• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 62 subscribers
  • Views 84 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77915: 10s pack with seperate top and bottom power paths

Thomas McGilvary
Prodigy 30 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ76942

My design consists of a 10S li-ion battery pack.     The power distribution is seperated between the top 5 cells and the bottom 5 cells.   The discharge is uneven between the top and bottom, with the top discharging at double the rate of the bottom.       

This results in low battery cutoff by the top, when the bottom still has ~50% charge left.  At this point, the battery is disconnected from the "tool"  

I would like to use the BQ77915 for balancing during the charge cycle.   If CC/CV charging is done at 1A, then the lower 5 cells will achieve full charge before the top, and therefore balancing is needed to bypass the "entire" charge current for the bottom cells while the top cells continue to charge.

Is my thinking correct here?

Thanks for any assistance!

  • 0
    Prodigy 30 points

    Also, to bypass the 1A charge current, I assume I need external fets?

  • 0 in reply to Thomas McGilvary
    TI__Mastermind 19420 points

    Hi Thomas,

    I'm curious why the load on the cells is significantly different based on the group. I would expect that all the cells are discharging at the same rate (ignoring subtle differences in the cells themselves).

    Unfortunately, I don't think cell balancing will work in this situation. The devices will balance all of their own cells to the same level, but they do not have the functionality to consider the other device's cells. So, cells 1-5 will all match each other and cells 6-10 will match each other, but cells 1-5 and cells 6-10 will not necessarily match.

    In your case this would most likely end up with the lower cell group charging up to the OV threshold and the CHG FET being disabled before the upper cell group can reach full charge. Over time this difference would only get worse.

    This is the main drawback with a stacked system that does not have the ability for a single MCU to control the cell balancing.

    You may consider looking into the bq76942, which is a 3-10s monitor with support for both autonomous cell balancing and MCU controlled cell balancing.

    Also, yes, for any balancing current on the bq77915 more than 50mA you will need external FETs.

    Regards,

    Max Verboncoeur