Rainbow Electronics DS2788 - Manual (Page 14)

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8 Active Empty: The active empty curve defines the tem-
perature variation in the empty point of the discharge
profile based on a high-level load current (one that is
sustained during a high-power operating mode) and
the minimum voltage required for system operation.
This load current is programmed as the active empty
current (IAE), and should be a 3.5s average value to
correspond to values read from the Current register
and the specified minimum voltage, or active empty
voltage (VAE) should be a 250ms average to corre-
spond to values read from the Voltage register. The
DS2788 reconstructs the active empty line from cell
characteristic table values to determine the active
empty capacity of the battery at each temperature.
Reconstruction occurs in one-degree temperature
increments. Active empty segment slopes are stored
the same as described for the full segments.
Standby Empty: The standby empty curve defines the
temperature variation in the empty point in the dis-
charge defined by the application standby current and
the minimum voltage required for standby operation.
Standby empty represents the point that the battery
can no longer support a subset of the full application
operation, such as memory data retention or organizer
functions on a wireless handset. Standby empty seg-
ment slopes are stored the same as described for the
full segments.
The standby load current and voltage are used for
determining the cell characteristics but are not pro-
grammed into the DS2788. The DS2788 reconstructs
the standby empty line from cell characteristic table
values to determine the standby empty capacity of the
battery at each temperature. Reconstruction occurs in
one-degree temperature increments.
Cell Stack Model Construction
The model is constructed with all points normalized to
the fully charged state at +50°C. The cell parameter
EEPROM block stores the initial values, the +50°C full
value in mVh units, and the +50°C active empty value
as a fraction of the +50°C value. Standby empty at
+50°C is by definition zero and therefore no storage is
required. The slopes (derivatives) of the 4 segments for
each model curve are also stored in the cell parameter
EEPROM block along with the break point temperatures
of each segment. Table 1 shows an example of data
stored in this manner.
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
14 ______________________________________________________________________________________
Table 1. Example Cell Characterization Table (Normalized to +50°C)
TEMPERATURE
CELL MODEL
PARAMETERS
15 BYTES
(EEPROM)
LOOKUP
FUNCTION
FULL(T)
AE(T)
SE(T)
Figure 13. Lookup Function Diagram
Manufacturer’s Rated Cell Capacity: 1220mAh
Charge Voltage: 4.2V Charge Current: 500mA Termination Current: 50mA
Active Empty (V, I): 3.0V, 500mA Standby Empty (V, I): 3.0V, 4mA
Sense Resistor: 0.020
SEGMENT BREAKPOINTS
TBP12 = -12°C
TBP23 = 0°C
CALCULATED VALUE
+50
°C NOMINAL
(mAh)
SEGMENT 1
(ppm/
°C)
SEGMENT 2
(ppm/
°C)
SEGMENT 3
(ppm/
°C)
SEGMENT 4
(ppm/
°C)
Full 1214 488 549 1587 2686
Active Empty 854 1526 2686 3113
Standby Empty 244 183 916 244