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Problem with a BQ78PL116 current measuarment

Other Parts Discussed in Thread: BQWIZARD, BQ78PL116

Hello!

I have a problem with BQ78PL116 current measuarment. I am working with 24V 8Ah battery and the problem comes up when I want to charge it with more than 10A current. BqWizard shows the wrong value (and none errors) but only when I am trying to charge it, and that is not a problem with discharge.

I would say that its properly calibrated and set up, but obviously I am doing something wrong. Hardware problem is less possible, because I tried it on 5 differents boards.

Does anyone have an idea how to resolve this problem? Where can i get more information?

Thanks in advance !
Vladimir B.

  • The device should support more than 10A charging current. What value sense resistor are you using? Can you send your tmap file?
  • Hi Thomas,

    thanks for your quick response! (please advise how to send you the Tmap-File)

    we made some further measurements and this is what we found out:

    Our BMS is using a 0.5mOhm shunt. After calibration of temperature and zero current/coulomb we made a gain calibration with -20A. The offset displayed in the calibration tool was -217 for current and -201 for coulomb. The gain was 0.896 for current and 0.886 for coulomb.

    When applying a load of -20A, the shunt voltage was corresponding to the current with a value of -11mV (our shunt seems to be 0.55mOhms in real). This value we could measure also between pin 9 and pin 10 respective between pin 6 and pin 7 of the BQ78PL116.

    The strange behaviour comes up when charging the battery with currents higher than 10A: The measured current value becomes the more inaccurate the higher the charging current is. At currents of 15A the measured value is about 14A, when increasing to a charge current of 25A, the measured value is very unstable and varies between 11 to 13A which is even less than when applying 15A charge current(!).

    The values we measured on the pins of the BQ were strange too: at a charging current of 25A we expected the values to be at 14mV. The voltage between pin 9 and pin 10 was exactly the shunt value of 14mV, however, the value between pin 6 and pin 7 was 6.2mV with a corresponding value of 11.1A displayed by the BQ.

    For us, it seems that the value of the low pass filter (R67, R68, C3) between the shunt and the BQ on its pins 6 and 7 is seeing a high input impedance of the BQ while the current is negative (discharge) and a much lower impedance when the current is positive (charge).

    Our schematics are more or less identical with the TI-EVM so we are pretty sure that we did not have a hardware issue with our PCB here.

    Does anyone have an idea what is going wrong here?

    Vladimir B.
  • So, the last report:

    After hours and hours spent on searching the defect, we found it (and we still can´t explain it but it works). The problem was cable lenght. Our battery had a very large cable and we couldnt connect it direct to the BMS so we took some other thiner / longer cable and used it as extension. We dont know why but that new cable was a problem.. after we made it shorter (10 cm) we could charge our battery with 25 A without problem and strange behaviour.

    If anyone can explain what really happened, i would be really grateful.

    Vladimir B.