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BQ27520-G4: Capacity doesn't show 100% when battery is fully charged

Mitja Reppold
Prodigy 70 points
Part Number: BQ27520-G4
Other Parts Discussed in Thread: BQ25601,

Tool/software:

Hello there, 

I am using the BQ27520-G4 Fuel Gauge in combination with the BQ25601 Battery Charger.
Unfortunately the Gauge doesn't behave like I'd expect: When the battery is charged the gauge either shows 100% capacity way to early or never reaches the 100%.
I've tried multiple versions of the Golden Image but none of them really worked well.
In the one where I achieved the best results I've modified the following values:
 

  • TEMPS: set to internal Temp Sensor
  • LDMD: constant load - since this fits better than const. current, though I have not really a constant load either
  • CHG Inhibit Temp low/high as well as Suspend Low and High Temp: Charging is never suspended oder inhibited so I set those values to its min/max
  • Taper Volt: I reduced this to 50 mV since I hoped to achieve better capacity values with a more narrow taper voltage
  • Taper Current: Set to the taper current the charger is using
  • OT Chg, Chg Recovery, Dsg, Dsg Recovery: Set to its max values (not used)
  • Charging Voltage: Set to the charging voltage the charger uses (4100mV)
  • Design Capacity: Set to the Capacity of the battery
  • Terminate Voltage: Set to the voltage, which leads to a shutdown of the device
  • Qmax Cell 0 and 1: also set to the Capacity of the battery
  • Chem ID: Set to the best fitting according to the Gauging Parameter Calculator

Are there any more values I have to modify? Or is there anything else what I am missing? 

Tank you very much!

  • 0
    TI__Guru 61516 points

    Please set Taper Current 10% higher than the cut-off current from your charger.

    Change the Taper Voltage back to the default value.
    Set the Charging Voltage to your charger's constant voltage.

    Make sure that when you charge, the current drops below Taper Current for at least 80 seconds (2x40s) before the charger stops while actual cell voltage is within Taper Voltage of Charging Voltage. That's what the gauge uses to detect a full charge.

    Set Cell 0/1 V at Chg Term to the cell voltage when the current drops below Taper Current. This voltage, in combination with the Taper Current, is what tells the gauge what 100% SOC means.