Rainbow Electronics DS2781 - Manual (Page 13)

DS2781: Two-Cell Standalone Fuel Gauge IC
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MODELING CELL STACK CHARACTERISTICS
In order to achieve reasonable accuracy in estimating remaining capacity, the cell stack performance
characteristics over temperature, load current and charge termination point must be considered. Since the behavior
of Li-ion cells is non-linear, these characteristics must be included in the capacity estimation to achieve an
acceptable level of accuracy in the capacity estimation. The FuelPack™ method used in the DS2781 is described
in general in Applications Note AN131 “Li+ Fuel Gauging with Dallas Semiconductor Devices”. To facilitate efficient
implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2781. Full and empty points are retrieved in a look-up process which re-traces piece-wise
linear model consisting of three model curves named Full, Active Empty and Stand-by Empty. Each model curve is
constructed with 5 line segments, numbered 1 through 5. Above 40C, the segment 5 model curves extend infinitely
with zero slope, approximating the nearly flat change in capacity of Li-Ion cells at temperatures above 40C.
Segment 4 of each model curves originates at +40C on its upper end and extends downward in temperature to the
junction with segment 3. Segment 3 joins with segment 2, which in turn joins with segment 1. Segment 1 of each
model curve extends from the junction with segment 2 to infinitely colder temperatures. Segment slopes are stored
as µVh PPM change per ºC. The three junctions or breakpoints that join the segments (labeled TBP12, TBP23 and
TBP34 in figure 12) are programmable in 1C increments from -128C to +40C. They are stored in two’s complement
format in locations 0x7C, 0x7D, and 0x7E respectively The slope or derivative for segments 1, 2, 3 and 4 are also
programmable. One the lower (cold) end of each model curve, segment 1 extends from breakpoint TBP12 to
infinitely to colder temperatures.
Figure 12. Cell Model Example Diagram
Full: The Full curve defines how the full point of a given cell stack depends on temperature for a given charge
termination. The charge termination method used in the application is used to determine the table values. The
DS2781 reconstructs the Full line from cell characteristic table values to determine the Full capacity of the battery
at each temperature. Reconstruction occurs in one-degree temperature increments. Full values are stored as ppm
change per ºC. For example if a cell had a nominal capacity of 1051mAh at 40ºC, a full value of 1031mAh at 18ºC
(TBP34) and 1009mAh at 0ºC (TBP23), the slope for segment 3 would be:
TBP12 TBP23 TBP34 40C
100%
Segment 1
Derivative
[ppm / C]
Active
Empty
Stand-by
Empty
FULL
Cell Characterization
data points
Seg. 2 Seg. 3 Seg. 4 Seg. 5
FuelPack is a trademark of Dallas Semiconductor.