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BQ34Z100-G1: Quit current, charge current and discharge current

Gorka Lorea
Expert 1620 points
Part Number: BQ34Z100-G1

Hi,

I am using a BQ34z100-g1 with an 8s2p LiFePO4 batteries. I have one doubt about the realtion between quit current, charge current and discharge current.

We have a system with a permanent current consumption of 40mA. Each 2/3 hours the current consumption rise to 3-4A during 20-25s. The battery is always powering the device so the consumptions seing  from the battery are the same. 

The charge procedure of the battery force to sometimes charge it with 30mA during 10-20 minutes to balance its cells. In normal operation the charge current is 2A in CC and when in CV it drops to 80mA when we program the charge final. 

Which values  of quit current, charge current and discharge current should we program?

Thankyou,

  • 0
    TI__Mastermind 25955 points
    Hi Gorka,

    Does the system ever relax?

    The default for Quit current is 40 mA. If the system is always over 40 mA with a 40 mA setting for quit current, the system will never enter relaxation mode from a current-flowing mode. The quit current will need to be increased from default settings.

    What is your cell capacity?

    It is critical that he battery voltage be relaxed during OCV reading to get the most accurate results. Please ensure that the current is not higher than C/20 when going into relax mode.

    If you choose 50 mA for your quit current, for example, please ensure that your cell capacity is in excess of 1000 mAh.

    Sincerely,
    Bryan Kahler
  • 0 in reply to Bryan Kahler
    Expert 1620 points

    Hi Bryan,

    In its relax mode, the system is consuming 40mA. As I told you before each 2/3 hours the current consumption rises to 3-4A during 20-25s.

    The battery capacity is 3200mA and we have an 8s1p configuration so the cell capacity is 3200mAh.

    So, do you think the correct quit current is 50mA?

    And the charge current? Because it is said in some documents of IT algorythm that charge current should be higher than quit current but in this case the charge procedure of the battery force to sometimes charge it with 30mA during 10-20 minutes.

    Thankyou very much,

     Best regards,

  • 0 in reply to Gorka Lorea
    TI__Mastermind 25955 points
    Hi Gorka,

    The quit current will need to be larger than 40 mA, with margin. From the information provided, 50 mA appears to be a sensible value to try.

    Charge current needs to be higher than the quit current. This just means that the device will not be reporting in the 'charge' mode.

    Sincerely,
    Bryan Kahler
  • 0 in reply to Bryan Kahler
    Expert 1620 points

    Hi Bryan,

    Ok, I wil fix the quit current in 50mA.

    If charge current is fixed, for example, at 75mA, is there any problem if the charge procedure of the battery force to sometimes charge it with 30mA during 10-20 minutes?

    Thankyou very much,

    Best regards,

  • 0 in reply to Gorka Lorea
    TI__Mastermind 25955 points
    Hi Gorka,

    Thank you for the clarification. With such a low current, to ensure the device is in the charge state during charging, we will need to change values discussed above.

    Please try setting the quit current to 15 mA, discharge current threshold to 35 mA and the charge current threshold to 25 mA. To allow the device to switch between the modes quickly, you may also try reducing the chg and dsg relax times to 1s.

    Sincerely,
    Bryan Kahler
  • 0 in reply to Bryan Kahler
    Expert 1620 points
    Hi Bryan,

    So the configuration of the BQ34z100-g1 should be:
    Quit current: 15mA
    Charge current: 25mA
    Discharge current: 35mA

    I have read in documents of IT algorithm that the device start taking OCV readings after 30minutes in relax mode (and if dV/dt<4uV/s). Is this time configurable?

    Best regards,
  • 0 in reply to Gorka Lorea
    TI__Mastermind 25955 points
    Hi Gorka,

    We need to ensure that the device switches modes properly. The answer changes depending on the following questions:

    Could you please send your charge profile? What is overall capacity of the cells?

    Are you having trouble with charge detection?

    If this balancing action is slight (which it now appears to be against the capacity of the system) and isn't always the entry or exit point to charging, please set the quit current to be larger than 40 mA and the discharge current threshold and charge current threshold larger than quit current. Reduce the chg and dsg relax times 1 s and let me know if you see the device flipping between chg and dsg modes sporadically. If so, increase the chg and dsg relax times.

    If erratic behavior is observed, please send the results for review.

    Sincerely,
    Bryan Kahler
  • 0 in reply to Bryan Kahler
    Expert 1620 points

    Hi Bryan,

    I answer to your questions as well as possible

    Could you please send your charge profile? What is overall capacity of the cells? 

    We have 2 potential batteries to use in my system, both are the same chemistry: 

    - LiFePO4, 8s2p , 6400mAh: 8 cells, two rows in paralell. 

    - LiFePO4, 8s1p , 3200mAh: 8 cells, one row. 

    Here you have one charge example of the model 8s1p. 

    However, this is the ideal charge of the battery. The real application of the system is powered by a photovoltaic panel so the energy source is not guaranteed. Thus means that sometimes the battery wiil be charged with 1A, sometimes with 500mA and sometimes with 80mA. 

    It also means that sometimes current through battery will be zero (if the battery is charged and photovoltaic energy source is available) but we do not know how long. 

    Are you having trouble with charge detection?

    No, I have never had problems with charge detection.

    If this balancing action is slight (which it now appears to be against the capacity of the system) and isn't always the entry or exit point to charging,

    I do not understand exactly what you want to say here. 

    Regards,

  • 0 in reply to Gorka Lorea
    TI__Mastermind 25955 points
    Hi Gorka,

    This is good news. 30 mA is quite small compared to the overall capacity of the system. Even though the device will not always be in charge mode while charging, it should continue coulomb counting. As long as every charge cycle doesn't end at 30 mA, but sometimes above the quit current and charge current thresholds, FC will remain to be detected a large portion of the time and the gauge will continue to learn and update.

    Please set the quit current to be larger than 40 mA and the discharge current threshold and charge current threshold larger than quit current. If you run into any issues, please send me log. Data on this variable charge rate application would be helpful in such analysis.

    Sincerely,
    Bryan Kahler