SLUA450A January   2008  – November 2022 BQ27421-G1 , BQ27425-G2A , BQ27425-G2B , BQ27441-G1 , BQ27505-J2 , BQ27505-J3 , BQ27505-J4 , BQ27505-J5 , BQ27520-G4 , BQ27530-G1 , BQ27531-G1 , BQ27545-G1 , BQ27546-G1 , BQ27741-G1 , BQ40Z50 , BQ40Z50-R1 , BQ40Z50-R2

 

  1.   Theory and Implementation of Impedance Track™ Battery Fuel-Gauging Algorithm in bq2750x Family
  2.   Trademarks
  3. 1Summary of the Algorithm Operation
  4. 2Parameters Updated by the Gas Gauge in More Detail
    1. 2.1 Modes of Algorithm Operation
    2. 2.2 Update of Chemical Depth of Discharge (DOD)
    3. 2.3 Update of Qmax
    4. 2.4 Update of Resistance
    5. 2.5 Update of Temperature Model
    6. 2.6 Update of DataRAM.Remaining Capacity (RM) and DataRAM.Full Charge Capacity (FCC)
    7. 2.7 Update of DataRAM.Remaining Capacity( ) and DataRAM.State Of Charge( ) Values
  5. 3Real Application Example
    1. 3.1 GSM Smart Phone Application
  6. 4Revision History

2.3 Update of Qmax

Maximal chemical capacity Qmax for a battery cell is stored in Data Flash as DF.Qmax Cell x, where x = 0,1, the cell number. The GG updates Qmax based on two DOD readings made before and after charge, or discharge. For example, DOD1 is taken during relaxation, then a discharge mode starts, and PassedCharge is integrated. Following this, another relaxation mode is entered, and DOD2 is taken;

DOD1 and DOD2 are calculated from OCV readings in a well-relaxed state, as exemplified in Figure 2-2 for subsequent DOD0 measurement. A well-relaxed state is detected if dV/dt<4 μV/s or maximal waiting time of 5 hours is exceeded. The first condition is satisfied in typical batteries after approximately 1 hour if DOD is between 0 and 80%, and 3–4 hours if DOD is above 80%. At low temperature relaxation takes a longer time.

In order to ensure high accuracy of the DOD measurement, the Qmax calculation does not occur if the temperature is above 40°C, or below 10°C. It also does not occur if at least one of voltage measurements for DOD1 or DOD2 was taken in the cell voltage range between 3737 mV and 3800 mV because of very flat OCV(DOD) dependence in this range. These limits are chemistry dependent and are specified for different chemistries separately.

Qmax is calculated as Qmax = PassedCharge / (DOD2 – DOD1)

The data flash constant DF.Update Status increments by 1 when the first Qmax update takes place (e.g., from 0 to 1 if no resistance update were made or from 1 to 2 if a resistance update was made).

PassedCharge has to be more than 37% of DF.Design Capacity for an update to occur. For the first cycle (with DF.Update Status = 0), 90% of DF.Design Capacity is required because this cycle takes place in the factory settings and Qmax is learned for the first time.

In order to prevent Qmax fluctuations, a first-order smoothing filter is applied to all Qmax readings except in the first cycle. Readings with lower PassedCharge are assigned lower weights in the smoothing.