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BQ34z100 SoC is decreasing to 94% in a week

Ilja Lavrukov
Prodigy 160 points
Other Parts Discussed in Thread: BQ34Z100, BQ34Z100EVM

Hello,

I have BQ34Z100 with high capacity 4-cell LiFePO4 battery. I came through the learning cycle and Update Status is reached 06. I am charging the battery with automatic charger till 14.6 volt and this voltage is normally lowering till 13.6V during 1 day of relaxation. The problem is that BQ34z100 State of charge is decreasing to 94% in a week with no load (I checked it once in two days), and is staying constantly on this level after it. Now the battery voltage is 13.4V (BH is set) and SoC is not decreasing any more during 2 weeks.

I was thinking the problem is because of cleared BH flag during relaxation. I lowered Cell BH Clear Volt Threshold value till 3250 (it was 3400 before), but the problem still exist.

What the problem could be? Could it be something wrong in Data Flash parameters?

GG is attached

2425.SoC decreasing to 94.txt
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[Header]
bq EVSW Version = 0.9.86
DeviceName = bq34z100 v0.06
Time = 11.09.2013 9:27:37
[Safety(Configuration)]
OT Chg = 55.0
OT Chg Time = 2
OT Chg Recovery = 50.0
OT Dsg = 60.0
OT Dsg Time = 2
OT Dsg Recovery = 55.0
[Charge Inhibit Cfg(Configuration)]
Chg Inhibit Temp Low = 0.0
Chg Inhibit Temp High = 45.0
Temp Hys = 5.0
[Charge(Configuration)]
Suspend Low Temp = -5.0
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Thanks in advance,
Ilja

  • 0
    Prodigy 100 points

    Hello, Ilja 

                            Do you use the bq34z100EVM to do that ?   I  also use the bq34z100,but the circuit I refer

    to the datasheet,there are some problems.   So, now I am serching that the circuit which I can refer.   Thanks!

  • 0 in reply to lys79129
    Prodigy 160 points

    I used circuit based on the circuit of bq34z100EVM, but a little bit modified (for higher currents) and simplified (removed unused features). Pull-up resistors on I2C line are on the host side.

  • 0 in reply to Ilja Lavrukov
    TI__Guru 73830 points

    Ilja,

    The gg file is setup properly to support LiFePO4 cells. I checked the OCV table and 94% SOC would be correct for a 4-cell stack voltage in the 13.4V to 13.6V range. Have you determined whether something is loading the cells or is the drop due to resting?

    Regards

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 160 points

    Hello Tom!

    Nothing was loading the cells, the voltage dropped due to resting.

    Is it normal that battery SoC drops due to resting when BQ34Z100 learning cycle is finished? Or it still depends on battery voltage? By the way, I have found in specification that this battery self-discharge rate is =< 3% (Monthly), but the battery SoC stayed on the same level (94%) after some weeks.

    Best regards,

    Ilja

  • 0 in reply to Ilja Lavrukov
    TI__Guru 73830 points

    Ilja,

    The device will continue to run OCV checks during rest and adjust RemCap and SOC as the voltage drops. You have the ChgDoDEoC bit set in the Operation Cfg B register, so the device will not reset the DOD point. You could try reducing the Taper Current to allow the cells to charge fuller and this should help to reduce the amount that the cell voltages drop after charge complete.

    Regards

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 160 points

    Hello Tom,

    I reduced Taper Current tlll 200 mA (actual 2000 mA in X10 mode) and went once again through calibration process.

    I don't think the problem is in Taper Current parameter, because I am charging the battery with automatic charger and charging stops itself at  about 14900 mV. The SoC dropped again from 100% till 95% during weekend.
    I offer you to review screenshots of Data RAM table taken at 0% (discahrged) after 1 night relaxation, 100% charged after one day relaxation, 100% one more day passed and 95% 3 more days later.
    Values changing in DOD0 and SelfDischargeCurrent are confusing me! Are they normal, taking into account that no load was connected?

  • 0 in reply to Ilja Lavrukov
    Prodigy 160 points

    Hi Tom,

    You still haven't answered to my previous post!
    Two weeks more past since my previous measurement when the battery SoC dropped till 95%. Now the battery SoC is 94% and voltage is 13340 mV, and I think this battery parameters will not decrease a lot while no load or charger is connected!
    Do you still thinking that battery SoC decreasing from 100% till 94% during some days is normal and it is nothing to do than just leave it!

    Your reply is appreciated!

  • 0 in reply to Ilja Lavrukov
    TI__Guru 73830 points

    Ilja,

    That is normal. The device will wait for 48 hours before running OCV measurements when using LiFePO4 cells. After that time, it will measure the voltage, compare it to the OCV table in the device and adjust SOC as needed. This correlates with what you are seeing. The ChemID's are setup such that the rested voltage should be above the 100% OCV voltage, so the issue could be caused by one or more cells not getting fully charged or leakage is draining them. I would measure each of the cells and see how they compare to one another. You will probably find that one is low.

    Regards

    Tom