BQ76952: BQ76952 cell balancing questions

Hi Miles,

I have added comments below:

1. If set 100mA as the internal balancing current, how many channels could perform the cell balancing at the same time?
   1. You cannot really reach 100mA of balancing current using the internal cell balancing, since the switches are approximately 28-Ohms (typ), so with external 20-Ohm series resistors and a 4.3V cell, this results in maximum 63mA balancing current.  The actual average balancing current will be somewhat lower, since this current is disabled periodically to allow for cell voltage checks and COV/CUV checks to occur.

   2. The number of channels that can balance at the same time depends on their thermal conditions.  The 63mA I mention above will dissipate ~112mW within the device while balancing.  The device can autonomously balance non-adjacent cells, for a maximum of 8 cells simultaneously.  If that were done in this case, then 896mW would be dissipated within the device.  Given the theta-ja of the device is 66degC/W, this results in a temperature rise of approximately 59degC at the device junction.  if you keep the device junction at a maximum of 85C, then the ambient temperature should not exceed 26degC.  Given this doesn't seem practical, it is then best to either reduce the current (by making the series input resistors larger) or reduce the maximum number of simultaneous cells that can be balanced (using the Settings:Cell Balancing Config:Cell Balance Max Cells data memory parameter)

2. If cell balancing is performed, what is the maximum duration time of the balancing?
   1. The device can be programmed to autonomously balance, in which case it can continue balancing until the cells are balanced.  Note that it interrupts balancing periodically to measure cell voltages and to perform COV/CUV checks.  If manually controlled balancing is performed, then the device will only continue balancing for up to 20-sec, unless the host re-initiates the balancing.  This is to avoid a case where the host started manual balancing, then the host locked up and never responded again, and the balancing continued indefinitely.  So in the manual case, the host must send a new balancing command every 20-sec to keep balancing going.  When balancing is on/off, what is the turn-on/turn off time of this activity?

3. When balancing is on/off, what is the turn-on/turn off time of this activity?
   1. When balancing is enabled and active, it will still be disabled periodically automatically to allow for cell voltages to be measured and COV/CUV checks to be performed.  The COV/CUV checks occur on the balanced and adjacent cells every 1-sec while balancing is active.  The balancing is disabled for measurements depending on the speed of the measurement loop (see Settings:Configuration:Power Config[CB_LOOP_SLOW_1:0] data memory parameter).

Hi Miles,

The application note https://www.ti.com/lit/pdf/sluaa81 may be helpful in understanding the BQ76952 cell balancing.

1. The number of channels which can balance at the same time will require a thermal calculation for the design involved.  See https://www.ti.com/lit/an/spra953c/spra953c.pdf and the example in 2.1 of the application note above.

2. The maximum time allowed for balancing is determined by the system design.  The maximum time which can be set for the Cell Balance Interval is 255 seconds, but this is an evaluation time.  If autonomous balancing is configured it balances to voltage thresholds, see the technical reference manual section 10.1

3. I don't know that we have a good description of this timing since there are so many variations possible.  Most commands will take a ms or 2 to operate. Since balancing operates in the measurement loop, it could take the remainder of the loop to complete the activity.  See the examples in the sluaa81 application note section 5 and table 4-2 in the technical reference manual.