I'm working on designing an open source 12V LiFePo4 BMS, 100A maximum charge and discharge, coupled to an Arduino Zero and 2.8" TFT LCD for control and status of the pack. I'm designing this system using the BQ76920 / BQ78350-R1 combo.
I'm trying to figure out the application information regarding external balance FETS as referenced in slua749.pdf page 7, where it says:
External cell balancing can be used with the bq769x0 family. An external FET is switched to draw current
from the cell through a resistor. Control for the FET comes from the voltage across one of the Rc input
resistors. When P-channel balance FETs are used the upper resistor is used, see Figure 5. When N-
channel balance FETs are used, the lower resistor is used, see Figure 6. A FET with a defined R DS(ON) at
approximately 1⁄2 the cell voltage is desired. These FETs will typically have a low maximum V GS , so the
gate voltage will usually need to be protected by a zener diode. The gate voltage should be connected
through a resistor to limit the current when the diode conducts. During normal operation the zener will not
conduct. During a heavy load event such as a short circuit, the cell inputs will drop near battery- while the
IC VCn pins will initially be at their normal voltage as shown in Figure 7. The zener diodes will prevent the
high voltage from reaching the gate and most of the input resistor voltage will be dropped across the gate
resistor. The gate resistor current will contribute to the drop of the Cf capacitor voltage, so the gate
resistors should be large. When the short circuit is released, the voltage will reverse on the input filter
resistors and gate protection zeners will conduct in the opposite direction.
1. When it states that the RDS(on) should be half of the cell voltage (charged should be around 3.6v), does this translate into roughly 1.8Ohms or 1.8mOhms?
2. Is it possible to run the external balancing circuit in parallel with an identical circuit that uses an LED with current limiting resistor instead of a power resistor to indicate that the cell is being actively balanced?
3. What would be a nominal value for the gate protection zener diodes using N-FET?
4. Would using 2 0.002Ohm 5W sense resistors in parallel utilize the full current sensing range of the AFE without overloading it with up to 100A currents?