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BQ28Z610: Learning Cycle Not Successful - Log Provided For Review

Justin McKenrick
Prodigy 180 points
Part Number: BQ28Z610
Other Parts Discussed in Thread: GPCCHEM

Hi there,

We've designed a battery controller board that includes the BQ28Z610 gas gauge IC but are having some issues completing a successful learning cycle.  We've followed the procedure detailed in Application Report SLUA777 several times now; attached is the log file of our most recent attempt.  We used chem ID 0450 after running through the gpcchem tool with our chosen battery cells (Tenergy model 34009-0 in a 2s configuration).  Also attached are the data flash settings we used for our most recent attempt.  The firmware on the BQ28Z610 has also been updated to the most recent release (version 20, build 25).

Right now we are attempting another learning cycle attempt using chem ID 0457; this was another chem ID that we found as a potential match for our cells after using the gpcchem tool.  In the mean time, would it be possible to have the attached log file reviewed in order to identify where we might have gone wrong during the learning cycle process?

Thanks,

Nick Medor

Learning Cycle 082819 Log and GG File.zip

  • 0
    TI__Mastermind 42535 points

    Hi Nick,

    Please allow me 3 days until Friday to get back to you.

  • 0 in reply to Batt
    TI__Mastermind 42535 points

    Hi Nick,

    In your log, you never reach charge taper termination, therefore you do not get a good DOD at EOC and therefore you don't learn Qmax. Also, your current cuts out intermittently, this should not happen. Then during dsg, you never dsg completely. Your cell is always at 6600mV or so, that means you never reach V at chg Term, therefore you will never learn capacity. Please follow slua777 for help, 

  • 0 in reply to Batt
    Prodigy 180 points

    Hi Batt,

    Thank you for the feedback.  I'm working on a better way to charge the cells.  Do you have any recommendations?  I was using a power supply set to 7.4V and current limited to 500mA, but as you noticed, the current does not taper and is intermittent towards the end of charge.  I was not using a dedicated charger IC for performing the charging step of the learning cycle; the power supply was connected directly to the gas gauge circuitry.

    You mention that I never discharge completely and that the combined cell voltage is ~6600mV.  The 6600mV appears at the end of charge (when it should be over 7V).  Did you mean to say that I never charge completely?  And did you have a chance to also look over the provided data flash settings to see if anything seemed off?

    Thanks,

    Nick

  • 0 in reply to Justin McKenrick
    TI__Mastermind 42535 points

    Hi Nick,

    If you are limited to 500mA, then you are artificially limiting the current and voltage and cannot perform the learning cycle. You need a charger that can charge your battery to Vmax of the battery DS.

  • 0 in reply to Batt
    Prodigy 180 points

    Hi Batt,

    I've experimented with different charging setups and reviewed both my battery cells' datasheet and the bq28z610 manual and found the following:

    - I need to charge the cells initially at a constant current until the cells (which again are connected in a 2s configuration) are at 3.65V each followed by a constant voltage charge towards the end of charge.  The constant voltage charge needs to stop once current is 0.01C mA, or 32mA.  I set the current limited power supply to a lower voltage (~6.9V) and was able to achieve the desired taper current without the charging current going in and out at the end of charge, but the Update Status would still not increment.

    - I noticed that my battery cells are consistently off from one another by about 300mV.  I believe this might be why the learning cycle is not progressing; section 4.1 of SLUA777 states that Qmax will not be updated if the cell voltages are inside the flat voltage region.  I think that at the end of charge and discharge there is always one cell that could be charged or discharged more due to the 300mV cell imbalance and that this cell could be stuck in the flat voltage region.  This would also explain why the pack voltage does not reach the desired voltage and why the charger power supply voltage needed to be reduced in order to achieve a taper charge current.

    - I've tried setting GaugingConfiguration[CELL_TERM] = 1 within the Impedance Tracking register followed by a cell discharge in an attempt to correct the cell imbalance, but it hasn't fixed it.  Section 7.1 of the bq28z610 manual also states that the cell balancing algorithm will gradually decrease any cell imbalances while in a fully charged state.  I had the cells fully charged and connected to the charger for about 3 total days without the cell imbalances being resolved.

    Could the cell imbalance be the cause for the unsuccessful learning attempts as I described, and if so, am I on the right track in trying to resolve the cell imbalance?  And do you have any other feedback based on the information I've provided?

    Thanks,

    Nick

  • 0 in reply to Justin McKenrick
    TI__Mastermind 42535 points

    Hi Nick,

    Cell balancing itself is only enabled in a learned pack with update status of 0x0E. Otherwise it will not be enabled. If you are having problems with a learning cycle, please use the previous attached document to complete that first and then attempt cell balancing. Otherwise if you just want to test cell balancing please set update status to 0x0E and Ra Flags to 0x0000 and Rax Flags to 0x0055, reset the gauge and see if the balancing works. You will see the CB bit set if it's turned on.