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Original question:

BQ76952EVM: Missing Sub-commands, Example Code

BQ76952EVM: Practical limits to cell sizes for the BQ76952

Brent Singer1
Prodigy 70 points
Part Number: BQ76952EVM
Other Parts Discussed in Thread: BQ76952

Just wanted to touch base on a practical question about cell capacity and and balancing. How big can I go before I need to have move to external balance FETs?

At the moment i'm using LFP/3.2V cells at 6.5Ah each. But they get bigger, and I'm considering teaming a few. Do you have any reccomendations on how many Ah to put on a single BQ76952 chip?

And any updates on when silicon is available with the cell balancing enabled? 

Thanks in advance, Brent. 

  • 0
    TI__Guru* 81034 points

    Hi Brent,

    If you are using internal balancing the cell balancing current will be around 50 mA. This is assuming the recommended 20 ohm cell input resistors and a typical cell balancing input resistance of 25 ohms. With 4.2V on each cell, that will be ~65mA steady-state: 4.2/(20+20+25). Since the device duty-cycles to all measurements, the actual current will be lower, let's assume 50mA. Since you are using LFP which is 3.2V, the current will be lower than 40mA.

    Balancing can be configured to take place only during charging (many applications choose only to balance during charging to not waste any energy when disconnected from the charger) or it can also be configured to balance during rest. You can calculate the time to balance based on the size of the imbalance you would expect. This will also depend on how many cells you have in series if multiple cells need to be balanced.

    There are also power considerations for how many cells to balance at the same time with internal balancing. For your cells, the power dissipated (R*I^2) is around 40mW per each cell balanced. The device has a 66C/W junction vs. ambient theta-j. Balancing 8 cells at the same time will be about 320mW which will give a junction temperature rise of about 21C.

    The current prototypes available do not have cell balancing enabled. The devices with balancing will be available in 3Q 2020.

    Best regards,

    Matt