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bq274xx Design Capacity and Energy Configuration Values

Jim Gavin
Intellectual 920 points

I am using the bq27421 gauge with a 1000mAh (typ.) battery.  The 1000mAh is the capacity from 4.2V down to 3.0V.  My application requires a minimum battery input voltage of 3.5V, so I will be configuring the bq27421 with a Terminate Voltage of 3500mV.  That said, when I configure the Design Capacity and Design Energy do I need to adjust for the higher terminate voltage?  I was told by the battery manufacturer that I can expect 98% capacity from 4.2V to 3.5V, so should I use a design capacity of 980mAh and a design energy of 3,626mWh (3.7 x 980 = 3626)?

  • 0
    TI__Guru* 76230 points

    Hi Jim,

    If you are going to run a learning cycle so you learn your cells resistance, you will need your design capacity to be the same as what is in the manufacturers data sheet and you will need to discharge fully to 3. After learning is done you can now adjust design capacity to correspond to the capacity of 3.5 V (i.e 3626mAh). Design capacity is used in the learning cycle algorthm and with state of health calculation. If state of health is an important parameter whichou monitor then to you then the design capacity needs to be adjusted after learning as i just described, otherwise it can be left with as is.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Intellectual 920 points
    Hi Onyx,

    When you say "run a learning cycle" are you talking about with the bq27421-EVM or with the bq27421 on the target application hardware?
    I should mention that my application is powered only by the battery and the battery is removable and charged in a separate charger device, so the battery is not connected to the bq27421 while charging. Because of this the bq27421 will have lost all previous RAM data, so I need to reconfigure it with the Design Capacity, Design Energy and Terminate Voltage every time the battery is inserted. This is the main reason for determining an accurate value for the Design Capacity. Also, because of the way it is used, I am not interested in the state of health calculation.

    With all that new information, what value should I be using for the Design Capacity?

    Thanks,
    Jim
  • 0 in reply to Jim Gavin
    TI__Guru* 76230 points

    Ok that means you do not plan to run a learning cycle to learn the resistance. This meas that if the gauge looses power due to the battery swapping, you loose gauging information and since you are just programming only those four parameters the guaging accuracy will keep drifting off as the cell ages because the gauge will keep using the default resistances each time a battery is inserted even though the battery's actual reistance might be higher due to aging. You might want to include resistances  table and Qmax in addition to those four parameters that is written to the gauge so the gauge maintains some form of accuracy as the cell ages.

     

    Also can you verify that your cells are lithium cobalt oxide cells because that is the kind of cells that guage was designed for. If your cells are not then you will have to do a chem id match to ensure that your cells are a match to the preprogrammed chem id on the device.

    For design capacity, you can use the what corresponds to 3.5V.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Intellectual 920 points

    Hi Onyx,

    Thank you for the information.

    I did verify that my cell is LiCoO2.

    Jim

  • 0 in reply to Jim Gavin
    Intellectual 845 points
    Hey Jim,

    Did you get your gauge - battery setup working ?

    I have some problem with mine and could use the code that you are using to write the new design capacity to the gauge. My effort so far is this: github.com/.../gauge.c

    - Thanks
    Chintan Pathak