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BQ76940: Disabling Features, External Chargers, and Multiple Packs

William Kalfus49
Prodigy 20 points
Part Number: BQ76940

I have a couple of questions pertaining to the operation of the BQ76940. Essentially, I would like to embed the chip into my application for use solely as a cell voltage monitor, rather than a charger/discharger or balancer. This being the case, I would like to know about operation under the following conditions:

1. Is it possible to operate the BQ76940 with no current detection, i.e. either by disabling something internally and/or by connecting SRP and SRN together?

2. Is it possible to leave the chip connected to the cells when using an external balance charger?

3. Is it possible to operate the chip without the charge/discharge FETs?

4. Suppose I am designing around using 12 cells, in two 6S packs connected in series. Would it cause any other problems if I use separate external chargers on the packs individually, while connected to the chip? Or would I need to use one big charger that can charge all 12?

Thank you!

  • 0
    TI__Guru 54715 points
    Hi William,
    1. Yes, the part can be operated without current detection. The CC can be left disabled. The OCD/SCD protections can not be disabled, but if SRP & SRN are connected to VSS they should not occur. In the statistically possible case when anything can happen if a SCD or OCD fault was set it could be cleared.

    2. Yes, cells could be monitored by multiple devices of different types.

    3. Yes, CHG and DSG are outputs and can be left un-connected. If you want the fast response of the OCD/SCD output you may want some logic on the pins for signaling. If you have a SCD event which does not turn off quickly and allow the cell voltages to recover the bq76940 will likely loose power and need to be re-booted after the event.

    4. If you use appropriate isolation there should not be a problem. A problem could occur if you have the 12S battery negative referenced to chassis GND. If you charge the lower 6 cells to 25V, then attempt to connect the charger to the upper 6 cells and the charger is referenced to the same chassis GND, you will short the lower 6 cells and have high current. Again, be sure to use appropriate isolation, check for unexpected ground paths and any industry standards which may limit architectures in your equipment.
    Also be aware that if the 12S battery is discharged and the user charges only one 6 cell group to full, from a usability standpoint the battery is not half charged but still fully discharged. The energy in the cells does not even out. The bq76940 will report the voltages of the connected cells, some high, some low.