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BQ25898: charging current bouncing during the CV stage

mike zhang
Intellectual 925 points
Part Number: BQ25898

Hi TI charger team,

We use the BQ25898 in our product as a charger for a single Li-Ion battery. Its circuit design is almost the same as its evaluation board. Seems like the charger circuit works fine except the charging current is bouncing during the CV charging stage. The bouncing happens at middle of the CV stage and lasts about 30min.(see pic below) and then goes away.

our input voltage is 12V/2A, charging current is set to 2.048A, charging voltage is set to 4.208V, termination current is 64mA. SYS current is almost zero during the test and the battery is discharged to 3.3V before charging.

please note that the input current is about 720mA when the charging current is 2.048A, so the input power is not overloaded during the charging event.

during CV stage the charging current should lowly decrease until charge completed. what goes wrong on this? why is the bouncing seen during the CV stage? how to fix the issue?

Thanks!

Mike

  • 0
    TI__Guru*** 143880 points
    Mike,

    How are you measuring the charge current? Is there a current meter or in between the battery and IC bat pin?
  • 0 in reply to Jeff F
    Intellectual 925 points

    Hi Jeff,

    we have a current sensing resistor of 5mOHM between battery+ and BAT pin. we monitor the voltage cross the resistor to determine the charge / discharge current.

    Mike

  • 0 in reply to Jeff F
    TI__Guru*** 143880 points
    Where is the BATSNS connected?
  • 0 in reply to Jeff F
    Intellectual 925 points
    it is connected very close to the battery+ terminal.
  • 0 in reply to mike zhang
    Intellectual 925 points
    Jeff, is there any thought on this? Mike
  • 0 in reply to mike zhang
    TI__Mastermind 46551 points
    Mike，

    Do you test with real battery or battery simulator?

    Thanks,

    Ning.
  • 0 in reply to Ning T
    Intellectual 925 points

    Ning, it's a real battery. Mike

  • 0 in reply to mike zhang
    TI__Mastermind 46551 points
    Mike,

    Could you please provide all the register values? Are you using IR compensation?

    Thanks,

    Ning
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 925 points

    Hi Ning,

    Below are reg# & value (all in HEX). The IR comp was not used.

    REG#    VALUE    DESCRIPTION

     00          66           input current limit: 2A

     01          01           default

     02          D1          start ADC & 1 sec conversion

     03          1A          min. Vsys: 3.5V

     04          20           fast charge current limit: 2.048A

     05          10           termination current limit: 64mA; pre-charge current limit: 128mA

     06          5E          charge voltage limit: 4.208V; bat pre-charge/recharge threshold: 3.0V/100mV

     07          8B          WD/charger-time settings: WD disabled/8hrs

     08          03          default. no IR comp

     09          44          default settings

     0A          74          boost settings default

     0B          02          charge status. 02 no charge; 56 fast charge

     0C          00          fault status. no fault

     0D          92          input voltage limit: 4.5V

     0E          58          Vbat reading: 4.064V

     0F          58          Vsys reading: 4.064

     10          4D          VTS reading

     11          E0           Vbus reading

     12          xx            Ichg reading. xx based on charging current

     13          26           Ilim reading: 2A

     14          05           device info: bq25898

    Thanks for helping us on this!

    Mike

  • 0 in reply to mike zhang
    Intellectual 925 points

    Hi there, is there any thought why the bouncing happens? Mike

  • 0 in reply to mike zhang
    TI__Mastermind 46551 points
    Mike,

    Could you please share the schematic and charging profile measurement?

    Thanks,

    Ning.
  • 0 in reply to Ning T
    Intellectual 925 points

    Hi Ning,

    Here is the sch:

    what do you mean charging profile measurement?

    Mike

  • 0 in reply to mike zhang
    Intellectual 925 points

    Hi Ning,

    Here was one of the test measurement:

                   Time                     Charging current sensing voltage (mV)       BATSEN Voltage (V)

                   2:22pm                10.8                                                                     3.82

                   2:47                      10.0                                                                     3.985

                   3:03                      6.0                                                                       4.006

                   3:03                      4.9 min.~10.7 max. (bouncing started)        bouncing as well

                   3:33                      6.5 (bouncing stopped)                                   4.196

                   4:31                      1.0                                                                       4.202

                   4:48                      0.5                                                                       4.203

                   4:48pm                0.0 (charge completed)                                   4.187

    Please note that the charging current sensing resistor is 5 mOHM and charge voltage limit is set to 4.208V and the charge current limit to 2.048A. the battery was discharged to 3.35V at 1A before the charging test.

    Hope it helps.

    Mike

  • 0 in reply to mike zhang
    TI__Mastermind 46551 points
    Mike,

    Charging profile refers to Figure 15. Battery Charging Profile on page 24 of the d/s. It is much easier to understand the curve than descriptions.

    Have you tried to charge a different model battery and does it behave the same?

    Thanks,

    Ning.

    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 925 points

    No, I have not...

  • 0 in reply to mike zhang
    TI__Mastermind 46551 points
    Mike,

    Please use your board to charge a different model battery and see if the behavior is the same or not.

    Thanks,

    Ning.

    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 925 points

    hi Ning, we did tested different batteries and have the same issue...

    thanks!

    mike

  • 0 in reply to mike zhang
    TI__Mastermind 46551 points
    Mike,

    Please share the real charge profile test results to illustrate the issue.

    Thanks!
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 925 points

    hi, we got the issue resolved. it's a thermal issue. thank you for your effort and trying on helping us!

    mike