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BQ40Z80: QMAX not updating at discharge relax

Reed Christiansen
Prodigy 100 points
Part Number: BQ40Z80
Other Parts Discussed in Thread: BQSTUDIO

Hello,

I am seeing a persistant issue on a new design utilizing the BQ40z80 where Qmax is not updating after the discharge relax - I have tried to satisfy all conditions:

 -delta capacity > 37 %  (100%  delta capacity in my case (FC to FD) )

- temperature - 23 C

- cell voltage outside of flat region (OCVFR not set)

 - offset error< 1% of design capacity (12 ah - scaled to 1.2 ah to to scale of 10)  ?      Deadband is set to 2, scale is 10, current shunt  is .5 mohms.   

Could the 5 hour relax period be causing enough offset error to acumulate? ?

What do you think the issue is.  At this point I have more than 100 cycles across 20 different prototypes and I have yet to see a discharge QMAX update.  

It is is the accumulated offset error - is there a way to change the offset error limit to higher than 1%.  Accuracy is not the limiting issue here.  Cycle life on this battery is expected to be under 100 cycles - most important is getting those qmax updates in to track the degredation. 

fast qmax is enabled - however, I have not seen a qmax update occur after discharge.

after a full discharge and greater than 5 hour rest- FD flag set, REST set, VOK (set!), QMAX not set. - see below

Same battery after a full charge and rest -FC set, REST set, VOK cleared, QMAX set 

- 

  • 0
    TI__Guru** 105600 points

    Hello Reed,

    Can you share the .gg file and the BQStudio logs from your tests if possible? From your screenshots I see you have completed learning and gotten a field Qmax update, so it had happened before.

    The current sense of 0.5mOhm is outside of our recommended specification which can lead to disqualification faster for accumulated error.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 100 points

    Attached is the GG file.    I don't have BQ studio logs for this test using this GG file.  I will generate some next week.

    .5 mohm is forced due to high currents (up to 160 amps).

    Is there any way to adjust the disqualification threshold for accumulated error?  

    Can I play games with the current deadband?   Set Deadband to 1 (or 0) .      is the disqualification affected by Coulomb Counter Deadband?


    bq40z80_golden_k.gg.csv

  • 0 in reply to Reed Christiansen
    TI__Guru** 105600 points

    Hello Reed,

    Yes you would need 0.5 mOhm resistance for those peak currents.

    To change the disqualification from error accumulation you would need to change the CC Deadband, the regular Deadband only limits the reported value in the Current() register for better user experience with small loads. I do not think the disqualification if from the CC Deadband though, normally it requires long periods before Qmax with lots of passed charge and no relax for it to cause a disqualification. The logs should help determine what is occurring.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 100 points

    ser3_charge_discharge_qmax_test.log

    Hi Wyatt - thanks for clarification on the CC deadband

    See log file for charge, relax (qmax update), discharge, relax(no qmax)

    Thanks.

  • 0 in reply to Reed Christiansen
    Prodigy 100 points

    Here is a second log file - basically repeating the first test.  No change in behavior.

    ser3_charge_discharge_qmax_test_A.log

  • 0 in reply to Reed Christiansen
    TI__Guru** 105600 points

    Hello Reed,

    I took a look at the log, it looks like cell 3 is getting a Qmax update and it occurs after the discharge, can you clarify the behavior you are seeing? My understanding was no Qmax updates are occurring from the discharge cycle and charge was getting Qmax updates. It looks like Qmax may be occurring but it is calculating the same value since these are only changing by 2mAh.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 100 points

    Hi Wyatt,

    I did not notice the qmax change on cell 3.   I have mainly been relying on the QMAX bit to indicate a Qmax update has occured after discharge.

    what is the relationship between actual qmax updates and the qmax bit? 

    I did not notice the qmax change on cell 3.   

  • 0 in reply to Wyatt Keller
    Prodigy 100 points

    Hi Wyatt,

    I did not notice the qmax change on cell 3 after discharge.    I have mainly been relying on the QMAX bit to indicate a Qmax update has occured after discharge.

    what is the relationship between actual qmax updates and the qmax bit? 

    Looking at the point in the log where the cell 3 qmax updates.   

    IT status goes from 0x0b18

    REST=0

    VOK=1

    QEN=1

    QMAX=1 (?)

    to 0x0911

    REST=1

    VOK=0

    QEN=1

    QMAX=0 (?)

    So, it looks like QMAX toggled to "off" after the qmax update occurs.   This is oposite of what I expected.

  • 0 in reply to Reed Christiansen
    TI__Guru** 105600 points

    Hello Reed,

    The Qmax bit in GaugingStatus will toggle with every Qmax update, so when Qmax toggles from 0->1 or 1-> this means an update has occurred. It is defined this way in the TRM as well:

    Sincerely,

    Wyatt Keller