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BQ79616EVM-021: Experiencing strange cell voltages during balancing

Part Number: BQ79616EVM-021
Other Parts Discussed in Thread: USB2ANY

Hi team,

I have experienced some strange issues whilst testing auto/manual cell balancing of my 16 cell battery pack with the EVM using the USB2ANY and BQAutoEval-1.0.4 GUI. The cell voltages are all measuring correctly.

The open-circuit cell voltage for each cell was roughly 4.08V. Whilst performing cell balancing with a 5-second duty cycle, the individual voltages 8 cells that aren't being balanced increase to 4.29V. I measured the voltage of affected cells on the battery pack side, and the multimeter shows the same ~4.29V measurement.

The OV protection threshold was set to 4250mV. With 'stop balancing on fault' enabled, I can't perform cell balancing. I have stopped testing since, please advise if there is something I have not set up correctly or if this may be a board issue.

Thank you.

  • Please see the attached screenshot as a reference. Is this cell voltage increase behaviour expected?

  • Hi Jason,

    There should be some change in the cell voltage during balancing since you are pushing extra current through the resistor in the path to the cell. As for "stop balancing on fault", this is expected behavior because it shows on the right side you have a OT UT fault red, so CB will stop on any fault present in the fault summary.



  • Hi Taylor,

    Thanks for the response. Just to clarify:

    1. Apologies for the confusion - the OT UT fault in the screenshot was triggered as a test and is unrelated. The "stop balancing on fault" is a concern when the cell voltage exceeds the overvoltage threshold, which triggers the OV UV fault.
    2. Given these are Li-ion cells, the maximum charge voltage is 4.2 V so I set overvoltage threshold to 4250mV as I'd want to limit the cell voltage exceeding 4.25 V.

    In regards to the change in cell voltage during balancing, is such a large (0.2 V) increase expected given the open-circuit voltage is 4.08 V?

    Also, is it safe to balance the Li-ion cells (nominal 3.6 V) if the measured cell voltage exceeds 4.3 V during balancing? I suspect this may be dangerous as the cell voltage should not exceed 4.3V in any case.


  • Jason,

    Ok are you sure there are no other faults in the fault tab showing red or masked that may cause stop balancing on fault to occur? Please double confirm the fault tab and if you masked CRC or something else.

    As for 4.3V Li-ion, it really depends on the exact cells you have but I expect they should be ok. As for 200mV, this would depend on your cabling and current level which may affect the level of voltage difference. This does seem high, so you may have a setup issue or higher R someone along the path you may try to measure out to check.



  • Hi Taylor,

    I measured the resistance along the path at several points. The resistance between cell TP and the respective battery terminal - the resistance was only 0.921 ohms (the path includes the PCB traces, connectors and wires to the EVM).

    I tested the EVM and battery using the following process, please let me know if anything is out of the ordinary:

    1. Connect the EVM board to the USB2ANY.
    2. Plug the USB2ANY into computer USB port, open BQAutoEval 1.0.4 GUI.
    3. Ensure the S1 and S2 switches are in the open position. Remove the J16 and J14 jumpers.
    4. Connect the battery connector (J15)

    5. Wake up the EVM. Auto-address the stack. Start polling
    6. Mask CUST_CRC then reset all faults.
    7. Change AUX ADC (for BAT) to "Continuous". Enable TSREF. Set the voltage protection and temperature protection.

    8. Enable cell balancing (auto or manual).

    By step 7, there are no issues with cell measurements and there are no faults present. However, as soon as I enable any kind of balancing (auto or manual), the cell adjacent cell voltage increases by 200mV.

    The stop balancing does not occur when I set the voltage threshold to 4350mV or greater. It is only triggered by the OV threshold fault, and OT if balanced for too long.

    I also performed some diagnostic tests - the large voltage change issue also occurs during the CBFET Check. I have attached the results and .csv file for reference.

    Please let me know if I have incorrectly set something up, or if this might be a board issue?


  • Hi Jason,

    Yes I ran a test with some cells on the EVM and saw similar voltage change and would also be the same when turning on the CBFET check, it would be expected drop of 200mV with 200mA through a 1 ohm R right? So you mean stop balancing on fault only works if you have a OV threshold less than 4350mV? Can you clarify the exact steps when it works and doesn't?



  • Hi Taylor,

    I attached the results here. It didn't attach to my last post.

    To clarify, note I have only tested so far when the cells have an average open-circuit voltage of 4.08 V. When auto-balancing all 16 cells, the cells that are balancing have a voltage drop of 200mV with a measured cell voltage of ~3.86V; the adjacent cell has a measured cell voltage of ~4.29V.

    Now, with "stop balancing on fault" enabled, I ran two cases:

    1. OV Threshold set to 4250mV (given the charge limit for the Li-ion cells is 4.2V): as expected, the balancing stops as OV fault triggers due to the adjacent cell measuring 4.29V.

    2. OV Threshold set to 4350mV: the balancing operates as expected because the OV fault is not triggered.

    EVM Diagnostic Test Results.pdfBQ7961x 0x00 graphdata diagnostic test.csv

  • Hi Jason,

    So I am unclear then on the issues - it seems cases 1 and 2 are operating as expected right?



  • Hi Taylor,

    My main concern was that I wanted to always have OV threshold set to 4250mV - hence a fault would be triggered whilst balancing.

    I now understand the higher voltage measured during balancing is due to higher R in the path, so I think this issue can be resolved if I just set the threshold higher to 4350mV when balancing. Thanks for your help!