Rainbow Electronics DS2784 - Manual (Page 17)

DS2784: 1-Cell Stand-Alone Fuel Gauge IC with Li+ Protector and SHA-1 Authentication
CAPACITY ESTIMATION ALGORITHM
Remaining capacity estimation uses real-time measured values, stored parameters describing the cell
characteristics, and application operating limits. Figure 2 describes the algorithm inputs and outputs.
Figure 2. Top-Level Algorithm Diagram
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FuelPack is a trademark of Dallas Semiconductor, a wholly owned subsidiary of Maxim Integrated Products, Inc.
Capacity Look-up
Available Capacity Calculation
ACR Housekeeping
Age Estimator
Learn Function
Cell
Model
Parameters
(EEPROM)
FULL(T) (R)
Active Empty (T) (R)
Standby Empty (T) (R)
Remaining Active Absolute
Capacity (RAAC) mAh
(R)
Sense Resistor’
(RSNSP) (1byte EE)
Voltage (R)
Temperature (R)
Current (R)
Accumulated
Current (ACR) (R/W)
User Memory (EEPROM)
16 bytes
Aging Cap (AC)
(2 bytes EE)
Charge Voltage
(VCHG) (1 byte EE)
Remaining Standby Absolute
Capacity (RSAC) mAh (R)
Remaining Active Relative
Capacity (RARC) % (R)
Remaining Standby Relative
Capacity (RSRC) % (R)
Age Scalar (AS)
(1 bytes EE)
Min Chg Current
(IMIN) (1 byte EE)
Empty Voltage
(VAE) (1 byte EE)
Empty Current (IAE)
(1 byte EE)
Average Current (R)
MODELING CELL CHARACTERISTICS
To achieve reasonable accuracy in estimating remaining capacity, the cell performance characteristics
overtemperature, load current, and charge-termination point must be considered. Since the behavior of Li+ cells is
nonlinear, 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 DS2784 is described in general in
Application Note 131: Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor Battery Monitor ICs. To facilitate
efficient implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2784. Full and empty points are retrieved in a lookup process which retraces a piece-wise
linear model consisting of three model curves named full, active empty, and standby empty. Each model curve is
constructed with 5-line segments, numbered 1 through 5. Above 40°C, the segment 5 model curves extend
infinitely with zero slope, approximating the nearly flat change in capacity of Li+ cells at temperatures above 40°C.
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. The three junctions or
breakpoints that join the segments (labeled TBP12, TBP23, and TBP34 in Figure 3) are programmable in 1°C
increments
from
-128°C to +40°C. The slope or derivative for segments 1, 2, 3, and 4 are also programmable over a range of 0 to
15,555ppm, in steps of 61ppm.