Rainbow Electronics DS2788 - Manual (Page 12)

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8 Capacity Estimation Algorithm
Remaining capacity estimation uses real-time mea-
sured values and stored parameters describing the cell
characteristics and application operating limits. Figure
11 describes the algorithm inputs and outputs.
Modeling Cell Stack
Characteristics
To achieve reasonable accuracy in estimating remain-
ing capacity, the cell stack performance characteristics
over temperature, load current, and charge termination
point must be considered. Since the behavior of Li+
cells is nonlinear, these characteristics must be includ-
ed in the capacity estimation to achieve an acceptable
level of accuracy in the capacity estimation. The
FuelPack™ method used in the DS2788 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 DS2788. Full
and empty points are retrieved in a lookup process that
retraces piece-wise linear model consisting of three
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
12 ______________________________________________________________________________________
CAPACITY LOOKUP
AVAILABLE CAPACITY CALCULATION
ACR HOUSEKEEPING
AGE ESTIMATOR
LEARN FUNCTION
VOLTAGE (R) FULL FULL(T) (R)
ACTIVE EMPTY AE(T) (R)
STANDBY EMPTY SE(T) (R)
REMAINING ACTIVE ABSOLUTE
CAPACITY (RAAC) mAh (R)
REMAINING STANDBY ABSOLUTE
CAPACITY (RSAC) mAh (R)
REMAINING ACTIVE RELATIVE
CAPACITY (RARC) % (R)
REMAINING STANDBY RELATIVE
CAPACITY (RSRC) % (R)
TEMPERATURE (R)
CURRENT (R)
AVERAGE CURRENT (R)
ACCUMULATED
CURRENT (ACR) (RW)
AGE SCALAR (AS)
(1-BYTE EE)
AGING CAPACITY (AC)
(2 BYTES EE)
SENSE RESISTOR PRIME (RSNSP)
(1 -BYTE EE)
CHARGE VOLTAGE (VCHG)
(1-BYTE EE)
MINIMUM CHARGE CURRENT (IMIN)
(1-BYTE EE)
ACTIVE EMPTY VOLTAGE (VAE)
(1-BYTE EE)
ACTIVE EMPTY CURRENT (IAE)
(1-BYTE EE)
CELL PARAMETERS
16 BYTES
(EEPROM)
Figure 11. Top Level Algorithm Diagram
FuelPack is a trademark of Dallas Semiconductor Corp., a
wholly owned subsidiary of Maxim Integrated Products, Inc.