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BQ34z100-G1 Open circuit capacity estimation inaccurate

Sam Parkinson
Prodigy 85 points

Hi,

I am using a 2S LI-ion pack, I have created a golden image which I am using to program my fuel gauge IC's.

I have configured the gauge to report 0% capacity when the pack reaches 7V (actual 0% capacity for the pack is closer to 6.5V) this helps the product to shut down early and leave enough capacity in the pack for a few months of non-use.

I have submitted my battery chemistry to the online tool and it has recommended chemistry 1193 which I am using.

I am happy with the charge / discharge capacity tracking it correctly predicts 0 and 100%.

However when a new battery is inserted to the fuel gauge (and a reset command is issued to the gauge) it is incorrectly estimating the capacity based on the open circuit voltage.

With an external battery measuring from 7 to 7.4V it reports 0% capacity (with no load on battery) It only begins to increase from 0% when the pack voltage exceeds 7.4V which is incorrect.

Can you please help me to understand what parameters are used to determine the open circuit capacity estimation on a cold connection?

From my limited understanding, providing there is no current draw on the battery, the resistance tables should not be used and instead it should rely on a combination of the chemistry file and the value programmed into "Cell terminate voltage" which is currently 3500 (7V)

I have attached an srec file any help would be greatly appreciated.0100_0_16-bq34z100G1-1193.zip

  • 0
    TI__Mastermind 34775 points

    Hi Sam,

    Your Cell Terminate Voltage is set to 3500mV and you have 2 cells. The Gauge is simulating based on the cell resistance and a min current load (even though there's no load present) that cell terminate voltage will be reached at ~7.4V  (3.7V/cell) OCV. Hence it will show 0% SOC. It's not practical for the gauge to give an SOC based on no load current.

  • 0 in reply to Damian Lewis
    Prodigy 85 points

    Thanks Damian,

    That makes sense, I repeated the testing by drawing 1A from the pack and issuing a reset command, this enabled the fuel gauge to display sensible capacities between 7 and 7.4V.

    Would I be correct in assuming that it does not matter what the discharge current is when estimating the capacity of a new battery providing it is non zero?

    Is there a recommended discharge current to provide an accurate first guess at the capacity of a cold connected battery?

    I also assume that the first guess happens very shortly after the battery is connected (probably no load) so will need to be manually reset once the current is stable?

  • 0 in reply to Sam Parkinson
    TI__Mastermind 34775 points

    Hi Sam,

    I'm glad I could help clarify things. Yes the remaining capacity estimation is based a current or power consumption. As the gauge goes through charge and discharge cycles, it learns the system load profile and battery discharge profile to make accurate prediction of capacity and state of charge. The discharge current should be close to your application load current, but if you're not sure a load current of C/5 to C/10, where C is one series cell or parallel cells capacity. There's no need to manually reset later. As the cell voltage crosses one of the DOD points it will update or if the cell is left to relax after connected. 

    For new batteries we recommend running a learning cycle:

    1. Configure the dataflash with appropriate parameters suitable for customer's application; configure pack's chemistry and perform calibration.
    2. Activate impedance track.
    3. Charge the pack to charge termination.
    4. Allow the pack to rest until the OCVTAKEN flag sets (~ 2hrs).
    5. Discharge the pack at a C/10 to C/5 rate to the Termination Voltage.
    6. Allow the pack to rest until the OCVTAKEN flag sets (~ 5hrs). 
    7. Fully charge the pack to charge termination.
    8. Allow the pack to rest until the OCVTAKEN flag sets (~ 2hrs).
    9. Discharge the pack at a C/10 to C/5 rate to the Termination Voltage.
    10. Allow the pack to rest until the OCVTAKEN flag sets (~ 5hrs).