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Li-ion Battery CapacityDetection
Author: Arul Prabahar A.
ASCNT 2012
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• Project Titled “ Design and Development of
low cost real-time monitoring system forharmful gas detection” funded by Department
of Information and Technology (DIT).
ASCNT 2012
Introduction
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• Rechargeable batteries
• Fuel gauge IC
•
BQ27510• Focus on steps involved in capacity detection using TI’s
Impedance Track Algorithm
ASCNT 2012
Li ion Battery
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Presentation Flow
• Basic aspects of capacity detection
• Steps involved in single cell capacity detection
• Multi cell capacity detection
ASCNT 2012
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Capacity detection• Capacity: The battery capacity represents the maximum
amount of energy that can be extracted from the battery(Ah, Wh)
•
Importance of determining Remaining Capacity• Rechargeable batteries
• Strong Correlation between the battery open circuitvoltage and residual capacity- Temperature dependence
• More reliable when load current is low
• Discharge curve for known load current- low accuracy
ASCNT 2012
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Capacity detection contd.
• Discharge curve is affected by battery chemistry,ageing, anode used ,temperature and loadcurrent
• Fuel Gauge chips-Capture parameters, Algorithms
ASCNT 2012
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Battery Under Test
• Li ion battery with capacity approximately
1500mAh – Discharge curve method• Use of Fuel gauge
ASCNT 2012
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Steps Involved
• Step 1 Chemistry Cycle
•
Step 2 Learning CycleTools Used
• BQEasy TI Tool for data capture
• Math Cad - mapping
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• Battery Chemistry
• TI Data Base
•
ADCs Calibration• “rel-dis-rel” cycle – o/p log file from BQEasy
• Mathcad Tool – Matches with TI database
ASCNT 2012
Step 1-Chemistry cycle
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• load current
• battery voltage
•
time elapsed• time stamp
• temperature
• different flag status
ASCNT 2012
Chemistry cycle
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Voltage vs Time plot of logged data
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Step 2: Battery learning cycle• The Chemistry information as input
• Learn the battery capacity and impedance of the
battery at different stages• Generate Image using BQEasy tool
• This image can be used to program any BQ27510chipset to determine the capacity of a given Li-Ion
battery model
ASCNT 2012
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ASCNT 2012
• I2C interface
• O/P Remaining time, Current drawn, Battery
voltage , SOC etc.
• Battery Capacity – 1439mAh @ 2.9V cutoff
Results
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Multi cell in series Application
• Assumption - all the multi-cells are made of
identical cells with same characteristics• Multi-cell treated as single cell
• High precision voltage divider
ASCNT 2012
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ASCNT 2012
Battery Under Test
• Li ion battery with capacity approximately
11.1V , 6600mAh (3 cell series 3 cell parallel)• Voltage divider (1:3 ratio) compensate for 3
cell series
•
3 cell parallel handled in learning curve
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Voltage vs Time plot
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• Battery Capacity – 6461mAh @ 9V cutoff
Results
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Hardware Implementation
• Battery good flag to enable power regulators
• Kelvin connection for the current sense
•
LDO (3V) to supply power to fuel gauge
ASCNT 2012
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Conclusion
• Developed a technique to use TI’s BQ27510
Single cell System side Fuel gauge IC for multi
cell applications.
• Future work – Temperature compensation to
increase the accuracy
ASCNT 2011
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Thank You
ASCNT 2012