single-cell gauging 101 1. 2 what is fuel gauging technology? fuel gauging is a technology used to...
TRANSCRIPT
Single-Cell Gauging 101
1
2
What is Fuel Gauging Technology?
• Fuel Gauging is a technology used to predict battery capacity under all system active and inactive conditions.
• Battery capacity – Percentage– time to empty/full– milliamp-hours– Watt-hours– talk time, idle time, etc.
• Other data can be obtained for battery health and safety diagnostics.• State of Health• Full Charge Capacity
73%Run Time
6:23
3
Outline
• Battery chemistry fundamentals• Classic fuel gauging approaches
– voltage based– coulomb counting
• Impedance Track and its benefits
Single-Cell Gauging 101Part 3: Impedance Track Benefits
5
• Voltage based gas gauge: Accurate gauging under no load • Coulomb counting based gauging: Accurate gauging under load• Combines advantages of voltage and current based methods• Real-time impedance measurement• Calculate remaining run-time at given average load using both
open circuit voltage and impedance information.
Impedance TrackTM Gas Gauge
V = OCV(T,SOC) - I*R(T,SOC, Aging)
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• OCV profiles similar for all tested manufacturers
• Most voltage deviations < 5 mV
• Average SOC prediction error < 1.5%
• Same database can beused for batteries from different manufacturersfor the same chemistry
Comparison of OCV=f (SOC, T) profiles
100 50 0 SOC %
15
5
-5
-15Volta
ge D
evia
tion
(mV)
100 75 50 25 0 SOC Error %
4
1.33
-1.33
-4
100 75 50 25 0 SOC %
4.2
3.93
3.67
3.4
Open Circuit Voltage (OCV) Profile
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How to measure OCV ?
• OCV measurement allows SOC with 0.1% max error
• Self-discharge estimation is eliminated
Cel
l Vol
tage
(V)
0 0.5 1.0 1.5 2.0 2.5Time (hour)
4.2
4.1
4.0
3.9
3.8
3.7
System ON System ONSystem OFF
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How to measure impedance?
•Data flash contains a fixed table: OCV = f (SOC, T)•IT algorithm: Real-time measurements and calculations
during charge and discharge.
100 75 50 25 0 SOC %
4.2
3.93
3.67
3.4
Open Circuit Voltage Profile
OCV
VBAT
IRBAT
AVG
BATBAT I
V-OCVR =
V = OCV(T,SOC) - I*R(T,SOC, Aging)
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Issue for Traditional Battery Capacity Learning
+
Shipping
Fully Discharge
+
Fully Charge
+
Half Discharge
• Involve a lot of test equipment and time• User may never fully discharge battery to learn capacity• Gauging error increase 1% for every 10 cycles without learning
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• Charge passed is determined by exact coulomb counting• SOC1 and SOC2 measured by its OCV• Method works for both charge or discharge exposure
Learning Qmax without Full Discharge
Q
Cel
l Vol
tage
(V)
0 0.5 1.0 1.5 2.0 2.5 3.0Time (hour)
4.2
4.1
4.0
3.9
3.8
P1
P2
2SOC1SOC
max
Start of Charge
Measurement PointsOCV
0 0.5 1.0 1.5 2.0 2.5 3.0Time (hour)
Q
P2
P1Start of Discharge
Measurement PointsOCV
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.
current
........resistance updates
Total capacity updates
currentintegration
Cooperation between integration and correlation modes
discharge relaxation charge
currentintegration
voltage correlation
SOC updates
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Impedance Track Fuel Gauge
Advantages• Combine advantages from both voltage based and coulomb counting
• Accurate with both small current (OCV) and large load current
• Throw out inaccurate self-discharge models (use OCV reading)
• Very accurate with new and aged cells
• No need for full charge and discharge for capacity learning
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Fuel Gauging Benefits
• Accurate report of remaining run time• Better Power Management• Longer Run Time
– Power management– Accuracy: less guard band needed for
shutdown– Variable shutdown voltage with
temperature, discharge rate, and age based on impedance
• Orderly shutdown – Automatically save data to flash with
reserve energy when battery dies• Fuel gauging enables mobile applications
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Run Time Comparison Example Impedance TrackTM gauge shutdown vs. OCV shutdown point
• Systems without accurate gauges simply shutdown at a fixed voltage
• Smartphone, Tablets, Portable Medical, Digital Cameras etc… need reserve battery energy for shutdown tasks
• Many devices shutdown at 3.5 or 3.6 volts in order to cover worst case reserve capacity
• 3.5 volt shut down used in this comparison
• Gauge will compute remaining capacity and alter shutdown voltage until there is exactly the reserve capacity left under all conditions
• 10 mAH reserve capacity is used
• Temperature and age of battery are varied
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2000
2500
3000
3500
4000
4500
0 20 40 60 80 100 120 140 160 180
Fuel GaugingOCV vs. IT Use Case exp – NEW battery w/ variable load mix
Conditions:• New Battery
• Room temp (25°C)• 10 mAh reserve capacity for
shutdown
Run Time in Minutes
Vo
ltag
e
Impedance TrackTM Gauge Shutdown @ 3.295V168 minutes run time
Extended runtime with TI Gauge:
+40%
OCV
Shutdown @ 3.5V 120 minutes run time
4.2
3.6
3.0
2.4
Bat
tery
Vol
tage
(V
)
Open Circuit Voltage (OCV)
EDV
Qmax
I•RBAT
Quse
Cell voltage under load
16
2000
2500
3000
3500
4000
4500
0 20 40 60 80 100 120 140 160
Fuel Gauging OCV vs. IT Use Case Exp – OLD battery w/ variable load mix
Conditions• Room temp (25°C)• 10 mAh reserve
capacity for shutdown
Run Time in Minutes
Vo
ltag
e
Impedance TrackTM Gauge Shutdown @ 3.144V142 minutes run time
OCV
Shutdown @ 3.5V 90 minutes run time
Extended runtime with TI Gauge:
+58%
4.2
3.6
3.0
2.4
Bat
tery
Vol
tage
(V
)
Open Circuit Voltage (OCV)
EDV
Qmax
I•RBAT
Quse
Cell voltage under load
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2000
2500
3000
3500
4000
4500
0 20 40 60 80 100 120 140
Fuel GaugingOCV vs. IT Use Case Exp – NEW battery COLD w/ variable load mix
Run Time in Minutes
Vo
ltag
e
Impedance TrackTM Gauge Shutdown @ 3.020V117 minutes run time
OCV
Shutdown @ 3.5V 53 minutes run time
Conditions Batty• Cold (0°C)
• 10 mAh reserve capacity for shutdown
Extended runtime with TI Gauge:
+121%
4.2
3.6
3.0
2.4
Bat
tery
Vol
tage
(V
)
Open Circuit Voltage (OCV)
EDV
Qmax
I•RBAT
Quse
Cell voltage under load
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2000
2500
3000
3500
4000
4500
0 10 20 30 40 50 60 70 80 90
Conditions(0°C)• Cold (0°C)
• 10 mAh reserve capacity for shutdown
Run Time in Minutes
Vo
ltag
e
Gauge shutdown at 3.061 volts:
82 minutes run time
OCV
Shutdown @ 3.5V 21 minutes run time
Extended runtime with TI Gauge:
+290%
Fuel GaugingOCV vs. IT Use Case Exp – OLD battery COLD w/ variable load mix
4.2
3.6
3.0
2.4
Bat
tery
Vol
tage
(V
)
Open Circuit Voltage (OCV)
EDV
Qmax
I•RBAT
Quse
Cell voltage under load
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Fuel Gauging – Impedance TrackTM Advantages
• Dynamic (learning) Ability– Temperature variability in applications
• IT takes into account cell impedance changes due to increase/decrease in temperature
• IT incorporates thermal modeling to adjust for self-heating– Load variation
• IT will keep track of voltage drops due to high load spikes
• Aged Battery– IT has the ability to adjust for changes in useable capacity due to cell aging
• Increased Run Time– A lower terminate voltage can be utilized with an IT based gauge
• Flexibility– Cell Characterization– Host system does not need to perform any calculations or gauging algorithms
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Unused Battery Capacity implications
• Cell cost: avg $0.15 for every 100mAh• Lower Terminate Voltage (TV) = Larger Battery Capacity• 500mV lower of TV ~ 5% increase on capacity for new battery →
save ~$0.10 for 1500mAh battery• 500mV lower of TV ~ around 50% increase on capacity for aged
battery → save ~$1.00 for 1500mAh battery and extend run time!• Money saving opportunity for manufacturer while still extending
end-user runtime
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Cost of an inaccurate gauge
• Assuming customer discharge and charge once every day → three month use time = 90 day ~ 90 cycles → Battery internal impedance almost doubled → Aged battery scenario
• Non-Impedance Track based gauge → inaccurate gauging results due to battery aging→ much shorter run time or even system crash
• Battery warranty by operators could be one year or even two years.• Customer return entire units due to faulty gauging results → Returns
within warranty period cost company money• Impedance Track based gauge can extend the battery run time and
eliminate some costly return due to faulty gauging results
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Summary
• Accurate gauges for portable electronics are as critical to long run-time as reducing your design’s power and having a beefy battery.
• A variety of gauges are available with different approaches and different trade-offs.
Finish
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Back Up Slides:Impedance Track Reference
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Single Cell Impedance Track (IT) Basic Terminology and Relationships
• OCV – Open Circuit Voltage• Qmax – Maximum battery chemical capacity
(SOC1/SOC2 is correlated from OCV table after OCV1/OCV2 measurement)• SOC – State of Charge
• (* From Full Charge State)• RM – Remaining Capacity
• (SOC start is present SOC, SOC final is SOC at system terminate voltage)
|SOC|SOC
PassedQQ
21max -
=
max
*
Q
PassedQ1SOC -=
maxfinalstart Q)SOCSOC(RM ×= -
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• FCC – Full Charge Capacity is the amount of charge passed from a fully charged state until the system terminate voltage is reached at a given discharge rate
• FCC = Qstart + PassedQ + RM
• RSOC – Relative State of Charge
FCC
100RMRSOC
Single Cell Impedance Track (IT) Basic Terminology and Relationships
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Single Cell Impedance Track (IT) Fuel Gauge Introduction
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Gauging Error definition
• Reference points– at charge termination SOC =
100%– at EDV SOC=0– Charge integrated from fully
charged to EDV is FCCtrue
• From these reference points, true SOC can be defined as
SOCtrue= (FCCtrue-Q)/FCCtrue
• Reported SOC at all other points can be compared with true SOC.
• Difference between reported and true SOC is the error. It can be defined at different check points during discharge.
15%
3%
0%
Error check-points
0 1 2 3 4 5 6
3
3.5
4
4.5
Capacity, Ah
Vol
tage
, V
EDV
Qmax
Q
FCC
Check point at 0% is not meaningful – EDV is the voltage where system crashes!
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Single Cell Impedance Track (IT) Error Definition and Calculation
• Relative State of Charge (RSOC) Error
RSOC Error = RSOC calculated - RSOC reported
(RSOC reported is the RSOC reported by bq275xx Impedance Track TM algorithm)
100
FCC
PassedQQFCCRSOC start
calculated
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• Remaining Capacity (RM) Error
RM calculated = FCC - Qstart - PassedQ
(RM reported is the RM reported by bq275xx Impedance Track TM algorithm)
FCC
RMRMErrorRM reportedcalculated
Single Cell Impedance Track (IT) Error Definition and Calculation
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Example error plotsTrue vs reported RSOC RSOC error
0 20 40 60 80 1008.5
9
9.5
10
10.5
11
11.5
12
SMB RSOCtrue RSOC
RSOC
Vol
tage
0 10 20 30 40 50 60 70 80 90 1004
3
2
1
0
1
2
SMB RSOC
RSOC
RS
OC
err
or
Remaining capacity test Relative RemCap error
0 1 2 3 4 58.5
9
9.5
10
10.5
11
11.5
12
SMB remaining capacitytrue remaining capacity
Capacity, mAh
Vol
tage
0 12.5 25 37.5 50 62.5 75 87.5 1003
2.13
1.25
0.38
0.5
1.38
2.25
3.13
4
SMB RSOC
SOC
Rel
ativ
e R
emC
ap e
rror
, %