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BQ25190: Battery SOC Estimation Using the BQ25190's ADC

Gilhwan Cha
Prodigy 40 points
Part Number: BQ25190

Tool/software:

Hi, this is Gilhwan Cha.
I am trying to estimate the SOC of a Li-ion battery by measuring its voltage through the internal ADC of a BQ25190EVM.

I measured it through TIcharger_gui:
Here are the results of my experiment.

It was 3.486V without charging, and I changed the ICHG Resisotor to 250mA, 500mA, 750mA, and 1000mA for charging experiments.

Q1. Is the current difference between the value set by ICHG and IBAT measured by ADC due to the load current (I2C, LED, etc.)?

Q2. As the current value to charge the battery increases, the VBAT measured by the ADC also increases, is this error due to the characteristics of the battery? I have difficulty estimating SOC.

Thanks.

  • 0
    TI__Genius 17585 points

    Hi Gilhwan,

    The ADC for VBAT will measure voltage at the VBAT pin which may vary from voltage at the battery cell depending on resistance between the two points. This means that while charging the VBAT pin will be

    (I_CHARGE_CURRENT * R_TRACE_RESISTANCE = V_VOLTAGE_DIFFERENCE)

    higher than the actual cell voltage. While discharging it will be 

    (I_DISCHARGE_CURRENT * R_TRACE_RESISTANCE = V_VOLTAGE_DIFFERENCE)

    below the actual cell voltage. For the most accurate reading it is recommended that charging be disabled and the load current placed on the battery be minimal.

    Gilhwan Cha said:
    Q1. Is the current difference between the value set by ICHG and IBAT measured by ADC due to the load current (I2C, LED, etc.)?

    This may be due to a couple items and has a few factors to take into account. First, current limiting due to functions such as TREG, VINDPM, VDPPM, ILIM, TS, and CV regulation can cause IBAT to be reduced from the configured ICHG. The load current can have an impact in triggering VINDPM, VDPPM, and ILIM protections so this may be a factor. Additionally, it is important to note that ICHG has a +/-10% accuracy specification, so it is normal to deviate by less than 10% accuracy from the charge current.

    Gilhwan Cha said:
    Q2. As the current value to charge the battery increases, the VBAT measured by the ADC also increases, is this error due to the characteristics of the battery? I have difficulty estimating SOC.

    This is likely due to the explained difference in voltage between the VBAT pin and the battery cell. As charge current increases, the I_CHARGE_CURRENT component increase causes the V_VOLTAGE_DIFFERENCE to increase as well. We recommending pausing charge for a brief period then measuring the ADC voltage in order to get the most accurate reading.

    Best Regards,

    Juan Ospina