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BQ34Z100-G1: Can't maintain state of charge

Andrea Chung
Prodigy 20 points
Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQSTUDIO

I have a battery connected to the BQ34Z100-G1 as shown in the datasheet. I have performed a successful learning cycle (update status has changed to 6). I currently have a few concerns:

1. The SOC will show 0% until at least one full charge and discharge has been performed. Not sure if this is normal operation

2. When I unplugged the charger when it wasn't at full the SOC jumped back down to 0%. When I plugged it back in it started going up again, but assumed that the point where I unplugged the charger was 0%.

3. While charging, at some point the SOC showed as 100% even though it wasn't. I unplugged and plugged the charger back in and the SOC jumped back down to 0% and showed 0% until the battery was fully discharged.

TLDR I'm having trouble maintaining the correct SOC. It seems to reset to 0% whenever the charger is plugged in and only seems to show accurate SOC when charging. Also, if it gets stuck at 0% even though it's at half capacity, the SOC won't update until full discharge. Is there a fix for this?

  • 0
    TI__Guru** 102465 points

    Hello Andrea,

    Can you pull your .gg file before running your tests and another .gg file after while logging the tests with bqStudio? This will help us determine what may be going on.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    Here is the .gg file before running any tests and after a charge and discharge cycle.

    4276.before.gg.csv

    7444.after.gg.csv

  • 0 in reply to Andrea Chung
    TI__Guru** 102465 points

    Hello Andrea,

    Can you attach the logged data during the test? On the main register tab in bqStudio you can click "Start Log".

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    Here are the logs from today's tests. The first one got interrupted by error. I was charging the battery and it was showing 100% (even though it was fully discharged) and at some point the charger got confused and thought it was at 100% and stopped charging. The second file is a log of a almost complete charge cycle. Throughout the cycle it faced the same issue as the first one where it thought the charge was 100% even though it wasn't and would stop charging.

    5556.test.log

    8270.test2.log

  • 0 in reply to Andrea Chung
    TI__Guru** 102465 points

    Hello Andrea,

    Can you give more of the system specs? (Battery chemistry, charge voltage, GPC CHEM report) I'm confused from looking at the log and gg file, the gg is configured for 20S NiCd pack, but the voltage is showing 2500mV, also the voltage steps are in 20mV which I haven't seen before which indicates there's something wrong with you voltage divider ratio.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    Yes the voltage division is off. Someone else ran the learning cycle and used that division. We have the BQ34Z100-G1 connected to an MCU. The battery voltage goes through a 606.8k, 16.5k physical divider and is then scaled up in the firmware using the scaling factor of 7.5551515 (converting to V not mV). The battery is 20 cell NiCd, 7000 mAh, 168 Wh. It's rated at 24V, but I've seen it go all the way up to 28.5V

  • 0 in reply to Andrea Chung
    TI__Guru** 102465 points

    Hello Andrea,

    Your voltage should report the actual stack voltage, not a scaled voltage that your MCU converts. This will cause the gauge to reference the wrong DOD-OCV points. Your voltage is low enough you shouldn't need to do the modifications.

    You can reference this app note for how to change the cell count also so the voltage scaling can work: www.ti.com/.../slua760

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    I changed the cell count to match the datasheet you sent. I did 20/7.55515 which is 3 when rounded up. As it was charging it showed a SOC. When it reached 100% I unplugged the charger and connected a smaller load and suddenly it jumped back down to 0%. Plugged in the charger again and it didn't respond. It's like it forgot everything it learned once the load resistance decreased. It's similar to the error that I was having before. Here is the log file of when this occurred (SOC hits 0% at line 3582).

    3cell_charge.log

  • 0 in reply to Andrea Chung
    TI__Guru** 102465 points

    Hello Andrea,

    I'm not sure I'm following the scale procedure you are doing. Scaling only needs to be done if your voltages are over 60V. You shouldn't need to scale any of the voltages, I think that's what is leading to your SOC errors. The OCV table is not correct because the voltage scale is off.

    Sincerely,

    Wyatt Keller