Rainbow Electronics DS2788 User Manual

Page 13

curves named full, active empty, and standby empty.

Each model curve is constructed with five line seg-

ments, numbered 1 through 5. Above +50°C, the seg-

ment 5 model curves extend infinitely with zero slope,

approximating the nearly flat change in capacity of Li+

cells at temperatures above +50°C. Segment 4 of each

model curves originates at +50°C on its upper end and

extends downward in temperature to +25°C. Segment

3 joins with segment 2, which in turn joins with segment

1. Segment 1 of each model curve extends from the

junction with segment 2 to infinitely colder tempera-

tures. Segment slopes are stored as µVh ppm change

per °C. The two junctions or breakpoints that join the

segments (labeled TBP12 and TBP23 in Figure 12) are

programmable in 1°C increments from -128°C to

+25°C. They are stored in two’s complement format,

TBP23 at 7Ch, and TBP12 at 7Dh. The slope or deriva-

tive for segments 1, 2, 3, and 4 are also programmable.

Full: The full curve defines how the full point of a given
cell stack depends on temperature for a given charge

termination. The charge termination method used in the

application is used to determine the table values. The

DS2788 reconstructs the full line from cell characteristic

table values to determine the full capacity of the battery

at each temperature. Reconstruction occurs in one-

degree temperature increments. Full values are stored

as ppm change per °C. For example, if a cell had a

nominal capacity of 1214mAh at +50°C, a full value of

1199mAh at +25°C, and 1182mAh at 0°C (TBP23), the

slope for segment 3 would be:

((1199mAh - 1182mAh) / (1214mAh / 1M)) /

(25°C - 0°C) = 560ppm/°C

1 LSB of the slope registers equals 61ppm so the full

segment 3 slope register (location 0x6Dh) would be

programmed with a value of 0x09h. Each Slope register

has a dynamic range 0ppm to 15555ppm.

D
S

Stand-Alone Fuel-Gauge IC with

LED Display Drivers

______________________________________________________________________________________

100%

SEGMENT 1

DERIVATIVE

(ppm/

°C)

FULL

ACTIVE

EMPTY

STANDBY

EMPTY

SEGMENT 2

SEGMENT 3

SEGMENT 4

+50

°C

+25

°C

TBP23

TBP12

SEGMENT 5

CELL

CHARACTERIZATION

CELL

CHARACTERIZATION

Figure 12. Cell Model Example Diagram