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BQ25750: Integrating a DALY smart BMS with the BQ25750 Charger IC

Mahmood Fares
Prodigy 20 points
Part Number: BQ25750

Tool/software:

Hello,

I am designing a 6S2P UPS using the BQ25750 to charge the batteries and perform power path management for the system rail as well. Within the design I want to integrate one of DALY's smart, active balancing BMSs. I have been trying to workout how to wire the system, considering that their BMS is wired in series with the cells (the load current passes through the cells and the BMS for monitoring), as shown in the attached wiring diagram . 

Looking at the wiring schematics of the BQ25750 in the datasheet, I should use the FB and FBG terminals for accurate battery voltage monitoring, and as mentioned in the datasheet the FBG pin is grounded to PGND during charging. Considering the inclusion of the BMS as highlighted in the wiring, how should I connect the BQ25750 to ensure it senses the correct voltage of the cells? If I connect B- to FBG of the IC, during charging FBG would be shorted to PGND, effectively shorting out the BMS, since  P- of the BMS shall be connected to PGND.

On the other hand if I connect FBG to the P- terminal of the BMS, or effectively to PGND, the voltage sensed by the IC would not accurately represent the battery voltage as it would also include the voltage drop across the BMS. In furtherance, the datasheet makes reference to PGND and AGND in Figure 9-1, but without specifics; just that AGND is connected to the sensing terminals. Could this be part of the solution?

On a final note, will the inclusion of this BMS interfere somehow with the BQ25750's ability to sense VAC abnormalities and to directly switch the load to battery power seamlessly. The UPS is meant to supply a critical load and switching shall be almost instantaneous (max 1ms of 10% droop is allowed - System rail shall be 24V and the maximum load would 3A)

Your suggestions for solutions to this matter would be much appreciated.

Regards,

Mahmood

  • 0
    TI__Mastermind 31400 points

    Hello Mahmod,

    Thanks for working on this. You wouldn't need to connect FBG to any point of the BMS. The FBG pin provides a path to GND though the IC for the resistor feedback divider. Here's an example of the resistor feedback divider from the EVM:

    In this case, I think BAT would be connected to P+ on the diagram.

    Mahmood Fares said:
    will the inclusion of this BMS interfere somehow with the BQ25750's ability to sense VAC abnormalities

    No, this shouldn't change the BQ25750's functionality.

    Mahmood Fares said:
    The UPS is meant to supply a critical load and switching shall be almost instantaneous

    For a fast and reliable supply for a critical load, I'd recommend using the BQ25750's automatic reverse mode to switch system power from the input to the battery.

    We also have a FAQ for the BQ25750's power path switchover for more information.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 20 points

    Hello Ethan,

    Thanks for your reply. 


    My concern now is regarding the accuracy of the monitored voltage for the Battery Cells. If the voltage divider is effectively only monitoring P+ with respect to ground, and the BMS is actually placed in series with the cells, then the overall voltage being monitored is that of the cells + the voltage drop across the BMS. Surely the drop within the electronics of the BMS wouldn't be significant (considering my load wouldn't exceed 3A at this stage of the design); nevertheless it's a variation from the real voltage across the cells themselves. Your insights would be appreciated on how to mitigate this loss of accuracy.

    On another note, please refer to my calculations below for Rbot, which show that the equation for Rbot should have -RFBG rather than +RFBG as represented in the datasheet. If confirmed from your side, kindly see to it that the datasheet is revised with an update for everyone's future benefit. Thanks. 

    Datasheet Extract:

    Regards,

    Mahmood Fares

  • 0 in reply to Mahmood Fares
    TI__Mastermind 31400 points

    Hello Mahmood,

    Thanks for letting me know that the BMS is in series with the battery cells. In that case, VBAT in the diagram needs to be tied to the output of the BQ25750. A BMS in series with the battery should have no system issues.

    A BMS will likely have a resistance from the charger to the battery cell. This resistance should increase the time the charger spends in the taper charge phase, but this should not be a critical problem.

    Mahmood Fares said:
    On another note, please refer to my calculations below for Rbot, which show that the equation for Rbot should have -RFBG rather than +RFBG as represented in the datasheet

    Yes, this is correct. I'm working on updating this in the datasheet.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway