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Full charge capacity not correct

Benn
Prodigy 170 points
Other Parts Discussed in Thread: BQ27541

Dear All

I use bq27541 to management battery pack which use 2 cells (750mAh) in parallel.  But the full charge capacity seems to be not correct.

After updated chemistry ID 0238 (provided by TI) and run a learning cycle to create dfi file, the full charge capacity of this battery pack reached to 1339mAh.

It’s a big difference between its design capacity (1500mAh). Are there something wrong with my data flash setting?

The cell datasheet, battery pack schematic circuit diagram and gg file are attached.

 

Best regards

Benn

bq27541 project.zip
  • 0
    TI__Genius 14985 points

    Hi Benn,

    Hopefully my answer to your other post will help you understand this.  You should compare the learned Qmax value to the cell's datasheet design capacity.  Probably it will be slightly bigger, in fact.  But FCC is compensated depending on load and temperature and will usually be smaller than Qmax.

    http://e2e.ti.com/support/power_management/battery_management/f/180/p/75067/273373.aspx#273373

  • 0 in reply to dMax
    Prodigy 170 points

    Do you mean the design capacity=1500mAh, Qmax=1391mAh and FCC=1339 is normal? 

  • 0 in reply to Benn
    Expert 2135 points

    Benn,

    In my experience, those numbers are very typical.

    The datasheet for those cells has very specific charge and discharge current/voltage values in order to achieve the nominal capacity value of 720mAh-750mAh. And the min. cap. rating is 700mAh so your cells could possibly only have 1400mAh under the best circumstances (probably closer to the nominal value though). Lastly, as the cells age, the capacity will also drop. Especially if stored fully charged IMHO.

    If you are using charge or discharge rates above the ones specified in the datasheet, your capacity will be lower than the nominal rating. If the temperature is lower, the cap. will be lower. And, if you have any inter-cell resistance (straps or wires), the capacity drops for the pack.

    Lastly, the capacity values in the datasheet are based on specific charge/discharge voltage levels. If yours are different, the capacities will be different.

    For comparison, I'm using a TI gas gauges on 4S 2300mAh LiFePO4 packs. While the cells are nominally rated for 2300mAh, they only have a new capacity of about 2150mAh because of the cutoffs I selected, the inter-cell resistance, and the charge/discharge rates they see during use. As the cells have aged, the capacity has dropped to about 1950mAh.

  • 0 in reply to John M
    TI__Genius 14985 points

    John is right.  Let's look at your configuration a little closer:

    I see in your gg file that your taper current is 75mA which means your charger probably stops charging at a quit current of less than 37.5mA/cell (not 14mA/cell from the cell datasheet).  Also, Qmax is defined as the chemical capacity from full charge to 3.0V (not 2.5V from the cell datasheet).  You also have a Terminate Voltage of 2700mV, which is used for the FCC calculation.

    All of these might be small contributions but could add up to reduce your actual capacity (Qmax & FCC) slightly below the theoretical design capacity of your pack.