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BQ76940: cascading of BQ76940

Anand Kishor Verma
Prodigy 10 points
Part Number: BQ76940
Other Parts Discussed in Thread: CSD19535KTT, BQ76930, TIDA-01093, TIDA-010030, CSD19536KTT, TIDA-00449

Hi I am Anand & doing BMS for EV 2W Application.

Please me know if cascading is possible with BQ76940 i m using 16 series cell Battery, since this is a 15 cell part.

For sixteen parallel cell and 16 series 90 AH battery what should be the rating for cell balancing components (MOSFET + Resister) and what is recommended cell balancing current with Li-Fe po4 batteries.

what should be the rating for charging and discharging Mosfet for 30 amp charging and 130 amp discharging max, pls do suggest TI MOSFTE if any (i see CSD19535KTT).

  • 0
    TI__Guru 54715 points

    Hi Anand,

    Stacking is not supported directly with part to part communication, both parts must talk to the host using an I2C level translator where needed and other signals combined as needed.  An example is shown in TIDA-01093 using 2 BQ76930's to support up to 20 cells. 

    Cell balancing is up to the system design and should consider the mismatch of the cells, any load difference on the cells, thermal conditions of the battery, and the frequency of the battery charging and balance algorithm. For example if part of the cells are hotter than the others in use and have higher self discharge, but the system is charged infrequently, a higher balance current may be needed if balanced only during charge.

    MOSFETs are often selected about 2x the battery voltage.  The current rating of the CSD19535KTT seems to handle the current you note, but with thermal design considerations you may need 2 or more in parallel to keep the operating temperature in a safe range. TIDA-00449 and TIDA-010030 show examples of the similar CSD19536KTT used in parallel.  Information on TI MOSFETs may be found at http://www.ti.com/mosfet.  Other industry references may be helpful with thermal designs.