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Original question:

BQ77915: Cell Balance Vstart LiFePO4 Compatibility

BQ77915: LiFePO4 battery compatibility

Renan Santos Adriano
Mastermind 22380 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ40Z80, BQ40Z50, BQ77905, BQ76940, BQ76952, BQ4050

Hi Team,

My customer would like to know further if the BQ77915 (BQ7791504) is compatible to be use for LiFePO4? Can we set the UTC higher than 50°C? By changing the RTS_PU value this change affects all the other temperature trip points and scales OTD,UTC,and UTD with the largest impact to OTD. Do we have a computation on this?

Looking forward to your inputs.

Regards,

Renan

  • 0
    TI__Genius 10830 points

    Hi Renan,
    We're about to start working on a new "flavor" of BQ77915 (BQ7791506) to address LFP batteries (OVP: 3.8V, UVP: 2.3V, UTC:0).
    UTC can only be factory programmed to -5 or 0C. I need to check if we have a model for calculating the temp trip points.

  • 0 in reply to David Hien
    TI__Mastermind 22380 points

    Hi David,


    Thank you for this response.

    Do we have any suggestion aside from BQ77915 for LiFePO4? Currently they are working with BQ7790508, but when external balancing is added to the BQ7790508 the Open-Wire protection does not work anymore, which makes the IC useless. Also the fixed OTC level at 50C is too low, should be 55C.

    Looking forward to your feedback.

    Regards,

    Renan

  • 0 in reply to Renan Santos Adriano
    TI__Guru 54715 points

    Hi Renan,

    You can adjust the temperature trip points by changing the resistor network attached.  The temperature thresholds in the BQ77915 data sheet are a ratio.  The part sees a resistor ratio of VTB determine by RTS_PU/(RTS_PU + RTS).  You can adjust the trip points by changing the RTS_PU as described in the data sheet section 10.1.1.5.  You can also adjust the ratio with a resistance in series with the thermistor RTS, or in parallel with RTS.  The parallel resistor affects mostly the low temperature points since that is where the thermistor impedance is highest, and the series resistor adjusts mostly the over temperature points since that is where the thermistor resistance is lowest, but it is a single resistor network and all trip points will be affected.

    The BQ77905 and BQ77915 are the newest multi-function protectors.  Depending on your cell count you might use a gauge which is more configurable such as the BQ4050, BQ40z50, or BQ40z80.  It is a more complex part.  For higher cell counts you might use a monitor and MCU such as one of the BQ76940 or BQ76952 family devices, again the implementation is more complex.