Rainbow Electronics DS2788 - Manual (Page 15)

Application Parameters
In addition to cell model characteristics, several appli-
cation 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
m h o s . R S N S P s u p p o r t s r e s i s t o r v a l u e s o f 1 Ω to
3.922m Ω. 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 19.52mV and can range
from 0 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. The value is stored as a 1-byte
value with units of 50µV 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 19.52mV and can range from 0 to 4.978V. VAE
is located in the parameter EEPROM block.
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 0mA to 2550mA in 10mA steps. IAE is
located in the parameter EEPROM block.
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 (6.25µVh). Setting AC to the manufacturer’s
rated capacity sets the aging rate to approximately
2.4% per 100 cycles of equivalent full capacity dis-
charges. Partial discharge cycles are added to form
equivalent full capacity discharges. The default estima-
tion 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 rat-
ing. Setting AC to a lower value, accelerates the esti-
mated 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 unaged battery. A
value of 95% is recommended as the starting AS value
at the time of pack manufacture to allow learning a larg-
er 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 be
exercised when writing AS to ensure that the cumula-
tive aging estimate is not overwritten with an incorrect
value. Typically, it is not necessary for the host to write
AS because the DS2788 automatically saves AS to
EEPROM on a periodic basis. (See the Memory section
for details.) The EEPROM-stored value of AS is recalled
on power-up.
Capacity Estimation Utility
Functions
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 AC. AS is then decremented by one, resulting
in a decrease in the scaled full battery capacity of
0.78%. See the AC register description for recommen-
dations 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 “learn cycle.” First, the active empty point
must be detected. The learn flag (LEARNF) is set at this
point. Second, once charging starts, the charge must
D
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2
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Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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