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TIDA-00792: AGV Battery Design Using Reference BMS

Yusuf KAYA
Intellectual 630 points

Part Number: TIDA-00792
Other Parts Discussed in Thread: BQ76200, , BQ76940, BQ78350-R1

Hi,

I have designed a BMS using BQ78350-R1, BQ76940 AFE and Bq76200 Mosfet Driver and my BMS is nearly the same as TIDA-00792 Reference Design. The Li-Ion battery pack with this BMS is working properly, everything about the battery pack is well. However, I have designed this battery pack for AGV (Automated Guided Vehicle) device. The AGV is operated by Lead Acid Battery without any problem but it is not operated by our Li-Ion battery pack. When we monitors the motor drivers inside the AGV using motor driver interface, drivers cut off the output because of the Over Voltage Alert. However, output of the battery pack is never cut off.

I think, in this case It is necessary to explain, the AGV' s motor drivers generate electric energy and it wants to charge it back to battery pack. (regenerative system). I am concerning that the problem arises from here. Because when I read the datasheet of the motor drivers, it says that the electric energy generated by motor drivers must be charged to battery pack or consumed across a shunt resistor.

Do you have any suggestion to solve problem or experience about that?

Thanks in advance.

  • 0
    TI__Guru* 81035 points

    Hi Yusuf,

    I don't have direct experience with this type of system, but it sounds like you are experiencing an over-voltage fault because the motor driver is trying to charge the pack beyond the cutoff voltage. This will cause the CHG FET to open and the DSG FET will remain closed. It seems like you will need to find another place for this energy to go when the Li-Ion pack is full since it is very dangerous to over-charge the Li-Ion pack.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 630 points

    Hi,

    Thank you for your message.

    I am monitoring battery status using bq studio interface. Charge and discharge mosfets are logic 1 and there is no action when over voltage protection occurs in motor drivers. So It is not easy to understand why the drivers can not deliver the electric energy back to battery pack.

    Thank you for your interest. If you have another suggestion, please let me know.

  • 0 in reply to Yusuf KAYA
    TI__Guru* 81035 points

    Hi Yusuf,

    That is not correct. A true over-voltage fault will disable the CHG FET. See Table 1 in the BQ76940 datasheet. 

    Either way, you do not want to charge the battery if it is in an over-voltage state. This is not safe and this is the reason there are protections to prevent this.

    Best regards,

    Matt