DS2780 Standalone Fuel Gauge IC
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Sense Resistor Prime (RSNSP[1/): RSNSP stores the value of the sense resistor for use in computing the
absolute capacity results. The resistance is stored as a 1-byte conductance value with units of mhos (1/. RSNSP
supports resistor values of 1 to 3.922m. RSNSP is located in the Parameter EEPROM block.
RSNSP = 1/RSNS (units of mhos; 1/
Charge Voltage (VCHG): VCHG stores the charge voltage threshold used to detect a fully charged state. The
voltage is stored as a 1-byte value with units of 19.52mV and can range from 0V to 4.978V. VCHG should be set
marginally less than the cell 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. It
is stored as a 1-byte value with units of 50V (IMIN * RSNS) and can range from 0 to 12.75mV. Assuming RSNS =
20m, 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 19.52mV and can range from 0V to 4.978V. 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 200V.
It can range from 0 to 51.2mV. Assuming RSNS = 20m, 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 cell capacity which is used to estimate the decrease in battery capacity
that occurs during normal use. The value is stored in 2-bytes in the same units as the ACR (6.25Vh). When set to
the manufacturer’s rated cell capacity the Aging Estimation rate is approximately 2.4% per 100 cycles of equivalent
full capacity discharges. Partial discharge cycles are added to form equivalent full capacity discharges. The default
aging estimation results in 88% capacity after 500 equivalent cycles. The Aging Estimation rate can be adjusted by
setting the AC to a value other than the cell manufacturer’s rating. Setting AC to a lower value, accelerates the
Aging Estimation rate. Setting the AC to a higher value, retards the Aging Estimation rate. The AC is located in the
Parameter EEPROM block.
Age Scalar (AS): AS adjusts the cell capacity estimation results downward to compensate for aging. The AS is a
1-byte value that has a range of 49.2% to 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 the learning of a larger capacity on batteries that have an initial capacity greater
than the rated cell capacity programmed in the cell characteristic table. The AS is modified by the Aging Estimation
introduced under Aging Capacity and by the Learn function. The host system has read and write access to the AS,
however caution should be exercised when writing it to ensure that the cumulative aging estimate is not over
written with an incorrect value. Usually, writing the AS by the host is not necessary because it is automatically
saved to EEPROM on a periodic basis by the DS2780. (See the Memory section for details.) The AS value stored
in EEPROM is recalled on power-up.
CAPACITY ESTIMATION UTILITY FUNCTIONS
Aging Estimation
As discussed above, the AS register value is adjusted occasionally based on cumulative discharge. As the ACR
register decrements during each discharge cycle, an internal counter is incremented until equal to 32 times the AC.
The AS is then decremented by one, resulting in a decrease in the scaled full battery capacity of 0.78%
(approximately 2.4% per 100 cycles). Refer to the AC register description above for recommendations on
customizing the age estimation rate.
Learn Function
Since Li+ cells exhibit charge efficiencies near unity, the charge delivered to a Li+ cell from a known empty point to
a known full point is a dependable measure of the cell capacity. A continuous charge from empty to full results in a
