see the same current. However, they won't necessarily charge at the same rate. When one of them reaches it's full charge, measured by it's voltage, the charger has to stop to avoid overcharge of that cell. Good chargers may go then go into a balance phase where the fully charged cell is dscharged a little, and the cycle repeats.

    Er
    Last edited by ed_h; 03-13-2019 at 07:22 PM.
    For just a little more, you can do it yourself!

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