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BQ27427: SOC at charge complete

David Smail
Prodigy 95 points
Part Number: BQ27427


Using the BQ27427 for a wearable using a small Lithium Ion battery (150mAh) whose charging voltage is 4.2. I've switched the chemistry to1202 and made the following changes to these data registers

Design Capacity  150

Design Energy     600

Terminate Voltage 3500

CC Gain 0.3346 (switched the sign to positive based on a previous post to this forum)

All is good with the SOC while discharging... 0% occurs exactly at 3500 mV!!!

However, when the charge is complete (0 mA charging current and feedback from the charger to indicate a full charge), the SOC max value is 93%. Are there any other parameters that need to be changed so that 100% charge is reached from the SOC register? I'm not looking for perfection, but need the SOC to reach 100%.

Thank you in advance

  • 0
    TI__Guru 64266 points

    Yes, please make sure that the gauge detects a valid charge termination. Details are in the TRM www.ti.com/.../sluucd5.pdf, chapter 7.4.2.3.10 Taper Rate, Taper Voltage.

    The gauge expects CC/CV charging. In the CV phase, the current will taper off until the charger stops. The cut-off current, Taper Current, must be detected by the gauge. Taper Voltage defines the voltage threshold that the gauge uses to start monitoring for the Taper Current. Taper Rate is the 0.1h rate for the Taper Current, so effectively 10x the C rate.

    If your charger stops at 100mA and the Design Capacity is 1000mAh, then the taper current is 100mA and the C rate is 1000mAh/100mA = 10h. The Taper Rate, in 0.1h units, is therefore 100 [0.1h]. As the gauge requires the current to drop below this threshold for 2x40s, it's necessary to set the rate for a slightly higher current, e.g. 10% higher. So this would be 110mA in this example and the rate would be 1000mAh/110mA = 9.1h and therefore Taper Rate should be set to 91 [0.1h].

    Once the gauge detects charge termination, it will measure cell voltage (and temperature) and use this to calculate depth of discharge at end of charge, which is then used to calculate FCC. By definition SOC = 100% when RM = FCC, hence this is an important concept and mechanism for the gauge to know what 100% SOC means.