Rainbow Electronics DS2781 - Manual (Page 15)

DS2781: Two-Cell Standalone Fuel Gauge IC
15 of 30
Figure 13. Lookup Function Diagram
APPLICATION PARAMETERS
In addition to cell model characteristics, several application parameters are needed to detect the full and empty
points, as well as calculate results in mAh units.
Sense Resistor Prime (RSNSP): RSNSP stores the value of the sense resistor for use in computing the absolute
capacity results. The value is stored as a 1-byte conductance value with units of mhos. RSNSP supports resistor
values of 1
W to 3.922mW. RSNSP is located in the Parameter EEPROM block.
Charge Voltage (VCHG): VCHG stores the charge voltage threshold used to detect a fully charged state. The
value is stored as a 1-byte voltage with units of 39.04mV and can range from 0V to 9.956V. VCHG should be set
marginally less than the cell stack voltage at the end of the charge cycle to ensure reliable charge termination
detection. VCHG is located in the Parameter EEPROM block.
Minimum Charge Current (IMIN): IMIN stores the charge current threshold used to detect a fully charged state.
The value is stored as a 1-byte value with units of 50V and can range from 0 to 12.75mV. Assuming RSNS =
20m
W, IMIN can be programmed from 0mA to 637.5mA in 2.5mA steps. IMIN should be set marginally greater than
the charge current at the end of the charge cycle to ensure reliable charge termination detection. IMIN is located in
the Parameter EEPROM block.
Active Empty Voltage (VAE): VAE stores the voltage threshold used to detect the Active Empty point. The value
is stored in 1-byte with units of 39.04mV and can range from 0V to 9.956V. VAE is located in the Parameter
EEPROM block. See the Cell Characteristics section for more information.
Active Empty Current (IAE): IAE stores the discharge current threshold used to detect the Active Empty point.
The unsigned value represents the magnitude of the discharge current and is stored in 1-byte with units of 200
mV
and can range from 0 to 51.2mV. Assuming RSNS = 20m W, IAE can be programmed from 0mA to 2550mA in
10mA steps. IAE is located in the Parameter EEPROM block. See the Cell Characteristics section for more
information.
Aging Capacity (AC): AC stores the rated battery capacity used in estimating the decrease in battery capacity that
occurs in normal use. The value is stored in 2-bytes in the same units as the ACR (6.25
mVh). Setting AC to the
manufacturer’s rated capacity sets the aging rate to approximately 2.4% per 100 cycles of equivalent full capacity
discharges. Partial discharge cycles are added to form equivalent full capacity discharges. The default estimation
results in 88% capacity after 500 equivalent cycles. The estimated aging rate can be adjusted by setting AC to a
different value than the cell manufacturer’s rating. Setting AC to a lower value, accelerates the estimated aging.
Setting AC to a higher value, retards the estimated aging. AC is located in the Parameter EEPROM block.
Age Scalar (AS): AS adjusts the capacity estimation results downward to compensate for cell aging. AS is a 1-byte
value that represents values between 49.2% and 100%. The lsb is weighted at 0.78% (precisely 2
-7 ). A value of
100% (128 decimal or 80h) represents an un-aged battery. A value of 95% is recommended as the starting AS
value at the time of pack manufacture to allow learning a larger capacity on batteries that have an initial capacity
greater than the nominal capacity programmed in the cell characteristic table. AS is modified by the cycle count
based age estimation introduced above and by the capacity Learn function. The host system has read and write
access to AS, however caution should exercised when writing AS to ensure that the cumulative aging estimate is
Cell Model
Parameters
(EEPROM)
Temperature
Look-up
Function
FULL(T)
AE(T)
SE(T)