• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 469 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ40Z50-R2: Unable to find Cell Balancing failure cause

Akshay James
Prodigy 45 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ40Z50

We are designing a BMS for a 4S1P pack of 4900mAh. We got a CB error in PFStatus after using the battery pack for a few weeks.

CB_details

From the cell balancing times, it seem like Cell 1 has lowest capacity (CB time is 0) and Cell 2 has highest capacity.

But I am not able to find out the reason for the CB failure from the reference manual. Could anyone point me to the possible causes of CB failure?

Attaching the log and data memory files. Could anyone throw some light on this CB failure situation?

bat3_CB.log.csvbat3_data_memory.gg.csv

  • 0
    TI__Genius 11770 points

    Hello Akshay,

    Here is the explanation from the TRM:

    What it is doing is comparing the balancing times the gauge is calculating, and if the difference between the times is great enough for long enough, it triggers the PF. Ideally, the cells shouldn't be balanced for massively different amounts of times; it suggests something is wrong with the cells or the configuration somehow. From Lifetimes, it appears this is your culprit:

    It seems both trip conditions are actually violated here, but they likely have the same origin. Cell 1 balances much less than all of the other cells. I believe you don't have different cell chemistries, and since you seem to be using internal balancing, I wouldn't suspect this to happen. I would suggest double-checking the calculations for cell-balancing times. Is there any reason you can think of for cell 1 to be the lowest RSOC at almost all times? Maybe something else loading that cell? Similarly, has this happened in multiple packs consistently, or was it just one? 

    Thanks,

    Alex M.

  • 0 in reply to Alexander Mazany
    Prodigy 45 points

    Hello Alex,

    Thank you for the quick and detailed explanation.

    We have got this similar issue in 2 other battery packs recently. In both cases, cell1 cell balancing time is much less than other cells.bat1_cb_times            bat4_cb_times

    We have not yet found any possible cause of load on cell1 alone. We are still investigating it.

    We checked another working battery pack (which has no CB error or any PF error), and the cell1 balancing time is lower than other cells. We found that the 'cell1' balancing times are lesser in all our batteries.

    Since the series resistor value in our PCB is similar to that on the Evaluation board (RVCx = 100ohms) and we are using internal balancing, we have kept the default 'Balance Time per mAh' values unchanged. As per calculation also, the values are close to the ones already configured.
    cell balancing configuration

    1)   Are the above values recommended for using with Evaluation Board with SDI-INR21700-50E 4900mAh cells? Is there any dependency on the cell capacity?

    2)   I was wondering when bq40z50 calculates cell balancing time, cell1 balancing time will be lesser right (since Time/mAh is lower for cell1 that cells2-4)? 

    3)   When looking at the cell voltages from 'PF Status', all cells seem to be at similar voltage ranges. Should we try increasing the 'Delta Threshold' or 'Delay' values to avoid this PF triggering in such cases?

  • +1 in reply to Akshay James
    TI__Genius 11770 points

    Hello Akshay,

    I'll go through the questions then share my thoughts:

    1) Those values are good as long as you are using all default setup. My only concern is that these may have been calculated for a 3700mV nominal while your cell is 3600mV nominal. But there is no dependency on cell capacity, these are to figure out the time based on capacity.

    2) In theory you could expect cell 1 to balance for less total time since it is faster, but in practice it can differ. Even if it does take less time to balance per mAH, it could negate that by having larger capacities needed to balance. 

    3) You could increase all of the PF threshold in order to not trigger it, but if you really don't want it to trigger, why not disable it? I suppose you would lose the ability to catch an actual failure where it is actually needed, but increasing the thresholds will do the same. 

    Remember that there are two conditions for triggering this by default:

    • >40 Hr difference between any two cells for two cycles (Qmax update required)
    • Any cell exceeds 240 Hrs of balancing

    You are meeting the second condition on a few of these packs you have shared.

    With only 20 cycles in lifetime and an SOH of 97%, I don't think there should be so much balancing done. You could try making the balancing requirements a bit stricter about minimum RSOC. There is a chance for that to increase the difference between cell balancing though. The unfortunate thing about balancing is that it takes a very long time to test. Another thing to try would be reducing/adjusting the s/mAH values. Let me know if you have any more questions about this topic/implementation.

    thanks,

    Alex M.

  • 0 in reply to Alexander Mazany
    Prodigy 45 points

    Hi Alex,

    Thank you for the explanations.

    We will test your suggested methods and test a few units. We'll also update the s/mAh values for 3.7V cells.

    We will get back to you if we have any questions during the testing. We are also investigating the cause of cell1 loading.