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BQ25898: Current drop and different terminal current in different IR compensation

Kailun Liu
Intellectual 1676 points
Part Number: BQ25898

Hi team,

Our customer they are using BQ25898 in there mp product. But the met some problem. Could you please help to analyze why the current will drop suddenly when approaching the end of charging and why the terminal current is not aligned with the setting value 192mA? Thanks！

After setting the BAT_COMP to 140mR and VCLAMP=32mV, the actual voltage of the battery can be charged to 4.4V, but the charging terminal current stops when it reaches 500mA, and the charge IC register is read
A cut-off current of 192mA can be seen.

com8_multi_2023_07_25_16_38_31(1).csv

Set BAT_COMP=20mR40mR The following phenomena occur, when BAT_COMP=20mR, the charging will be switched around 2A, the current will suddenly drop to 1.4A, and finally complete the charge, the terminal current is 230mA.

BAT _COMP=40mR, the charging will be switched around 1A, the current will suddenly drop to about 500mA, and finally complete the charge, the terminal current is 230mA.

IR=20mA-488.csvIR=40mR-488.csv

  • 0
    TI__Genius 10240 points

    Hi Kailun,

    What was the source device issued the charging to full in the log line 1235 - D:chg: from chg_s to full_s?  Could you provide the exact impedance from the battery cell to the charger BAT pin? 

    Regards,

    Eric 

  • 0 in reply to ezhao
    TI__Intellectual 1676 points

    Hi ezhao,

    They use MCU to measure the voltage of bat. The exact impedance between battery to charger BAT is unknown. They just want to reduce the charging time. When they use IR compensation, will it make current drop suddenly when the Charge Voltage reach the threshold of VREG+IR compensation?

  • 0 in reply to Kailun Liu
    TI__Mastermind 24110 points

    Hi Kailun, 

    The current drop behavior you describe is expected. When the IR compensation algorithm is enabled the charger IC will step down the charge current during the constant voltage phase as shown in the graph below comparing expected charge current when IRCOMP is enabled verse disabled. 

    When IR compensation is disabled the current reduction in CV phase will be a more smooth curve. When IR compensation is enabled larger steps down in charge current, such as from 2A to 1.4A as you described, are expected.  

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    TI__Intellectual 1676 points

    Hi Garrett, 

    Thanks for your reply! Could you please tell me why the current will step down when IR compensation is enabled?

  • 0 in reply to Kailun Liu
    TI__Mastermind 24110 points

    Hi Kailun, 

    The step down behavior is simply a result of how the IR compensation algorithm works.

    When using the IR compensation feature charge current will go down in step sizes to the nearest 500mA during the CV phase of charge. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    TI__Intellectual 1676 points

    Hi Garrett, 

    Thanks for your support! Our customer have some questions about the step down behavior:

    1. There is only one step down in their charge profile but there are several step down in the fig you share. What may cause the difference?

    2. The step down value in customer data is about 0.6A, what will affect this value? 

    Thanks and look for your reply!

  • 0 in reply to Kailun Liu
    TI__Mastermind 24110 points

    Hi Kailun, 

    Please see comments relating to questions below. 

    1) The number of steps will depend on multiple factors such as settings in the IR comp, ICHG, and ITERM registers. There is nothing unexpected about observing 1 step down in current compared to the multiple shown in the example figure. 

    2)The step down in current will be the the nearest 500 mA and in some instances can as large as 1A due to round down logic of the algorithm. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    TI__Intellectual 1676 points

    Hi Garrett,

    Thanks for your help!