ds2786 Maxim Integrated Products, Inc., ds2786 Datasheet - Page 14

• Manufacturer Part Number: ds2786
• Description: Ds2786 Standalone Ocv-based Fuel Gauge
• Manufacturer: Maxim Integrated Products, Inc.

Figure 17. Voltage Breakpoint Register Format
INITIAL CAPACITY ESTIMATION
The DS2786 calculates Relative Capacity immediately upon power up. During initialization, the DS2786 makes a
voltage measurement and uses the OCV cell model data to determine a starting point for the Relative Capacity
Register. This estimation occurs regardless of the load on the cell. Any error induced from cell loading will be
removed at the next OCV adjustment. The initial voltage measurement used in determining the starting point is
stored in the Initial Voltage Register until the IC is power cycled.
Figure 18. Initial Voltage Register Format
NEW CAPACITY LEARNING
As the cell ages, the Initial Capacity Scaling Factor Register value may no longer accurately reflect the true
capacity of the cell causing error in Relative Capacity calculation while in coulomb-counting mode of operation. The
DS2786 has a learn feature that allows the IC to remain accurate as the cell changes. The DS2786 compares the
percent Relative Capacity difference between the last two OCV updates to the change in the coulomb count to
learn the new cell capacity. The Last OCV Register maintains the Relative Capacity percentage at the previous
OCV adjustment point used for learning the new cell capacity. Last OCV will be updated with a new value at each
OCV adjustment.
Example: Assuming a 15mΩ sense resistor, the DS2786 adjusts the Relative Capacity of a 1000mAH cell to 10%
based on an OCV measurement during an idle period of the application. The cell is then charged by 500mAH (to
60% expected) based on the internal coulomb count multiplied by the Learned Capacity Scaling Factor value of
55h. The next OCV adjustment determines the Relative Capacity should actually be at 65%, not 60%. The DS2786
then adjusts the Learned Capacity Scaling Factor value upward to (65% - 10%) ÷ (500mAH × 0.015Ω) = 5Eh
lowering the expected cell capacity by approximately 10%.
Figure 19. Last OCV Register Format
MSb
MSb
“S”: sign bit(s), “X”: reserved
“X”: reserved
2
S
11
MSB—Even Addresses 68h–78h
2
2
10
11
MSB—Address 14h
2
2
10
9
2
2
8
9
2
2
7
8
MSb
2
2
2
6
7
7
2
2
2
6
5
6
LSb
LSb
2
2
2
4
5
5
14 of 22
Address 16h
2
4
MSb
MSb
2
2
2
3
3
4
2
2
2
Units: 0.5%
LSB—Odd Address 69h–79h
2
2
3
LSB—Address 15h
2
2
2
1
1
2
LSb
2
2
2
0
0
1
2
X
0
X
X
Units: 1.22mV
Units: 1.22mV
X
X
LSb
LSb
X
X