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Accuracy of bqWizard readings

Heye Heegardt
Prodigy 140 points
Other Parts Discussed in Thread: BQWIZARD

Hi,

we suspect that the SM-Bus data traffic of the bqWizard compromizes the accuracy of the transmitted data due to too much processor load.

We discharged the same pack twice, first time with the bqWizard logging away heaps of data, the second time the pack discharged without bqWizard. Both times we measured the delivered charge externally. There was a difference of 150mAh.

Also we discovered strange readings of the bqWizard, which should have triggered error messages from the pack, but the pack behaved just fine.

Is there a limit on SM-Bus data traffic to ensure the processor is able to perfom correctly.

Kind regards, Heye

  • 0
    Prodigy 140 points

    Addition :

    Our device interrogates our 11-cell batterypack three times per second at a signal frequency of 50kHz occupying about 10% of the time. Compared to the approx. 17% bus time of the bqWizard this seems to be quite high. We are a bit concerned about the quality of the data we get from the bqWizard considering the compromised data we got while the pack was connected to the bqWizard and behaved fine.

    At what frequency does the bq78pl114S12 operate ? Although the datasheet mentioned 100kHz, we could easily reduce the bus frequency even further than 50kHz and reduce the bus time to 7%.

    Is our assumption of a correlation between SM-Bus load and processor performance correct ?

    Kind regards, Heye

     

  • 0 in reply to Heye Heegardt
    Prodigy 140 points

    Addition 2 :

    The strange effects only occur during periods of intense balancing activity.

    Kind regards, Heye

  • 0 in reply to Heye Heegardt
    Expert 2135 points

    There's probably a lot of VLAN traffic going on during balancing so I can see how packets might be ignored to respond to a host's request for data.

    But, IIRC, the bq78PL114S12 only updates its parameters every one to two seconds. Polling more often than that won't give you any additional data and will just add the the bq78PL112S12's comm load.

    I'd recommend repeating your capacity test but polling every 1 second and then doing the test again but polling every 5-10 seconds and seeing if there is a capacity difference then. I'd also recommend repeating the tests at least twice (i.e., w/o polling, with polling, w/o, with) just to make sure that the capacity difference you saw wasn't due to any changes in the pack (temperature, etc.) due to the first discharge.