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BQ78350-R1A: How to Set High Reserve Capacity without Resulting in Low Indicated SOH

Lon Schneider
Prodigy 200 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ76940,

Tool/software:

We have a 12S1P application for bq78350-R1A with bq76940 for which the cell capacity is ~1000 mAh and the customer wants 140 mAh in Reserve Capacity. When I set Design Capacity (and the initial value of Learned FCC) to 1000 mAh, FCC is correctly shown as 860 mAh both before and after EDV2, But SOH reads 86% because it calculates FCC/Design Capacity without compensating for the high Reserve Capacity. I had previously tried setting Design Capacity to 860 mAh, but the gauge then subtracted the 140 mAh Reserve from that, resulting in FCC = 720 mAh.  Please tell me how to set the gauge parameters so that FCC is ~ (Cell capacity - Reserve capacity), and SOH is not reduced due to the high Reserve Capacity.

Note: FCC_LIMIT is set. CEDV Gauging is not enabled. 

Also, I noted that although the discharge was valid (VDQ=1), and   DoD@EDV2 was modified during a discharge cycle, FCC remained at 860 mAh (not even differing by 1 mAH) throughout the discharge to EDV0. It seems that the only adjustment to FCC was subtracting Remcap from Design Capacity. 

Thank you,

Lon Schneider

iTECH

 lschneider@itecheng.com

  • 0
    TI__Guru 61536 points

    This is a fundamental property of this gauge and there is no way to change this. SOH is defined as SOH = 100 * FCC/DesignCapacity.

    This would have to be compensated for on the host uC. If you know that there's a reserve capacity, then you'd have to calculate SOH_scaled = SOH * DesignCapacity/(DesignCapacity - ReserveCapacity).

  • 0 in reply to Dominik Hartl11
    Prodigy 200 points

    I'm trying to learn how TI intends users to set Reserve Capacity and the parameters that affect its calculation. For that purpose, I had intended to include the following when I first replied to your response but failed to do so..Perhaps my settings are not correct, and that is how I ended up trying Design Cap >>>FCC  which cause SOH to be low at the start..

    The TRM says that the value of programmed Design Capacity "should not include Reserve Capacity." I initially took that to mean that Design Capacity should be programmed as (Pack Capacity - Reserve.) I tried that and SOH was 100%, but once EDV2 was reached (with actual capacity delivered being ~ 820 mAh at that point)  FCC dropped from 860 to 720 mAh. i.e. Overall the Pack Capacity was reduced by 2x Reserve Capacity. That is why I then tried using Design Capacity = Pack Capacity.

    With Design Capacity = Pack Capacity, at EDV2, FCC was calculated as 860 mAh , but SOH was calculated as FCC/Des Cap, resulting in SOH = 86% at best. 

    Then I went back to DES CAP = (Pack Capacity - Reserve). but disabled FCC_LIMIT. The next discharge resulted in Learned FCC going to 1182 mAh!!! 

    FCC went to 1042 mAh, Learned Capacity - Reserve..   

    After multiple attempts to apply the instructions in the TRM for handling Reserve Capacity and finding anomalies, I'd greatly appreciate it if you would provide specific instructions or confirmations for the settings related to Reserve Capacity at this rather high level.  

    • Reserve Capacity is 140 mAh.
    • The Pack is 12S1P with cell capacity of 1000 mAh at the intended load.
    • Design Capacity: Should it be set at (Pack Capacity - Reserve.) or simply at Pack Capacity?
    • EDV 0, 1, and 2 were set for min Vcell at 3 levels;  The 140 mAh reserve, 140 mAh + 3%*FCC, and 140 mAh+ 5%*FCC respectively.    CORRECT?
    • Battery Low % was set at 18.9%,. to include 5% of FCC plus 140 mAh Reserve, as indicated in the TRM.  Please confirm.
    • DoD@EDV2 was initially set at 13,271 corresponding to 18.9% of FCC
    • Learned FCC was initially set at 1000 mAh, the actual capacity of the pack if there were no Reserve.
    • Did I miss anything relevant?
    • (To conserve charge, the gauge is shut down by the on-board uC after every operation (Chg, DSG, or even reading LEDs for SOC.))
    • CEDV is not enabled, in part because the gauge must determine its SOC every time it is awakened from Shutdown
    • FCC_LIMIT will be enabled. (For testing, I turned it off to determine whether it affected the calculation of FCC.  It did!)

    Many thanks,

    Lon

  • 0 in reply to Lon Schneider
    TI__Guru 61536 points

    The reason why we recommend to set DesignCapacity to the cell's nominal capacity (without subtracting reserve capacity) is because several parameters are expressed relative to DesignCapacity. For example, cycle count depends on discharge as a percentage of DesignCapacity. Or filtering of measured voltage depends on the load exceeding a C rate (C/3 and C/5). It's also a question of consistency.

    The gauge subtracts ReserveCapacity from the calculated FCC after a qualified discharge.

    DesignCapacity must be set to the nominal capacity of the cell as per the cell's datasheet. In your case, 1000mAh.

    EDV 2, 1 and 0 must be set to the BatLow%, 3% and 0% level without reserve capacity.

    Initial FCC must be set to the capacity that you measure for a typical discharge load at room temperature from a full charge to EDV0 minus reserve capacity.