BQ76940: BQ76940

Hi Rajaneesh,
Your diagram will not work, the bq78350-R1 does not share the I2C bus with another host. While a generic I2C might be shared, in this case it is a dedicated bus and must not have other communication. Your MCU could talk to the SMBus port of the bq78350-R1 if you want to use the gauge.
Regarding your questions:
1. Yes, the bq76940 will support parallel cells in addition to series. The monitor sees all the parallel cells as one, the same voltage.
2. The bq76940 has protection features as you note. The bq78350-R1 provides additional protection features such as over and under temperature, over current charge, protections with longer times and calibrations, and in some cases multiple levels/times. It also provides a balancing algorithm, gauging and fault recovery.
3. No, you do not need the bq78350-R1 gauge, it provides its suite of features through parameter configuration. You could implement your specific set of desired features in your MCU. Reasons for the MCU might be custom behaviors, special interfaces or lower cost. Reasons for the gauge might be simpler development.
4. The bq78350-R1 gauge reads the coulomb counter and maintains passed charge information. If not using the gauge your MCU would need to do that.
5. You do not need to use external cell balancing with the bq76940, but currents are low with the internal balancing. Cell balancing may be needed to adjust for the leakage and offset currents of the bq76940 which could affect the cell voltage over time. If your system has other loads or imbalance sources such as temperature you may want higher balance current. The amount of balance current you want will also depend on how your balance algorithm is implemented and how often your system is cycled. The bq78350-R1 balances only during charge at the end when the IxR error of the cell should be low. If you use your MCU you may choose a different algorithm and have more time to balance. The bq76940 supports only passive balancing. Active balancing would be possible with the voltages obtained from the bq76940, active balancing is a more complex and expensive system.
6. The 2A adapter will charge a larger battery, charge time will be longer for larger capacity batteries than smaller. Generally the concern with cells is charging at too high of a current, but cells will have a cutoff current at full charge as well. Check with your cell supplier for charging requirements for your cells.

Hi Rajaneesh,
Either option should work well.
With Design 1 you will need to design your protection limits and behaviors in your code. OCD, SCD, OV and UV are available in the bq76940 with the timings available. For OCC and other limits you will need to read values from the bq76940 and set limits. This gives you the best flexibility to get the system behavior as you want it. You will also need to design gauge and balance algorithms. This approach gives you the best flexibility such as being able to design balancing to match the cells voltages after assembly. But this takes programming.
With Design 2 the protections will be designed in the bq78350-R1, you simply configure them with the parameters provided in the ranges allowed. There is a lot of flexibility, but the behavior is fixed within those limits. With balancing for example it will balance only during the end of charge, it won't balance at rest. From your diagram with this choice your MCU programming would be mostly for the display and/or user interface although you could still do your own gauging with the data from the bq78350-R1 rather than using its gauge calculations.

Hi Rajaneesh,
1. Correct, the bq78350-R1 would be the battery controller. The MCU would provide user interface and some limited controls, but generally the bq78350-R1 algorithms will operate as configured. The MCU can't make the bq78350-R1 balance outside its design.
2. Yes, the bq78350-R1 makes battery data available for the MCU to use for other decision making. See the technical reference manual for the behavior details and communication.