Datasheet
DS2786 Stand-Alone OCV-Based Fuel Gauge
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CELL CAPACITY ESTIMATION
The DS2786 uses a hybrid OCV measurement and coulomb counting algorithm to estimate remaining cell capacity.
During periods of charging or discharging of the cell, the DS2786 counts charge flow into and out of the cell. When
the application becomes inactive, the DS2786 waits for the cell voltage to relax and then adjusts the coulomb count
based on an open-circuit voltage cell model stored in device EEPROM. The resulting calculation is reported to the
system as a percentage value between 0% and 100%. As the cell ages, a Learn feature adjusts for changes in
capacity.
The Relative Capacity Register reports remaining cell charge as a percentage of full. Relative Capacity is reported
with a resolution of 0.5% and is limited to a value between 0% and 100%. The Relative Capacity Register is
updated each time the IC performs a current measurement or open-circuit cell voltage measurement.
Figure 10. Relative Capacity Register Format
Address 02h
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb LSb
Units: 0.5%
Prior to the first Learn operation, the Relative Capacity value is calculated by adding the IACR multiplied by the
Initial Capacity Scaling Factor (7Ah) to the Last OCV Relative Capacity (16h). After the first Learn operation, the
Relative Capacity value is calculated by adding the IACR multiplied by the Learned Capacity Scaling Factor (17h)
to the Last OCV Relative Capacity (16h).
Each Capacity Scaling Factor Register has a resolution of 78.125%/Vh and a maximum range of 0 to
19921.875%/Vh. During assembly, the Initial Capacity Register should be programmed to the capacity of the cell.
For example, an application using a 1Ah cell and 0.015 sense resistor would set the Initial Capacity Register to a
value of (100% ÷ (1Ah × 0.015)) ÷ 78.125%/Vh = 55h. The Learned Capacity Scaling Factor Register is
controlled by the DS2786. The power up value is 00h, and the register is updated with the calculated new cell
capacity value after every Learn operation.
Figure 11. Initial Capacity Scaling Factor Register Format
Address 7Ah
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb LSb
Units: 78.125%/Vh
Figure 12. Learned Capacity Scaling Factor Register Format
Address 17h
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSb LSb
Units: 78.125%/Vh