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BQ78350-R1: Gauging a battery pack that isn't fully charged or fully discharged often

Kevin Knicker
Expert 1101 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350

Tool/software:

Dear TI support team,

I was wondering if TI had any information regarding the challenges of an application, in particular, a Lithium Iron Phosphate battery pack that isn't fully charged or fully discharged often.

I know that gauging this chemistry poses challenges because of it's flat discharge profile, and that a full charge and a full discharge need to be accomplished in order to establish the upper and lower points of a curve.

This is done prior to battery packs entering the field.

However, in the field we have seen applications where the pack doesn't always get a chance to receive a full charge or get fully discharged.

These conditions lead to the state of charge drifting.

Do you have any tips or recommendations to combat this challenge?

As far as I know, properly recharging the battery pack to full charge can help correct any state of charge errors.

In particular, setting the RSOC hold on so that the pack holds at 99% until a full charge condition is reached.

Is this correct?

Appreciate your expertise and insight on this subject.

Regards,

Kevin

  • +1
    TI__Guru 61856 points

    The bq78350 is a CEDV gauge which relies on qualified discharges to measure FCC so you cannot implement a rarely discharged application with this algorithm, regardless of chemistry.

    TI's other algorithm, Impedance Tracking, also requires regular discharges but they don't have to be quite as deep as for CEDV. The chemistry becomes important for the Impedance Tracking algorithm. LiFePO4 is problematic, as you wrote, because it's difficult to determine depth of discharge with voltage measurements as the OCV vs. DOD curve is flat. It's not impossible but it's difficult and TI's algorithm has exclusion zones and relaxation rules to address these challenges.

    However, a truly rarely discharged battery can't be gauged with any TI algorithm as by definition there's nothing that the gauge can measure if there is no discharge. This always comes down to a compromise, where the application is designed to periodically, e.g. once a month, allow for a discharge that is deep enough for the algorithm to learn changes in chemical capacity and cell resistance. The exact parameters (time interval, depth of this discharge) can be optimized for an application. These depend on OCV flatness and accuracy requirements and application limitations.

  • 0 in reply to Dominik Hartl11
    Expert 1101 points

    Thank you, Dominik.

    Just as a follow-up: Would you agree there is no good way to put a value on potential sources of error in state of charge over a period of time due to drift?

    Those sources of error mainly being a lack of a full charge or full discharge.

    Lastly, the best way to correct any error over time would be to as you state, perform a full discharge and full charge?

    Best Regards,
    Kevin

  • +1 in reply to Kevin Knicker
    TI__Guru 61856 points

    The bq78350 will not change SOC if it doesn't measure a current (SOC = 100% (FCC - passed charge)/FCC)). So if passed charge is 0, then SOC will be 100% or if passed charge is constant, then SOC will stay constant.

    The error will increase as there will be a real change to FCC as the cell ages over time while the gauge has no clue how FCC changes if you don't run a qualified discharge for FCC updates.