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Cell balancing with bq77908A

Carl Gallenson
Prodigy 210 points
Other Parts Discussed in Thread: BQ77908A

I am trying to understand the voltages and power ratings associated with the external resistors that connect to the VC1-9 pins of the bq77908A.  The data sheet says balancing current must be held to 50mA maximum, but what voltages are assumed for that?  50 ohms is the minimum recommended external resistor, so if 50mA was flowing through that resistor, it would mean 2.5V was being dropped across that resistor.  Is that correct?  Should one design using the maximum cell voltage for this calculation?

I am trying to determine the required power rating of these resistors, as I am currently using 0603 size (100mW) resistors and I am unsure of what currents will flow with a value of 50 ohms or 100 ohms.  The EVM document (SLUU854) states in section 7.3 that "balancing current is approximately the cell voltage divided by twice the resistor".  How is that derived and is that a good rule of thumb?  Or would you just add the external resistor to the Rds on of the internal FET, which varies with the voltage, and divide your maximum cell voltage by that value? 

If an out of balance cell is at 4V, and we use Rds on of the internal FET = 10 ohms with external resistor = 50 ohms, is the current going to be 4V/60ohms = 66.7mA?  Or would you assume 4V/2*50 = 40mA?  I wish to size my resistors for the maximum possible balancing current that can be handled with an 0603 size resistor.

Thank you,

Carl 

  • 0
    TI__Guru 54715 points

    The current limit is independent of the cell voltage.  When the balance current is flowing in the device, the pins are basically shorted together by the internal Rdson.  Current will flow from the top of the cell through one Rvcx, into the IC through VC(n), through the internal balance FET and out Rvc(n+1) through another Rvcx to the bottom of the cell.

    The recommended operating conditions don't all coincide, but they fit in the abs max ratings.  At the maximum recommended balance current is 50 mA and assuming 0 ohm internal balance FET Rdson, the minimum recommended 50 ohm Rvcx would give a cell voltage of 0.05 A x (2 x 50 ohm) = 5V which is the abs max for VC8.  Perhaps this is where the limit came from.

    Somehow you will need to account for tolerances and may want some margin in your design.  A good first estimate may be to consider the internal Rdson = 0 and calculate based on either the maximum nominal charge voltage or the protection limit and the current through 2 Rvcx resistors.  Then you might re-calculate with expected possible cell voltage rise due to temperature effects to be sure you are still within maximum ratings or other suitable  limits. If you are designing for power, use the appropriate equation.