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bq40z50: Registers Cell Imbalance

Kevin Knicker
Expert 1091 points
Part Number: BQ40Z50
Other Parts Discussed in Thread: BQ29312A

Dear TI,

I've run into a phenomenon where the bq40z50 will show a cell imbalance even though there isn't one.

In the photo below, note that Cell 3 is showing 0 mVolts and Cell 4 is showing 6051mV.

The cells themselves are actually measuring the same voltage (about 3647mV) as Cell 1 and Cell 2 are.

In the past I've seen this on the bq2084 and bq29312A chipset where if a proper cell connection sequence wasn't followed, this would happen.

Could that also be the case here?

As far as I know they had been reading proper voltage so I'm also wondering if something else could have caused this.

Thanks for any feedback,

Kevin

  • 0
    TI__Genius 14985 points
    Hi Kevin,

    Was this unit working fine and reporting all four voltages correctly and then suddenly began reporting these values? Does it recover if you reset the device or power cycle it?
  • 0 in reply to dMax
    Expert 1091 points

    Hi dMax,

    Yes, everything was normal when this happened.  Resetting or power cycling doesn't recover, so it appears to be internal to the bq40z50.

    I've also seen instances where the the bq40z50 will draw excessive current from the cells until they become very low and mismatched (I have a separate post on this issue).  After removing the board and connecting it to a fresh set of cells, the bq40z50 reports correct and matching voltages for all four cells.

    I think the main issue stems from the bq40z50 drawing excessive current from the cells.  I think it may be because the design we used didn't need LED's so they were pulled low based on an earlier TI recommendation.  It appears that has since changed and it is recommended that they either be floating or have a resistor in line to Vss.

    I feel as though the datasheet should probably be updated as it states unused LED pins can be tied to Vss.

    Best Regards,

    Kevin