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BQ27441-G1: Erroneous State of Health, and strange SoC behavior

Andreas Urke
Expert 1460 points
Part Number: BQ27441-G1
Other Parts Discussed in Thread: BQ24074

Hi,

While testing the BQ27441-G1A we are seeing an issue where gauge says State of Health status is Ready, yet the reported State of Health and Remaining Capacity is wrong: It reports approx. half of the design capacity, while we estimate the true value somewhere between 90 - 100 %. We have configured the BQ as recommended in the Quick Start Guide www.ti.com/.../sluuap7.pdf (except the NVM-part, yet we do not power off the BQ during these tests). We also notice in the final charge before going SoH goes to Ready, the State of Charge jumps from about 60% to 100 % midway during taper charging (while charging at about 500 mA of 1 A max, cutoff at 100 mA). It leads us to believe there is something wrong with our configuration, yet double-checks have not revealed anything. We are testing a battery with two cells in parallell and the following parameters: 5200 mAh design capacity, 19240 design energy, 2700 terminate voltage, and 520 taper rate (BQ24074 charger at defaults).

If it possible to point us in a direction of troubleshooting based on this description we would appreciate it. Regardless we have some questions:

1. The quick start guide section 4 states a guard around the taper rate is advisable. However, looking at other doc., implementations and forum posts, this is typically not utilized. Could you confirm if the guard is necessary? Is the 15 % in the example an appropriate value?

2. What accuracy can we expect of the SoC before any significant learning / charging cycles have been conducted?

3. When the SoH state reports Ready, can we use this to infer the SoC and other values are now fairly accurate?

Regards

( I re-posted this as the original post was marked with wrong part number)

  • 0
    TI__Guru** 102955 points

    Hello Andreas,

    We will look into this and provide feedback, it may be delayed due to Thanksgiving holiday.

    Sincerely,

    Wyatt Keller

  • +1
    TI__Guru 53616 points

    #1: The reason for the 15% adjustment for taper rate is that it is critical for the gauge to detect a full charge. The way this works is described in the TRM but in a nutshell, the average current must be below the taper current for 2*40s and if you set the taper rate so that the taper current is exactly the cut-off current from the charger, then the gauge won't detect a full charge because of the 2*40s requirement. If your charger cuts off at 100mA, which is C/52, which is a very high rate (and low current), then I'd set the taper rate to a taper current of more than 15% higher. Like 50% higher, 150mA, for a taper rate of 347. This is still a super low current (and will likely not add significant charging capacity so it's safe to ignore the extra passed charge). The goal is for the gauge to detect charge termination.

    The reason why this is so important is that the gauge uses measurements from charge termination detection to establish the SOC = 100% point (=maps it to the absolute DOD, called DODatEOC). Without this, the gauge won't be able to calculate meaningful FCC and therefore SOH.

    #2: This depends on the cell. Impossible to say (unless you accept up to +/-20 percentage points).

    #3: Only if the basic configuration is good. Which isn't the case here.

    From what you provided, it looks like the taper rate is too high. It's too high for reliably charge termination detection and it is too high for the gauge to be in charging state. Quit current, by default is set to a C/25 rate (250 0.1Hr rate) so the gauge won't even be in charging state once the current actually drops below C/25 (= 208mA) for 60 seconds.

    --> Set Taper Rate to 347 and Quit Current to 520. This will allow the gauge (most likely) to reliably detect charge termination and will fix the FCC (and SOH) problems. This depends on the actual current taper curve and assumes that the current stays above C/52 and below C/34.7 for at least 2*40s.

  • 0 in reply to Dominik Hartl11
    Expert 1460 points

    Thank you very much Dominik. A short follow-up: Is it necessary for the gauge to see the actual charger termination (as it cuts off at for example 100 mA), or is it sufficient for it to see the taper current for 2 x 40s? In other words, if we wanted to avoid tapering all the way to the cutoff, would the gauge be negatively influenced if we "manually" cut charging when SoC reaches 100 % (after taper current for 2 x 40 s) as opposed to letting it taper down to charger cutoff.

  • 0
    TI__Guru 53616 points

    From a gauge point of view, what matters is that the gauge detects a full charge where it sets FC in Flags = 1. You can stop charging right after this event.