DS2780 Maxim Integrated Products, DS2780 Datasheet - Page 11

• Manufacturer Part Number: DS2780
• Description: Stand-Alone Fuel Gauge IC
• Manufacturer: Maxim Integrated Products

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CAPACITY ESTIMATION ALGORITHM
Remaining capacity estimation uses real-time measured values and stored parameters describing the cell
characteristics and application operating limits. The following diagram describes the algorithm inputs and outputs.
Figure 10. 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
FuelPack is a trademark of Dallas Semiconductor.
Accumulated
Current (ACR) (R/W)
Average Current (R)
Aging Cap (AC)
(2 bytes EE)
Age Scalar (AS)
Sense Resistor’
(RSNSP) (1byte EE)
Charge Voltage
(VCHG) (1 byte EE)
Min Chg Current
(IMIN) (1 byte EE)
Active Empty
Voltage (VAE)
(1 byte EE)
Active Empty
Current (IAE)
(1 byte EE)
Voltage
Temperature
Current
(1 bytes EE)
Parameters
(EEPROM)
16 bytes
Cell
(R)
(R)
(R)
Available Capacity Calculation
ACR Housekeeping
Capacity Look-up
Learn Function
Age Estimator
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DS2780 Stand-Alone Fuel Gauge IC
FULL
Active Empty
Standby Empty SE(T)
Remaining Active Absolute
Capacity (RAAC) mAh
Remaining Stand-by Absolute
Capacity (RSAC) mAh
Remaining Active Relative
Capacity (RARC) %
Remaining Stand-by Relative
Capacity (RSRC) %
FULL(T)
AE(T)
(R)
(R)
(R)
(R)
(R)
(R)
(R)