BQ2085 Datasheet, PDF (44/56 Pages) Texas Instruments – SBS-COMPLIANT GAS GAUGE IC FOR USE WITH THE bq29311

bq2085
SLUS541 – OCTOBER 2002
EDV Discharge Rate and Temperature Compensation
If EDV compensation is enabled, the bq2085 calculates battery voltage to determine EDV0, EDV1, and EDV2
thresholds as a function of battery capacity, temperature, and discharge load. The general equation for EDV0,
EDV1, and EDV2 calculation is
EDV0,1,2 = n (EMF • FBL - | ILOAD | • R0 • FTZ)
(5)
EMF is a no-load cell voltage higher than the highest cell EDV threshold computed. EMF is programmed in
mV in EMF/EDV1 DF 0x84–0x85.
ILOAD is the current discharge load magnitude.
n = the number of series cells
FBL is the factor that adjusts the EDV voltage for battery capacity and temperature to match the no-load
characteristics of the battery.
FBL = f ( C0, C + C1, T )
(6)
C (either 0%, 3%, or Battery Low % for EDV0, EDV1, and EDV2, respectively) and C0 are the capacity-
related EDV adjustment factors. C0 is programmed in EDV C0 Factor/EDV1 DF 0x86–87. C1 is the desired
residual battery capacity remaining at EDV0 (RM = 0). The C1 factor is stored in EDV C1 Factor DF 0x8f.
T is the current temperature in °K.
R0 • FTZ represents the resistance of a cell as a function of temperature and capacity.
FTZ = f ( R1 , T0, T, C + C1, TC)
(7)
R0 is the first order rate dependency factor stored in EDV R0 Factor/EDV2 DF 0x88–0x89.
T is the current temperature; C is the battery capacity relating to EDV0, EDV1, and EDV2.
R1 adjusts the variation of impedance with battery capacity. R1 is programmed in EDV R1 Rate Factor DF
0x8c-0x8d.
T0 adjusts the variation of impedance with battery temperature. T0 is programmed in EDV T0 Rate Factor DF
0x8a–0x8b.
TC adjusts the variation of impedance for cold temperatures (T < 23°C). TC is programmed in EDV TC DF 0x8e.
Typical values for the EDV compensation factors, based on overall pack voltages for a Li-lon 3s2p 18650 pack,
are
EMF = 11550/3
T0 = 4475
C0 = 235
C1 = 0
R0 = 5350/3
R1 = 250
TC = 3
The graphs in Figures 8 and 9 show the calculated EDV0, EDV1, and EDV2 thresholds versus capacity using
the typical compensation values for different temperatures and loads for a Li-Ion 3s2p 18650 pack. The
compensation values vary widely for different cell types and manufacturers and must be matched exactly to the
unique characteristics for optimal performance.
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