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BQ27542-G1: Two issues: long charge time from 0% to 1%; SOC drops from 100% to 98% after end of charge.

user6074007
Prodigy 130 points
Part Number: BQ27542-G1

Hi guys,

I have met two SOC issues when doing the charging test of batteries based on BQ27542-G1. 

Test Information

Battery rated capacity: 3300mAh

Max Charge current: 1A

The batteries were charged after discharged to 2.9V. There was no current consumption after end of charge. 

Battery No.

Issue

Test condition

0

The capacity drops from 100% to 98% after end of charge

Ambient temperature is 40℃,

Charge current is 1A,

Note: In the charging log, the charge current changes to 0A sometimes due to the temperature protection.

4,5

The charging time from 0% to 1% is very long:

BAT4: 1 hour and 49 minutes

BAT5: 51 minutes

While the normal value is around 20 minutes

Ambient temperature is 35.5℃,

Charge current is 1A

Note :This is not the first time to charge the two batteries. In the charge cycles before and after this cycle, the charge times are all OK. 

The attached are the charging log, including the temperature, battery voltage and current and SOC. 

The gg file comes from another battery of the same batch of the above batteries . 

Can you help to find out the possible reason for such issues？Thank you very much!

Wolf

20210526, BAT0, 40centi degree, 1A charge current.xlsx20210513,BAT4,BAT5,35.5 centi degree.xlsxBY-01.gg.csv

  • 0
    TI__Guru 61536 points

    1. Capacity drop:

    This is due to the gauge re-calculating remaining capacity after you stop charging, when the voltage stabilizes. Note that you get a jump from 98% to 100% when the charge termination conditions are met. So at that point, the gauge was "surprised" (as in: it thought there was more charge needed to fully charge the cell). Then when it had time to take an OCV in relax, after charge, it recalculated that there could be more charge stored. The gauge sets its 100% SOC vs. DOD reference point (DODatEOC) when it detects a full charge, so if the chemistry is compatible with the cell and if the load configuration is correct and if the cell resistance tables are correct and if temperature is not extreme, you should not observe this jump.

    So to me this looks like either the gauge hasn't learned QMax/Ra accurately or that the chemistry itself is not a good match or that the cell itself is damaged (i.e. it doesn't behave like the model predicts).

    2. Long time for the gauge to start increasing SOC: This means that the gauge considered the cell severely over-discharged. The key is the 2.9V discharge - are you 100% sure that this is ok to do with these cells? It seems awfully low. If somehow the cell voltage, after you stopped discharging, dropped way below 2.9V, the gauge may consider the cell to be severely over-discharged and then it will take a long time to compensate this charge. You can control the gauge's behavior with Pack Configuration, RMHOLD0. If this is 1 (default), then the gauge will hold SOC at 0% while in an over-discharge condition and not increment (typo in the TRM) until the charge deficit is equalized. Your setting is 0x097F. Change this to 0x097D and this should not happen.

  • 0 in reply to Dominik Hartl11
    Prodigy 130 points

    Dominik，

    Thank you very much for the explanation.

    1. So you have meant that jump from 98% to 100% is only because termination conditions are met, regardless of the real capacity; and after end of charge, the gag recalculated the capacity and found the real capacity is only 98%?

    2. In fact in my test, 2.9V is the discharge termination voltage,  the voltage measured after discharge is 3.0V in most cases. Int most cases, the charging time from 0% to 1% is only about 20 minutes. And on the same batteries described, the issue doesn't happen again. But I am afraid this will happen in the end users, thus the terminal devices won't be able to boot up after a long time of charging. 

  • 0
    TI__Guru 61536 points

    #1: Yes.

    #2: I recommend terminating discharge at voltages higher than 3000mV. Most ChemIDs don't cover voltages <3000mV. You can try reading true remaining capacity - if this is negative, then the gauge will not increase SOC until this turns positive.