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Original question:

BQ27426: FuelgaugeIC can correctly match the true SOH range of the battery

BQ27426: Questions about VatChgTerm registers

Wang Dong
Intellectual 590 points
Part Number: BQ27426

Hello，

1, What is the purpose of the VatChgTerm register? Is it similar to reserve capacity?

2、Capacity between VatChgTerm to Terminal Voltage = Fuelgauge.FCC. Is the understanding correct?

3, How is VatChgTerm related to DODatEOC? What is the corresponding formula?

4、Is it feasible to have VatChgTerm=4400mV based on the battery information we are using? What is the reasoning?

5、What are the effects on the FuelGauge if VatChgTerm(4250mV --> 4400mV) is modified?Please help to elaborate.

6. What is the relationship between VatChgTerm and Taper Voltage?

7, The battery has V at Chg Term=4389mV and Taper Voltage=4339mV after the completion of gold learning.

According to FAE's recommendation, V at Chg Term=4250mV was manually modified in the generated FS file, and synchronized with Taper Voltage=4252mV.

The FAE reasoning is that if VatChgTerm is set too high, it may result in the aging cell not reaching 100% SOC. According to TI's experience the value should be set 100mV-150mV below the battery voltage.

7-1, Is the above reasoning correct? Can you tell the real reason?

7-2, Is it reasonable to manually modify V at Chg Term to 4250mV? What is the effect?

7-3, Is it reasonable to manually modify Taper Voltage=4252mV? What is the effect?

7-4. What is the correlation between V at Chg Term and Taper Voltage? What is the correlation with SOC?

  • 0
    TI__Guru 55826 points

    A1: The gauge uses V at Chg Term to calculate the depth of discharge for a full cell (DOD at EOC). It is the reference point for SOC = 100% (together with the taper rate, it defines FCC).

    A2: FCC is the total charge that the cell can supply from a full charge (DOD at EOC) until cell voltage drops below Terminate Voltage (+Delta Voltage) for a *specific* discharge scenario.

    A3: DODatEOC = f(VatChgTerm, taper current, T). The gauge models OCV as a function of DOD and T (this is the core of the ChemID and the algorithm). It is possible to calculate OCV if one has VatChgTerm, taper current and Ra(DODatEOC). The exact "f" is proprietary information.

    A4: VatChgTerm is a dynamic parameter. It should be set to the constant voltage of your charger initially. It's not necessarily the same as the charging voltage from the cell datasheet - it's application specific and depends on what you chose. The gauge will automatically measure and update it when it detects a full charge (which is one reason, why it's important for the gauge to detect a full charge).

    A5: The gauge will calculate a different DODatEOC and therefore FCC will change. Note that the gauge will only calculate DODatEOC at specific times so just changing VatChgTerm won't immediately have an effect. It's best set according to answer A4.

    A6: Taper Voltage is a threshold when the gauge starts monitoring the measured current during charging for charge termination detection. If the voltage is too low, the gauge will not consider the cell to be fully charged even if the current drops below the current given by the Taper Rate.

    A7-1: VatChgTerm is measured by the gauge and depends on your charging voltage. So if you set it to specific value, it will be overwritten by the gauge the next time the gauge detects a charge termination. Therefore the reasoning is not correct.

    A7-2: It's only reasonable, if your charger uses this voltage. Otherwise it's pointless. The effect is that FCC will adjust to reflect a charge termination at this voltage.

    A7-3 & 4: Taper Voltage should be set significantly below (at least 100mV below) the lowest charging voltage at charge termination in your application. If your charger can terminate charge with a constant voltage phase of 4250mV, then I would set Taper Voltage to 4150mV. Taper Voltage is only a threshold, it won't have a direct effect on gauging results. It just defines the voltage level that the gauge may consider a full charge.

  • 0 in reply to Dominik Hartl11
    Intellectual 590 points

    Hello Dominik，

    1、Your reply does not answer my question. My understanding is: Charge --> Battery Voltage > Taper Voltage --> Charge Current <Taper Current --> VatChgTerm updated --> DODatEOC simulation --> FCC change. Is the understanding correct?

    Wang Dong said:
    6. What is the relationship between VatChgTerm and Taper Voltage?

    2、In which scenarios are DODatEOC calculated? What conditions need to be met?

    Dominik Hartl11 said:
    A5: The gauge will calculate a different DODatEOC and therefore FCC will change. Note that the gauge will only calculate DODatEOC at specific times so just changing VatChgTerm won't immediately have an effect. It's best set according to answer A4.

    3、What is the basis for "at least 100mV below"?

    Dominik Hartl11 said:
    A7-3 & 4: Taper Voltage should be set significantly below (at least 100mV below) the lowest charging voltage at charge termination in your application. If your charger can terminate charge with a constant voltage phase of 4250mV, then I would set Taper Voltage to 4150mV. Taper Voltage is only a threshold, it won't have a direct effect on gauging results. It just defines the voltage level that the gauge may consider a full charge.
  • +1 in reply to Wang Dong
    TI__Guru 55826 points

    #1: Yes. The basic idea is correct. The details aren't complete. Please check chapter 7.4.2.3.10 Taper Rate, Taper Voltage in https://www.ti.com/lit/ug/sluubb0/sluubb0.pdf for details (it's not just Taper Voltage and Taper Rate that control charge termination detection).

    #2: The gauge will calculate DODatEOC at start of charge, charge termination detection and if temperature changes by more than 5deg.C (default). So if you just change VatChgTerm, you'll also have to trigger one of these events for DODatEOC (and FCC) to change.

    #3: It's based on what I think is a good margin. You can change it as you wish but make sure that it's not too small as the gauge may not reliably detect a full charge. Also, if you set so that the margin is too high, and other conditions from 7.4.2.3.10 are met, the gauge may erroneously detect a full charge.