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
