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BQ78350 coulomb counter went wrong after shutdown

Fred P
Expert 2235 points
Other Parts Discussed in Thread: BQ78350

Following a full discharge cycle matching the expected FCC by 0.1% (very good!!), I charged the pack to about 55% (~7300mAh).

I sent (twice) the shutdown command to my BQ78350. It went down properly, or at least appeared to do so.

I disconnected all the cells then reconnected them.

After booting the BQ, the coulomb counter went from about 7300mAh to about 10700mAh.  I saw it live in Battery Studio. SOC went up from about to 55% to about 80%.  SOC, FCC and coulomb counter all match but where does that extra 3400mAh come from???

It is not the first time I do a shutdown and cells disconnect.  It always appeared to stay at the same SOC after rebooting.

Now that I try charging the pack, I quickly get an overcharge fault because the coulomb counter reaches FCC+OC Threshold.

What could have happened?

How can I put the coulomb counter back on track again?

Thanks

Fred

  • 0
    TI__Expert 6470 points

    Hi Fred,

    You didn't mention anything about how you selected the correct Chem ID for your cells. Did you find your model in the list, or did you use some chemistry matching tool?

    In any case, it would seem that the cells you are using are not matched well with the initialization curve programmed into the gauge. On power-up, the bq78350 measures the cell voltages then assumes that they are relaxed as it looks up the corresponding best guess at the initial state of charge. It will not "remember" the state of charge from before as that is not something which is persistently stored in flash.

    Regards,

    Doug

  • 0 in reply to Doug Williams4811
    Expert 2235 points
    The right Chem ID is set in the BQ78350. As a matter of fact, I posted question recently about that (e2e.ti.com/.../396873).
    I will double check that it is correctly set.

    Anyhow, since that will happen only once in a lifetime, I can live with that wrong SOC at first boot.
    I have fully discharged the pack and at one point the SOC stepped down drastically. It went from around 30% to less than 10%. I did not see it live but noticed that it had stepped down.
    I am now charging the pack and it looks like the SOC is back on track, from a current perspective at least.
  • 0 in reply to Fred P
    Expert 2235 points
    I am using chem ID 0x2012 for the NCR18650B. This is all good.

    I guess the associated curve is the OCV for that cell.... right?
    I don't know for the NCR18650B but some cell have quite a flat spot that makes the SOC estimation from OCV very sketchy.
  • 0 in reply to Fred P
    TI__Expert 6470 points
    Yes, that's correct. When its in the flat region, the quality of the gauge's voltage calibration becomes quite important. Also, its important that the cells are completely relaxed.