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BQ76PL536A one string discharged

Rafael Garayoa
Prodigy 30 points
Other Parts Discussed in Thread: BQ76PL536A

Hello, we have successfully implemented a BMS with 18 BQ76PL536A devices monitoring 102s1p 20Ahr LiFePO4 cells. Two times it happened that the cells corresponding to a given BQ IC got discharged below 2.0V in less than 24hrs.

A first guess was that the bleeding resistors were unintentionally switched on; however they are set to limit the current to 100mA therefore it is not possible to fully discharge the cells in one day. 

The same behavior was seen on the testbench one or two times using a '536EVM with 18s1p 1.1Ahr LiFePO4 cells. Overnight one section of 6 cells was discharged below 2.0V.

In our battery the '536s are hot plugged to the battery, the cell voltages are applied in a random order to the '536 terminals. I remember reading some information that this is not a problem for the '536.

We are not able to reproduce the problem nor explain the reason for which the cells get discharged, I wonder if somebody has faced this problem or there are any ideas. Thank you.

  • 0
    TI__Genius 10785 points

    PL536 is hot pluggable.  You can randomly connect cells in any sequences.

    You have 1.1Ahr cell. If you turn on 100mA for 10hrs then it will bleed for 1A. So, it could be possible.

    Can you share the portion of schematic? I want to see cell balancing and cell input side.

     

     

  • 0 in reply to Roger Hwang
    Prodigy 30 points

    Hi Roger, thanks for your reply. Indeed for the 1.1Ahr cells it would be possible, but not for the 20Ahr. We have the same front-end as designed in the BQ76PL536EVM. I have to say that this issue has happened even in the EVM, the small cells (1.1Ahr) are attached to the EVM via the provided connectors and we have experienced this issue in the past. Please let me know if you require further information. Thank you!

  • 0 in reply to Rafael Garayoa
    TI__Genius 10785 points

    You are right. PL536 and 100mA bypass current don't explain on 20A hr cells.  It's hard to say what could cause that. Possibe one bad cell, something is shorted. I haven't seen it myself or my customers complained about issue like yours.