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BQ27441-G1: Coulomb Counter offset when scaled data

Ruben Artero
Prodigy 65 points
Part Number: BQ27441-G1
Other Parts Discussed in Thread: EV2400

Hi,

I have used battery TLI-1550 of Tdiran Batteries. And the charger is CC/CV with 100mA and 4.2V.

The charger reeduce current, and the taper current is 10 % of the target configuration current, 10mA.

In my application I need to have a resolution  of 100uA.

By default, bq gives a resolution of 1mA with sense resitor of 10mΩ.

So, I just read and follow steps of TRM (slua792). I had calculate the scaled factor like:

SCALED FACTOR = K = 1mA / 100uA = 10

New resistor is:

R = 10mΩ x 10 = 100mΩ 

So, in my design I rescaled data like this:

Design Capacity = 330mAh x 10 = 3300mAh

Design Energy = 3.7 x Design Capacity = 3.7 x 3300 = 12210

Terminate Voltage = 2500 (According TLI-1550 datasheet)

Taper Rate = Design Capacity / (0.1 x Taper Current) = 3300 / (0.1 x (10 x K) = 3300 / (0.1 x 10 x 10) = 3300 / 10 = 330

Full Charge Set = 98

Full Charge Clear = 95

Taper Voltage = 4100

Terminate Charge Voltage = 4190

All data is loaded in flash in correct way, because I can read it with EV2400 tool.

The problem is when the current is 100mA (I read it with a precision tester) the bq AverageCurrent() reports 820 when it should be 1000... this device has an error of 20% with scaled data. I can't admit this error for my application.

Do you know any technique to calibrate current flow through sense resistor? In the Battery Management Studio isn't a box for calibrate current, only has a box for calibrate voltage offset.

Or maybe, have you got other solution for my problem?

Thanks and regards,

Rubén.

  • 0
    TI__Guru** 105760 points

    Hello Ruben,

    Have you performed the board offset calibration in order to adjust the current? Also when are you reading the average current after the start of current flow? The average current will take some time to stabilize. For is the current reading from the gauge at different applied currents? Is the same offset always present?

    I would also double check with your sense resistor tolerances since this can compound the error. It should have a very low tolerance and low PPM for temperature.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 65 points

    Hello Wyatt,

    No, because in Battery Management Studio isn't any mode for make it possible.

    I found a calibration to performed the board offset and battery voltage.

    How can I calibrate the current that flow through resistor?

    Yes, I read the AverageCurrent() value after 60 seconds... and the value doesn't change.

    When I applied different current, the error is more or less constant, about 20%.

    I used a precision resistor with tolerance of 1% 1/2W 50 ppm. In order to comply what it says in paragraph 10.2.2.3 Sense Resistor Selection of bq27441 datasheet.

    Thanks!

  • 0 in reply to Ruben Artero
    Prodigy 65 points

    Hello,

    Can you let me know if the value of taper rate is correctly?
    In the TRM indicates that do not be scale, but I think that the Taper Current must be scaled, since the Coulumb Counter will meassure x10 in my application.

    You can see in this table of slua729.

    So, in my application the Taper Current is 10mA. So, I scaled x10 --> 100mA.

    As sluuap7 says:

    Can you check that parameter?

  • +1 in reply to Ruben Artero
    TI__Guru** 105760 points

    Hello Ruben,

    Since the unit is 0.1 Hr you do not need to scale the Taper Rate, you should scale the Taper Current. When Taper Current is scaled the scale you applied to the design capacity will cancel and you will be left with the same 0.1 hr rate.

    Sincerely,

    Wyatt Keller