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BQ27Z561-R2: Fuel gauge for application with very small active current

Aivaras
Prodigy 215 points
Part Number: BQ27Z561-R2
Other Parts Discussed in Thread: BQSTUDIO

Hello to everyone I'm in the process of evaluating and selecting the right fuel gauge for new application that will run from 1cell Li-pol battery.

Key points:

Application active current will range from 1mA to 30mA depending on the operating mode. 

Application Resting current will be <1mA

Battery capacity may range from 0.5Ah up to 27Ah and charging current accordingly from 0.5A up to 4A. We might consider lowering the maximum battery capacity if no suitable gauge will be found. However we would like to maintain 4A charge current. 

i2C communication

I use 10mR shunt resistor for maximum resolution. 

As you probably thought already, this will be long run time, small consumption environmental sensor. My first candidate to evaluate was BQ27Z561-R2 but now I see that resolution with which I can work is 1mA. It now gets difficult to arrange modes when gauge will switch to relax mode and back to Charge/Discharge mode, when I have 1mA resolution and application active current might be as low as 1mA. 

1) I select Chg Current Threshold level as 100mA

2) Dsg Current Threshold level as 1mA

QUAESTION

But then where should I put Quit current level? Make it equal to discharge threshold ,which is 1mA? Or do I get all this process completely wrong and Quit current and Relaxation mode is only used for learning cycle and I do not need to relate it to application running current ? 

According my calculations with 10mR shunt with BQ27Z561-R2 I have resolution of 0.374mA but TI BQ software lowest digit to enter is 1mA. How can I gain that extra resolution? 

Are there other gauges that are more suitable for my application? 

  • 0
    TI__Mastermind 28440 points

    Hi Alvaras,

    Regarding the quit current and DSG current threshold, the we recommend for the DSG current threshold to always be set higher then the quit current. If they are set equal, then this can effect how the gauge enters relax mode. Is it possible from the application side to increase the DSG current threshold to 2mA and quit current to 1mA? If quit current is set to 0mA, then it would be difficult for the gauge to enter relax.

    Also, I do not believe it is possible to enter higher resolution values to bqStudio, and that the inputs to parameters must be integers.

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Prodigy 215 points

    Thank you for your insight and information. 
    We are aiming for best battery life we can achieve so we can not increase our sensor current to 2mA just for the gauge nuances.   

    What if I will program gauge with DSG current threshold of 2mA, but the actual sensor will run in 1mA consumption?  There are different operational modes with higher consumption current but when senor will be running in 1mA mode, what will happen with the battery tracking? 

    Gauge will fail to enter Discharge mode and it will not track and will not update SOC and other parameters? At the time when sensor will enter high consumption stage gauge will detect discharge fact by the voltage difference will try to guesstimate new SOC or it will continue to show false SOC and will remain unaware of the battery drain that happened during 1mA operation stage? 

  • 0 in reply to Aivaras
    TI__Mastermind 28440 points

    Hi Aivaras,

    If the gauge is in relax mode and then the 1mA current is applied, the gauge will not leave relax mode since the current has not broken the DSG Current threshold. However, when in relax is where the gauge takes the majority of its measurements for calculation, where the accumulated charge over this time will still be considered and recalculate Remaining Capacity and FCC every time there is an OCV measurement. Remaining Capacity and FCC are the to major components of the SOC calculation. However, if the smoothing function is enabled, the gauge will purposely not change the FCC and Remaining Capacity during relax, which can be disabled.

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Prodigy 215 points

    So do I understand correctly that in my scenario the best solution that will work is:

    1) set Quit current threshold to 1mA

    2) set DSG current threshold to 2mA

    3) make sure smoothing function is disabled. Question - should I leave smoothing in general enabled Bit 12: SMOOTH and only disable Bit 14: RELAX_SMOOTH_OK? Should I also Enable Bit 11: RELAX_JUMP_OK ?

    How all of this is related to Sleep Current threshold and also Deep sleep current threshold?  Do I need to lower both of these thresholds to 1mA in order to make sure gauge will not sleep in the application running mode of 1mA ? and I presume it will then go to sleep when application hardware is not running and current is 0mA. 

  • +1 in reply to Aivaras
    TI__Mastermind 28440 points

    Hi Aivaras,

    Aivaras said:

    So do I understand correctly that in my scenario the best solution that will work is:

    1) set Quit current threshold to 1mA

    2) set DSG current threshold to 2mA

    Correct.

    Aivaras said:
    3) make sure smoothing function is disabled. Question - should I leave smoothing in general enabled Bit 12: SMOOTH and only disable Bit 14: RELAX_SMOOTH_OK? Should I also Enable Bit 11: RELAX_JUMP_OK ?

    RELAX_SMOOTH_OK should be disabled.

    RELAX_JUMP_OK should be enabled.

    SMOOTH should be enabled.

    Aivaras said:
    How all of this is related to Sleep Current threshold and also Deep sleep current threshold?  Do I need to lower both of these thresholds to 1mA in order to make sure gauge will not sleep in the application running mode of 1mA ? and I presume it will then go to sleep when application hardware is not running and current is 0mA. 

    The Charge, Discharge, and Relax modes are the gauging states of the device, while Sleep and Deep Sleep are power states for the device. Since the application has a 1mA current draw in some instances, I would not recommend using Sleep unless necessary since the minimum value for Sleep Current is 1mA and the gauge could potentially fall asleep. The conditions for Sleep can be found below:

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Prodigy 215 points

    Thanks again Anthony for your time and effort supporting my new design. 

    In the reference manual, I noticed mentioning  that while in SLEEP mode "The device continues to coulomb count and update current
    every 4 s". So what do I lose compared to NORMAL modes? Only the refresh rate from 1 sec intervals to 4 sec intervals? in that case I could accept less frequent updates and would use sleep mode and only disable DEEP SLEEP.

    Or I will loose something significant while using SLEEP mode when application is actually running? 


    Datasheet states power consumption in NORMAL modes is 60uA and SLEEP mode is 11uA. Our battery with gauge is designed as a pack and can be detached. I'm concerned that without any kind of sleep mode battery will start to lose its charge during the long term storage. 

  • 0 in reply to Aivaras
    TI__Mastermind 28440 points

    Hi Aivaras,

    Aivaras said:
    In the reference manual, I noticed mentioning  that while in SLEEP mode "The device continues to coulomb count and update current
    every 4 s". So what do I lose compared to NORMAL modes? Only the refresh rate from 1 sec intervals to 4 sec intervals? in that case I could accept less frequent updates and would use sleep mode and only disable DEEP SLEEP.

    This is correct, the main difference between NORMAL and SLEEP mode for this device is the rate at which sampling occurs. Current measurements will occur every 4 seconds, where voltage sampling is configurable since it requires the gauge to briefly wake up to conduct its measurements.

    Aivaras said:
    Datasheet states power consumption in NORMAL modes is 60uA and SLEEP mode is 11uA. Our battery with gauge is designed as a pack and can be detached. I'm concerned that without any kind of sleep mode battery will start to lose its charge during the long term storage. 

    I believe that the HIBERNATE mode would be worth looking into if there is long term storage. This can be found in section 3.5 of the TRM below:

    https://www.ti.com/lit/pdf/sluuc54 

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Prodigy 215 points

    But if I understand correctly there is no way to do this automatically, we need to know when battery will be disconnected and enable Hibernation manually from FW?  

  • 0 in reply to Aivaras
    TI__Mastermind 28440 points

    Hi Aivaras,

    I believe the battery would still need to be connected in this mode. In the statement from the TRM, "pack" refers to either the charger or the application where charge and discharge would come from. This can be seen from the PACK pin of the device.

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Prodigy 215 points

    One more question if I can. 

    Do you see any potential problems if I will leave SLEEP and DEEP_SLEEP currents equal? 

    I made a test where both thresholds are equal to 1mA and I can see that gauge stages from active to immediately both flags active SLEEP and DEEP_SLEEP successfully but this might by only coincidence and I potentially might be balancing on the coincidence. 

    When I make them equal, is it better to synchronize and make "Voltage time" and "Deep Sleep Voltage Time" to make those check at the same time? Or vice versa I need to make "Voltage time" smaller so I would check SLEEP condition prior DEEP_SLEEP will be estimated ? 

  • 0 in reply to Aivaras
    TI__Mastermind 28440 points

    Hi Aivaras,

    By this test, are you trying to get the gauge to enter DEEP_SLEEP automatically? I believe this can be done by setting the AUTO_DP_SLP_EN bit.

    I believe leaving these currents the same should be fine. I believe the process the gauge takes is that once it enters sleep mode from being under Sleep Current, if the Wake Comparator Current (taken from the periodic measurements in sleep) is lower then Deep Sleep Current, then the gauge should enter Deep Sleep.

    Regards,

    Anthony Baldino