Theory and Implementation of Impedance Track™ Battery Fuel-Gauging Algorithm in bq2750x Family

This application report outlines the theory of Impedance Track™⁽¹⁾ (IT) technology used in the bq2750x series of fuel gauge ICs for single-cell Li-ion application (e.g., smart phones, media players, and PDAs). The implementation of the IT algorithm in the bq2750x family is reviewed, and the setting of data flash constants associated with the fuel-gauging algorithm is described in detail.

⁽¹⁾_{Impedance
Track algorithm is protected by US Patents US6832171, US6789026, and
US6892148.}

Trademarks

Impedance Track™ is a trademark of Texas Instruments.

All trademarks are the property of their respective owners.

1 Summary of the Algorithm Operation

The gas gauge algorithm uses three types of information to calculate remaining capacity (DataRAM.Remaining Capacity( )) and full-charge capacity (DataRAM.Full Charge Capacity( )).

Chemical: depth of discharge (DOD) and total chemical capacity Q_max
Electrical: internal battery resistance dependence on DOD
External: load and temperature

DataRAM.Full Charge Capacity( ) is defined as the amount of charge passed from a fully charged state until the voltage defined in DF.Terminate Voltage flash constant is reached at a given rate of discharge, after subtracting the reserve capacity (DF.Reserve Capacity).

Note that DataRAM.Full Charge Capacity( ) depends on the rate of discharge and is lower at higher rates and low temperatures because the cell I*R drop causes the Terminate Voltage threshold to be reached earlier.