Datasheet

DS2780 Standalone Fuel Gauge IC
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Figure 11. Top Level Algorithm Diagram
MODELING CELL CHARACTERISTICS
In order to achieve reasonable accuracy in estimating remaining capacity, the cell performance characteristics over
temperature, load current, and charge termination point must be considered. Since the behavior of Li-ion cells is
non-linear, even over a limited temperature range of 10°C to 35°C, these characteristics must be included in the
capacity estimation to achieve a reasonable accuracy. See Applications Note AN131 “Li+ Fuel Gauging with Dallas
Semiconductor Devices” for general information on the FuelPack™ method used in the DS2780. To facilitate
efficient implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2780. Full and empty points are retrieved in a lookup process which re-traces a piece-wise
linear model. Three model curves are stored: Full, Active Empty and Standby Empty. Each model curve is
constructed with 4 line segments and spans from 0°C to 40°C. Operation outside the 0°C to 40°C model span is
supported by the model with minimal loss of accuracy. Above 40°C, the 40°C fixed points are extended with zero
slope. This achieves a conservative capacity estimate for temperatures above 40°C. Below 0°C, the model curves
are extended using the slope of each 0°C to 10°C segment. If low temperature operation is expected, the 0°C to
10°C slopes can be selected to optimize the model accuracy. A diagram of example battery cell model curves is
shown in Figure 12.
FuelPack is a trademark of Maxim Integrated Products, Inc.