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Original question:

BQ77915: pack+ and pack- inquiry

BQ77915: BQ77915

Karl Thorwart
Prodigy 150 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ77905

Hello all together,

Im using the bq77915 for multiple applications and it works fine except the short cut time is too long. But this is not my question here.

Now I want to develop a new BMS with the bq77915 or bq77905 that controls and balances a pack of 9 cells in series.

But it is not clear to me how the balancing between the chip for the lower cells and the chip for the upper cells works. I cannot see a connection for a communication or an overlapping of cell connections.

How does it work?

Many thanks

Katl Thorwart

  • 0
    TI__Expert 4140 points

    Hi Katl,

    Most of our team is out today, but someone should be able to respond to your question early next week.

    Thanks,

    Terry

  • 0 in reply to -Terry
    TI__Guru 54715 points

    Hi Karl,

    There is a cell balancing video in the product folder Videos section but mostly it is on the voltage operation with a mention of the interface between parts.  The interface is shown in figure 19 of the BQ77915 data sheet.   The 10M value for ROCDP for the top part is required so that it does not look for current to balance.  RCB is the primary communication from CBO of the bottom part to CBI of the upper part.  Fault status is also used in balancing control, it is communicated down with CTRC, CTRD.  The parts don't share voltages levels, they should use the same part number so the thresholds are the same for the 2 (or more) parts.  

  • 0 in reply to WM5295
    Prodigy 150 points

    Hi Willy,

    In the video I can see that the lower component recognizes that a charging current is flowing and then reports to the upper component that balancing is allowed. What happens if e.g. the lower cells are perfectly balanced at 4.2 volts and the upper cells are perfectly balanced at 4.1 volts. However, the charger continues to charge, since a charging cut-off of 4.2 volts per cell is set on it. The lower cell voltage will rise to over 4.2 volts per cell (Vov) until Vov switches off the charging path.
    In the event of overvoltage, the balancing remains active until the voltage falls below Vov - hysteresis.
    If I've described it correctly, I think I've understood the balancing of this chip.

    many thanks
    Karl

  • 0 in reply to Karl Thorwart
    TI__Guru 54715 points

    Hi Karl,

    Yes, I think you understand it.  The BQ77915 does not have a well calibrated voltage measurement with balancing managed by a MCU as is possible in a gauge.  The 2 chips will balance with the same thresholds based on the control.  If the cells on one BQ77915 part drop equally so that they remain balanced for that part there is no balancing action.  If/when the other part is pushed up to the OV balancing point, by charging, those cells will balance down.  I expect there could be a couple charge cycles where the cells monitored by that formerly higher voltage device have a wide difference, then are balanced to be lower than what was the lower device.  If the trend continues in the pack with one part discharging more than the other this likely cycles over the life of the pack.