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BQ78350-R1: State of charge drift

Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ34Z100-G1

Hello,

If battery in normal operation of our device never fully discharge, there might be a drift in the state of charge.

I need to estimate how much this drift may be after 100 , 1000 cycles of partial charge - discharge. form about 30% to 80% of full capacity.

Thank you.

  • Hi,

    This will really depend on your specific battery and how the total capacity of your battery changes over the number of cycles. The gauging accuracy will decrease over time due to the capacity changing and not getting updated. The capacity will decrease significantly as the battery ages, so the error can become large.

    Since your application will never reach 7% SOC to update the Full Charge Capacity, you may consider running characterization on several batteries over a large number of cycles to see how the capacity changes. If you have this information for your battery, you could manually update the 'Learn Full Charge Capacity' parameter periodically after a fixed number of cycles. 

    Best regards,

    Matt

  • Hello,

    except aging of battery cells, are there other factor that may contribute to state of charge drifting, like a difference in the current direction measured in or  out  of the battery.

    Thank you,

  • Yes, the accuracy of the charge measurement will also be a factor over time as error accumulates. When state of charge reaches the full charge voltage or if it reaches near full discharge (around 7%), it gives the gauge an opportunity to sync and remove the impact of cumulative inaccuracy.

    For your application, you may want to consider using an Impedance Track gauge like the BQ34Z100-G1. This type of gauge will have advantages for your application since it does not require full discharge to learn and update the capacity. There is a TI Reference Design available using the BQ769x0 with the BQ34Z100-G1 here: https://www.ti.com/tool/TIDA-010030

    And here is a video that describes Impedance Track gauging in a little more detail: https://www.youtube.com/watch?v=_8MzGy_tkEQ

    Best regards,

    Matt