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BQ24295: Circuit works fine except when measuring charge current

Daniel Hienzsch
Prodigy 40 points
Part Number: BQ24295

Hello Texas Instruments!

I have a BQ24295 circuit that seems to work flawlessly under all test conditions, except when I attempt to measure the charge current. By attaching a DMM in series with the low side of the battery, I am able to measure the load on VBAT when not charging, and I am able to measure the charge current when the battery is below the constant voltage portion of the charge cycle.  However after the battery rises to the constant voltage level of 4.20, and the charge current drops to around 250mA, the LED on VSTAT begins blinking rapidly, the current draw measured on the DMM rapidly cycles between 90 to 250mA (fairly useless metric at that point, I know), and the VBAT voltage measured with an oscilloscope shows a square wave from 4.21V to 3.8V, with a frequency of 19Hz, 33ms up, 17ms down.

With the DMM removed from the circuit and the battery connected directly again, VBAT showed 4.16 and the bench power supply connected to VBUS showed 597mA draw.

The registers read out these values the entire time...

REG[0] 0x5e
REG[1] 0x3b 
REG[2] 0x20
REG[3] 0x11
REG[4] 0xb2 
REG[5] 0x9c
REG[6] 0x93
REG[7] 0x4b
REG[8] 0xa4
REG[9] 0x80
REG[A] 0xc0

Do you know what may be occuring?  I've wracked my brain and feel I must be missing something obvious at this point.

  • 0
    Prodigy 40 points

    Quick update: after removing the DMM from the circuit and connecting the battery directly, the issue returned when the charge current got down to around 250mA, which is close to the default termination current programmed. Here are some scope capture that may be helpful.

    I've attempted adding scope screen captures to this, however I continually get an error when trying to upload the images.  I've created an imgur album with captions instead.

    https://imgur.com/a/gp73i3b

  • 0 in reply to Daniel Hienzsch
    TI__Genius 10240 points

    Hi Daniel,

    A DMM should not be part of the charging system as its impedance changes the battery's characteristics. We suggest use a small current sensing resistor in series to measure the charging current. 

  • 0 in reply to ezhao
    Prodigy 40 points

    Thank you for your note. I removed the DMM from the connection and replaced it with a sense resistor to calculate current draw. Unfortunately that hasn't resolved the issue.

    What I am discovering, is that the system holds the battery at VREG during the constant voltage portion of the charging cycle, the current drops steadily to the termination current, and then the charge terminates. However with the charge current removed, the battery voltage drops down below the 100mV < VREG recharge threshold, the BQ24295 waits for the 20ms deglitch period (seen in the low portion of the pulse wave scope trace linked in my previous post), and then reapplies the charge. The current jumps to the programmed charge current, then rapidly decreases again below the termination level, charging is stopped, and the cycle continues.

    I've tested this with a variety of different battery capacities, all the same Lithium Polymer chemistry...

    Capacity Termination Current Charge VREG Post Charge VBAT (No Load) Voltage Delta
    160 mAh 140 mA 4.224 V 3.901 V 0.323 V
    500 mAh 142 mA 4.224 V 3.980 V 0.244 V
    850 mAh 142 mA 4.224 V 3.993 V 0.231 V
    1500 mAh 130 mA 4.224 V 4.019 V 0.205 V
    2000 mAh 133 mA 4.220 V 4.009 V 0.211 V
    8000 mAh 125 mA 4.224 V 4.035 V 0.189 V

    Raising the VRECHG threshold from 100mV < VREG to 300mV < VREG seems to clean things up, however even with a termination current of 128mA, that leaves quite a bit of charge on the table for all but the largest battery capacities. Would the solution be to disable termination and continue charging until the charge current drops to the appropriate value? My understanding is that the ideal termination current should be around 5% of 1C. 

    Another question: the battery capacity for which the BQ24295 works most effectively doesn't seem to be listed in the datasheet, but using simple default settings, it would seem that it's optimized for capacities in the 5000 mAh range and larger. Is that so, and if so, is there a chip that works more effectively with smaller capacities?

  • 0 in reply to Daniel Hienzsch
    TI__Guru 51341 points

    Hi,

    It is ok to disable charge termination.

    In general, the device should work with 1 cell Li-Ion and Li-polymer batteries if the fast charging current setting needed is less than 3A.

    Thanks,

    Ning.