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BQ77915: BQ77915 Charger connection

Thomas McGilvary
Prodigy 30 points
Part Number: BQ77915

I am attempting to understand the balancing feature of the BQ77915.     When I hook up a LIPO charger(either a custom design or just a CC/CV COTs charger)   like the 

NPB-120-24TB    https://trcelectronics.com/products/mean-well-npb-120-24tb?utm_term=&utm_campaign=Sales-Performance+Max-4&utm_source=adwords&utm_medium=ppc&hsa_acc=9805199880&hsa_cam=21058039387&hsa_grp=&hsa_ad=&hsa_src=x&hsa_tgt=&hsa_kw=&hsa_mt=&hsa_net=adwords&hsa_ver=3&gad_source=1&gclid=CjwKCAiAxaCvBhBaEiwAvsLmWLc-1r7zRB7zxJ9hTWeLf8McHotUYb6ct9wuTNq-Ad2sVYph6h6ClBoCUwwQAvD_BwE

to a 5S lipo pack.

During CC phase the current will be 2A.   How does this effect the max current(50mA) on the internal balancing fets?  Will everything work OK?   Do I need to use external fets?    See attached image

 

  • 0
    TI__Mastermind 25005 points

    Hi Thomas,

    The internal balancing current is going to be determined by the cell voltage, the value of the input resistors (R_in), and the R_dson of the internal FET.

    The balancing current can be calculated as V_cell / (R_in + R_in + R_dson). The typ value for R_dson is 12ohms.

    I strongly recommend watching this video (link) on how cell balancing works on the bq77915. The algorithm can be a little confusing based on just the datasheet's explanation and the video has a lot of diagrams to help explain it better.

    Regards,

    Max Verboncoeur

  • 0 in reply to Max Verboncoeur
    Prodigy 30 points

    Thanks Max,    I watched the video and generaly understand the cell balancing.    More specifcally I wanted to understand the connection of a CC/CV charger to the battery pack when this circuit is implemented.    If the pack is not full, then the charger would implement constant current charging which would "force" 2A into the pack(and chip fets?).    With balancing active, where does the additional current go?  Does the constanct current of the charger need to match the balancing current of 50mA to prevent overheating?    Thanks

  • +1 in reply to Thomas McGilvary
    TI__Mastermind 25005 points

    Hi Thomas,

    The current from the charger will go through the cells. When the cell balancing FET is enabled, it will divert a small amount of current (determined by the equation from before) around the cell and through the IC. This makes the cell charge slightly slower than the rest of the cells to allow them to catch up. You won't need to match your charging current to the 50mA current, just pick the resistors so that the internal balancing current does not exceed 50mA.

    Regards,

    Max Verboncoeur