Rainbow Electronics DS2778 - Manual (Page 21)

DS2775/DS2776/DS2777/DS2778
2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
Protector and Optional SHA-1 Authentication
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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 50µV (IMIN x R
SNS
) and can range from 0 to
12.75mV. Assuming R SNS = 20m Ω, IMIN can be pro-
grammed from 0 to 637.5mA in 2.5mA steps. IMIN
should be set marginally greater than the charge cur-
rent 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 19.5mV and can range from 0 to 4.978V. VAE is
stored as an average of the cell’s voltages. VAE is locat-
ed in the parameter EEPROM block. See the
Modeling
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 rep-
resents the magnitude of the discharge current and is
stored in 1 byte with units of 200µV and can range from
0 to 51.2mV. Assuming R
SNS = 20m Ω, IAE can be pro-
grammed from 0 to 2550mA in 10mA steps. IAE is locat-
ed in the parameter EEPROM block. See the
Modeling
Cell Characteristics section for more information.
Aging Capacity (AC)
AC stores the rated cell capacity, which is used to esti-
mate the decrease in battery capacity that occurs dur-
ing normal use. The value is stored in 2 bytes in the
same units as the ACR (6.25µVh). When set to the man-
ufacturer’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 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 down-
ward to compensate for aging. The AS is a 1-byte value
that has a range of 49.2% to 100%. The LSb is weight-
ed at 0.78% (precisely 2
-7 ). A value of 100% (128 deci-
mal or 80h) represents an unaged battery. A value of
95% is recommended as the starting AS value at the
time of pack manufacture to allow the learning of a larg-
er 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 aging
estimation introduced under aging capacity and by the
learn function.
Batteries are typically considered worn out when the full
capacity reaches 80% of the rated capacity; therefore,
the AS value is not required to range to 0%. It is
clamped to 50% (64 decimal or 40h). If a value of 50%
is read from the AS, the host should prompt the user to
initiate a learning cycle.
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. The AS is automatically
saved to EEPROM. The EEPROM value is recalled on
power-up.
Capacity Estimation Operation
Cycle-Count-Based Aging Estimation
As previously discussed, 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 of the scaled full battery capaci-
ty by 0.78% (approximately 2.4% per 100 cycles). The
internal counter is reset in the event of a learn cycle.
See the
Aging Capacity (AC) section for recommenda-
tions on customizing the age estimation rate.
Learn Function
Because 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’s capacity. A continuous charge from empty to
full results in a learn cycle. First, the active-empty point
must be detected. The learn flag (LEARNF) is set at this
point. Then, once charging starts, the charge must con-
tinue uninterrupted until the battery is charged to full.
Upon detecting full, the LEARNF is cleared, the charge-
to-full (CHGTF) flag is set, and the AS is adjusted
according to the learned capacity of the cell.
Full capacity estimation based on the learn function is
more accurate than the cycle-count-based estimation
introduced under aging capacity. The learn function
reflects the current performance of the cell. Cycle-
count-based estimation is an approximation derived
from the manufacturer’s recommendation for a typical
cell. Therefore, the internal counter used for cycle-