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BQ77915: Managing the balancing of 5-series LIC supercapacitors in charge-discharge cycles

Peter Melling
Prodigy 10 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ24640

I want to know if the BQ77915 will manage LIC supercapacitors - 5 in series configuration. I plan on using BQ24640 CICV charger and limiting charge current to charge the cap stack but need to ensure balancing is maintained during the charge cycle.  I also plan on using load switches in the charge/discharge modes of the application.  As you will know, the LICs are rated 3.8v max discharging to 2.5v min so the stack will drop from 19v when charged to 12.5v min when discharged.  I'd like some feedback before ordering the EVM.  Thanks.

  • 0
    TI__Guru 54715 points

    Hi Peter,

    The BQ77915EVM has a BQ7791500 device installed.  For your levels it seems the BQ7791506 may be a closer match, it would target a 3700 mV full charge voltage.  You would need to change the IC for an effective evaluation with the caps.  Balancing with the -06 can start at 3500 mV.  See the data sheet for a description of the balancing, and the video on the product folder may also be  helpful.

    The BQ77915 will balance during charge, how effective it is will be determined by the rates.  With cells if an imbalance is not balanced out on one charge cycle it may be on the next, unless the cause continues to accumulate an offset which is too large for the duration of the charge with the balancing current selected.  The similar effect may occur with supercaps  also.  Supercaps have had a reputation for higher leakage during idle such that an imbalance may develop easily.  Check the specifications on the cap you have selected to see if that may be a concern.  The BQ77915 does not balance during idle unless you set it as an upper device. 

    I don't know that the part has been tested with supercaps, as with any design you should evaluate carefully if you plan to use it.