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BQ77915: Schematic review

Olli-Pekka Koski
Prodigy 10 points
Part Number: BQ77915


Hi,

We chose to use BQ7791501 for battery pack protection and cell balancing. We will use 32pcs of 4900mAh Li-ion cells so that 8pcs will be in parallel and four of these packs in series, making a 39.2Ah / 16,8V battery pack. Charger voltage is 16,8V and max current 17A.

33ohm resistors were chosen to set the balancing current to ~50mA as on the datasheet example. Current sense resistor of 1mohm is selected for a 35A maximum discharge current and 20A charging current.

What I haven't understood is why there are two different OCD values, OCD1 and OCD2? For example in our case the OCD1 threshold is -35mV so with a 1mohm current sense resistor the voltage will go below the limit at 35A if I'm correct. OCD2 voltage threshold of -60mV will never be achieved.

Please review the schematic attached.

bms_circuit_revA001.pdf

Kind regards,

Olli-Pekka

  • +1
    TI__Guru* 81035 points

    Hi Olli-Pekka,

    The schematic looks good - I do not see any issues. The different OCD thresholds are common - some applications may want to limit a higher current faster (some of the device number options show configurations like this). We have some battery monitors and gauges that have 3 or 4 different current thresholds.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 10 points

    Hi Matt,

    Thank you for your reply. Do you think 50mA balancing current is enough for 8 cells in parallel or should we use external FETs?

    Kind regards,

    Olli-Pekka

  • +1 in reply to Olli-Pekka Koski
    TI__Guru* 81035 points

    That's not easy for us to answer since it is really dependent on your cells and system needs. If your cells are really closely matched, you may not need to balance too often. With several cells in parallel, I often do see users choose to use external balancing because it might take too long to balance with 50 mA. Another thing to keep in mind is the BQ77915 only balances during charge, so you need to consider how much time your battery takes to charge.

    Best regards,

    Matt