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BQ21040: IC taking hours to charge above 4V

Swapniil Kanwar
Prodigy 160 points
Part Number: BQ21040

I was using commonly available TP4056 for charging my 1000mAh LiPo. Someone suggested me to use TI's BQ21040 as it is more trusted. 

I know that Lipo battery goes from 2.8V(0% SOC) to 4.2V(100% SOC) but I noticed a strange thing with both the charging ICs. Both the IC were able to charge from 3.2V to 4V in 1.5hr approx. But taking more than an hour to charge from 4V to 4.05V. ISET resistor in BQ21040 is 1Kohm, TS (temp sense pin) is pulled high. The schematic of BQ2140 circuit is exactly same as given in 1st page of datasheet. Charging is done with a mobile adapter 5V-1A. 

I want to ask that is it normal for LiPo to charge extremely slowly above 4V? How do commercial devices show 100% charge within an hour when 100% = 4.2V i guess! Can someone help me to suggest any other simple IC like this (which comes in standalone form) or any improvement to be made in my current circuit to charge it fast. Currently spending 2+hrs in charging ! 

I was also thinking to let it charge till 4V and in my software map that 4V to 100% SOC and 3V to 0%. Is this method suitable? 

  • 0
    Prodigy 160 points

    In addition to above query, BQ21040 datasheet says that CHG_bar pin will go high when charging current reaches below 10% of fast charge current. I have set ICHG resistor to 1k corresponding to fast charge current = 770mA. My Lipo is 1000mAh. I continuously measured the charging current. When the battery voltage reached 4.05V and charging current was 60mA still then the CHG_bar pin was low. Why is the CHG_bar pin not going high.  

  • 0 in reply to Swapniil Kanwar
    TI__Guru 70796 points

    Hello

    A couple of things to check:

    1.) Thermal Regulation and Charge Current -- The bq21040 will limit output current when die temp reaches 125C, this is separate from TS pin input.  This is not a fault condition and charge will complete but it will take longer, possible what you are seeing.  Recommend checking input current during the charge cycle it is possibly below ISET charge current.

    2.) Fast charge current with a 1k ISET resistor will be 540ma, for 770ma would require a 700 ohm ISET resistor.

    3.) CHG_bar pin will go low high when charge is complete, but this will required that unit is in Voltage regulation phase (V-bat=4.2V) and current at 10%.

    4.) Input voltage -- if input to output voltage difference is too low current is reduced and it will take longer to complete charge, check the input voltage.

    5.) Load on the battery during charging -- if the system is active and load applied to the battery it will take longer to complete charge.  The deglitch time from 10% charge complete is about 30ms, output current must be below 10% for 30ms to set charge complete.

    Also see Quick start guide for the BQ2404X family, it will apply to the BQ21040 also.

    BQ24040 Application Report