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Original question:

BQ76925: BQ76925 inaccurate and noisy current measurement

BQ76925: Drift in current measurement and fluctuation up to 400mA

Raam Kasinathan1
Prodigy 30 points
Part Number: BQ76925

Hello all,

I am using BQ76925 with 6 Li-Ion cells. I have two issues with the current measurements.

1. My current measurement is very fluctuating (up to 400mA).

2. when I reach zero current after the current is applied, the SENSEN and SENSEP are not equal.

I tried to recalibrate the SENSEN then only it becomes equal. How frequent should I update the SENSEN and should it be with respect to current or Temperature?

How to reduce the fluctuation and also the drift? is recalibration of SENSEN is the right solution?

Thank you in advance. 

  • 0
    TI__Guru 54715 points

    Hi Raam,

    The BQ76925 does not have a specification for the VIOUT with no current.  It will shift significantly with the reference selection, but will have tolerance after selection. The intended use of the VIOUT is to compare the value of SRP to SRN.  The closer together these are read the better the outcome of current measurement. 

  • 0 in reply to WM5295
    Prodigy 30 points

    Hello,

    Thank you for the reply. I think I should refine my question and also explain it a bit.

    At 0A,I get an offset of 2V when I use (REFSEL=1) 3V reference. As per the data sheet, when there is no current is drawn, both SRP and SRN should be equal(both should give comparably equal voltage).

    In my test, after drawing high current of 50A, when I remove the load, I see a difference between SENSEN and SENSEP, I read 1.7A even though I consume in micro amps lesser than the sensitivity of the sensor. But this difference between SENSEN and SENSEP gets corrected when I update the SENSEN again.

    According to the data sheet, p.no: 16 of BQ76925 data sheet, section 8.3.2.3. I read the SENSEN w.r.t. VSS at the beginning and read the SENSEP w.r.t. to VSS regularly. When there is an environmental change(word used in data sheet), I assumed that  it is the temperature change. I update the SENSEN whenever there is a 5 degree increase/decrease in temperature.

    In addition, I get a fluctuating reading, the distortion is up to 400mA which is pretty large for the current integration. 

    My question is

    1. How often should I update the SENSEN value?

    2. Should it also be calibrated with respect to the current applied, as it could cause the temperature increase? if possible, kindly tell me the interval.

    3. How can I get a stable reading without fluctuation, (I read the value after 6tau of the input filter)

    Thank you in advance.