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BQ20Z65-R1: Full charge RSOC appears less than 97%

Nitin Ghige
Prodigy 170 points
Part Number: BQ20Z65-R1

What are all possible causes that the some batteries shows Full charge RSOC less than 97%?

Sometime RSOC jumps from 95% to 100% due to charge termination and again jump back to 95% after few minutes of rest. 

Batteries are 4S2P. These are new batteries from manufacturer with Cyclecount = 0 .

1. How many full charge and discharge cycles recommended to get learning phase completed?  

2. I would like to understand how FCC and RC calculation works in Charging, Relax and Discharging modes.

  • 0
    TI__Mastermind 42535 points

    Hi Nitin,

    Can you please explain what you mean by full charge RSOC less than 97%? You can set FC = -1% so that FC is indicated only on charge termination. During relaxation DOD0 is again computed, once this is done RM will drop to match what is the appropriate DOD value of remaining capacity.

    1. We recommend 2 full chg dsg cycles for learning

    2. For a better understanding of how FCC and RM are computed please refer to http://www.ti.com/lit/an/slua364b/slua364b.pdf

  • 0 in reply to Batt
    Prodigy 170 points

    Thank you for quick response.

    My setting about FC is

    FC set % = 100.

    When I say Full charge RSOC, it meant the RSOC level where battery stops charging. When I keep battery for charging, after completion of charging I see battery RSOC less than 97%. The expected is that the battery should show 100% RSOC after charge completion. Our charger keep on charging till FC bit set. This observation is not consistent out of 22 batteries 5/6 batteries showed this.

    As you said during relaxation DOD0 again computed, what are the factors considered for compuatation? Is the last discharge cycle depth or current matters? For battery discharge purpose we use current 2A whereas in application our current is ~0.5A. Is the discharging current is very high for our 93Wh battery? will this affect anything? what is recommended discharge current for stable operation.

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    You can use varying rates of dsg. DOD effectively depends on load rate, V, I and T.

    In your case, FC = -1% would allow FC to set at charge termination if you would like your cell to reach full charge after chg taper. That would be a better setting for your charging case.

  • 0 in reply to Batt
    Prodigy 170 points

    I am not allowed to change any setting for now.

    I would like to understand why in some batteries the RSOC is not reaching to 100% at charge termination?

    Batteries show jump in RSOC at the end of charge, for example RSOC jumps from 95% to 100%. The same battery removed from Charger come back to 95% within few minutes.

    We have CSYNC bit set due to which once primary charge termination (ChargeCurrent<Taper current) occured, the RSOC jump to 100% regardless on anything.

    I wanted to understand why some batteries reaches to only 95-97% at full charge and jumps to 100% instead going in increment 97-98-99-100?

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    Hi Nitin,

    As I mentioned, some batteries have a higher internal resistance which means on charge relaxation their terminal voltage will drop. If the OCV for the terminal voltage corresponds to 95% the gauge will read that.

    If your gauge detects charge termination per your settings, both 0 and 100 are basically anchor points of RSOC. At either extremes a jump is allowed if the gauge detects the appropriate conditions.

  • 0 in reply to Batt
    Prodigy 170 points

    What are the possible reasons because of that my some battery's charge termination (based on taper current) occurs at less than RSOC 99% (say 94 or 95%).

    During charging, the battery RSOC appears ~95% when charge_current reaches to TaperCurrent. Once the current reaches TaperCurrent the RSOC jumps to 100%. If this happened, the battery when connected to load may show RSOC 95%.

    Thus a fully charged battery showing 95% instead 100%.

     

     

    In some batteries, The RSOC appears 99% when charge_current reaches to TaperCurrent.

     

    FYI..

    My charger keeps on charging cells beyond TaperCurrent. TaperCurrent is 280mA and my charger keeps charging cell till 200mA.

    My battery discharge operation includes the discharge till CUV sets. The battery keeps on discharging after Term Voltage (At Term Voltage RSOC jumps to 0%).  My Term voltage is 11.250V and CUV setting is 2.750V ( 4 cells).



    Is this extra charging (above Taper current) and discharging  (below Term Voltage) causing any issue?

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    OCV is a function of terminal voltage of the battery, once you stop charging and the cell starts relaxing, or at the moment the load hits the cell, an estimation of the RSOC is made by the gauge based on cell voltage, T and load current which determines the intrinsic resistance, this can certainly cause a jump in RSOC at high loads. This can be exacerbated by an old cell which has gone through many cycles as well.

    In your case, either might apply.

    Overcharging beyond taper current is OK, not really a concern. However, I'd suggest that you don't dsg the battery to CUV as you can cause metallization at the anode and reduce battery life. Terminate voltage and maybe 50 to 100mV below that is the max you should dsg.

  • 0 in reply to Batt
    Prodigy 170 points

    Thank you Batt for information provided.

    I understand that the RSOC jumps from 100% to 95% and vice versa is expected behavior.

    I have one more question :

    During constant power discharge I observed that the RemainingCapacity reported by battery is not linear, I see step increase at 88-89% RSOC during discharge.

    Can you please explain the behavior here? (The question is "why Remaining Capacity suddenly increased? The RemainingCapacity should always decrease during constant discharge.)

    Below graph shows RemainingCapacity and FullChargeCapacity during two discharge cycles.

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    Hi Nitin,

    I can't see the image that you attached. However, it is likely that when you cross the first grid point during dsg, the value of FCC changes, if this change decreases FCC (per IT simulation, calculated based off your V, I, T, and load), your RSOC can increase.

  • 0 in reply to Batt
    Prodigy 170 points

    Is it FCC or RM which changes at first grid point? I beleive RM changes and hence FCC changes.

    (FCC = Full Charge Capacity; RM = Remaining Capacity)

    What is the maximum change in RM/FCC expected after first grid point for a good battery?

    What is the specified accuracy for FCC/RM estimation? Consider that the battery is new with Cyclecount ~10 for aging point of view. I supposed to get the accuracy less than 15%.

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    Hi Nitin,

    IT estimates RM, what you get is a remcap simulation based on your load select along with V, and T.

    It really is not something that you can predict. Typically the capacity changes can be small. If your load model is correct along with a good chem ID and a learning cycle, the gauge can estimate capacity accurately within 1% of true capacity.

  • 0 in reply to Batt
    Prodigy 170 points

    Is the load power/current in full discharge cycle matters in estimating FCC?

    I observed that less load power estimates larger FCC whereas more load power estimates smaller FCC. What is discharge current recommended for better accuracy?

    I observed during battery cycling, with dischraging at 7.2W (~0.5A), 4S1P battery estimating FCC more accurate than 4S2P battery. I suspect the load current playing any role here.

    Regards,

    Nitin

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    That is correct. This is because the IR drop caused by the load from the OCV curve affects capacity. It simply means that you hit terminate voltage earlier at a higher load, reflecting less capacity.

  • 0 in reply to Batt
    Prodigy 170 points

    Our problem is : at the start of discharge the FCC estimation is less accurate. The FCC becomes more accurate after first grid point update.

    I beleive the first FCC estimation (when discharge onset) comes from relaxed cell voltages and load current. What should we do for better accuracy during the start of discharge (say RSOC 100 to 88%)?

    Is the first estimation of RM/FCC depend on previous discharge cycle depth/load current? OR the estimation become better with number of cycles executed?

    Regards,

    Nitin

  • 0 in reply to Nitin Ghige
    TI__Mastermind 42535 points

    Nitin,

    There's not much you can do. It's an artifact of both the load used and the OCV taken from the chemistry. You may get better estimates by choosing load select to use average current instead of avg I last run. However, the accuracy will get better as you pass more grid points during dsg.