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BQ40Z50-R2: Current measurements issue even after calibration

DudeFromRiga
Intellectual 305 points
Part Number: BQ40Z50-R2

Hello,

We have strange issue with bq40z50-R2 BMS IC current measurement (Rsense=2mOhm).

Problem description:

1) Calibrated (through Battery management studio GUI) BMS shows current values with ~15% error after cell pack is disconnected and connected again, or if we copy *.srec and *.gg files from calibrated board to other (with same component values) prototype board and use same cell pack as in board #1.
2) If we calibrate BMS again, then current measurment error is less than 1% (tested in both charge and discharge conditions, with different current values), until we do p.1)

What could be the reason of such behavior?

If you need any further details (step-by-step problem description, Schematics, etc.) , please let me know.

  • 0
    TI__Expert 6575 points
    Hi,
    1. On the same board, the calibration should not have much variation, unless there is significant change in the ground trace impedance when re-attaching cells.
    2. Typically users will use the same "CC Gain" and "CC Offset" values if they find that a sample (typically 20 boards) has similar calibration values. If the variation is large, then they may choose to calibrate all the boards individually. These variations are typically due to layout of the sense resistor traces, filter cap grounding, or variable ground trace impedance. Also, the [OFFSET_TAKEN] in "CC Auto Config" should be cleared (set to 0) and "CC Auto Offset" should be set to 0 before exporting the gg or srec to be programmed to other boards, since this value will be specific to each device and if [AUTO_CAL_EN] and [AUTO_NEST_ON] are set, the gauge will measure the "CC Auto Offset" before the device enters sleep mode.
    Regards,
    Swami
  • 0 in reply to Swaminathan Ramanathan75031
    Intellectual 305 points
    Hi Swami,
    Thank you for fast response.
    1. what is the "significant" change? - Is something like 10-15% from Rsense value? Whether, it's independent on Rsense, constant value in mOhms?
    2. Thank you for info about flags - didn't notice it eralier. After I save *.srec file, considering your recommendations, do I need to do extra actions with other prototype boards, after I program them with mentioned *.srec file?