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Single-Cell Gauging 101
1
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
Run Time 6:23
• Full Charge Capacity
73%
2
Outline
• Battery chemistry fundamentals
• Classic fuel gauging approaches
– voltage based
– coulomb counting
• Impedance Track and its benefits
3
Single-Cell Gauging 101
Part 2: Classic fuel gauging approaches
Goal: Full Use of Available Battery Capacity
100%
+
Charging Voltage Tolerance
80%
60%
Actual Useful Capacity
40%
20%
0%
Capacity
Shutdown Uncertainty
due to inaccurate gauging
• Only 80-90% of Available Capacity may actually be used!
• High Accuracy Gas Gauge Increases the Battery Run-time
5
Traditional Battery Pack-Side Gas Gauge
PACK+
Power
Management
Host Processor
TI OMAP
GPIO
I2C/HDQ
Gas
Gauge
PACKProtector
TS
Battery Pack
6
System-Side Impedance Track Fuel Gauge
PACK+
Power
Management
Host P
TI OMAP
GPIO
Gas
Gauge
TS
Protector
PACKPortable Devices
Battery Pack
7
What does the Fuel Gauge do?
• Communication between battery and user
• Measurement:
– Battery voltage
– Charging or discharging current
– Temperature
• Provide:
– Battery Run Time and Remaining Capacity
– Battery health information
– Overall battery power management (Operation mode)
8
How to Implement a Fuel Gauge?
• Voltage Based: SOC = f (VBAT)
• Coulomb Counting:
Q i dt
• Impedance Track: Real time resistance measurement
V = VOCV - I •RBAT
9
Voltage Based Fuel Gauge
Battery Voltage (V)
4.2
Open Circuit Voltage
(OCV)
I•RBAT
3.8
3.4
3.0
EDV
Battery Capacity
Quse Qmax
• Applications: low end cellular phone, DSC,…
• Pulsating load causes capacity bar up and down
V VOCV - I R BAT
• Accurate ONLY at very low current
?
10
Battery Resistance
V = VOCV - I •RBAT?
Impedance = f( Temperature, State of Charge, and Aging)
Resistance doubles after 100 cycles
10-15% cell-cell resistance variation
10-15% resistance variation from different manufacturers
11
Impedance Dependent on Temperature and DOD
Impedance is strongly
dependent on temperature,
State of Charge and aging
SOC =
Full Charged
Q
Qmax
Fully Discharged
DOD=1-SOC (State of Charge)
SOC=1 (Full charged battery)
SOC=0 (Full discharged battery)
SOC: State of Charge
DOD: Depth of Discharge
12
Impedance Differences for New Cells
Rhf
R1
R2
RSER L
C1
C2
Manufacturer 1
0.05
- Im (Z) -
- Im (Z) -
0.05
1 mHz
0.025
Manufacturer 2
1 kHz
1 mHz
0.025
1 kHz
0
0
0 0.062 0.084 0.11
R(Z) -
0.13
0.15
0 0.042 0.064 0.086
R(Z) -
0.11 0.13
• Low-frequency (1 mHz) impedance variation 15%
• At 1C rate discharge, 40-mV difference, causes maximum SOC error of ±26%
13
Battery Voltage (V)
Battery – Transient Response
3.905
Load Removal
3.880
• Different voltage at
different instants
3.855
3.830
0
1000
*C/3 rate current used for both tests
Battery Voltage (V)
• Complete relaxation takes
about 2000 seconds
2000
3000
Time (Second)
4000
• Voltage difference between
20 and 3000 seconds is
over 20 mV
3.325
3.300
3.275
3.250
Load Removal
0
500 1000 1500 2000 2500 3000 3500
Time (Second)
Rhf
R1
R2
RSER L
C1
C2
14
Voltage Relaxation and State of Charge Error
100
SOC %
20
SOC % Error
10
50
0
0
-10
-50
3.2
3.45
3.7
3.95
Battery Cell Voltage (V)
4.3
-20
3.2
3.45
3.7
3.95
Battery Cell Voltage (V)
4.3
• ±20mV difference
• Error depends on particular voltage at the moment of
estimation
• Maximum error reaches 15%, average error 5%
15
SOC Error of Voltage-Based Fuel Gauging
Error for a New Cell
Error Evolution with Aging
100
100
1515
13.13
Relaxation Error
Cell-to-Cell Variation
Total Error
11.25
%
Error
SOCSOC
Error – %
SOC Error – %
SOC Error %
11.25
9.38
7.5
7.5
5.63
3.75
3.75
1.88
0
00
0
300 Cycles
200 Cycles
100 Cycles
0 Cycles
87.5
7575
62.5
50
50
37.5
25
25
12.5
0
20
20
40
60
40 SOC – %60
SOC %
80
80
100
100
00
0
V = VOCV - I • R BAT
20
20
40
60
40
60
SOC – %
SOC %
80
80
100
100
?
• 20-mV relaxation measurement error
• 15% cell-to-cell resistance tolerance
• Battery resistance doubles every 100 cycles
16
Voltage-Based Fuel Gauge
• Advantages
– Learning can occur without full discharge
– No correction needed for self-discharge
– Very accurate with small load current
• Disadvantages
– Inaccurate due to internal battery impedance
– Impedance is function of temperature, aging,
and State of Charge
17
Coulomb Counting Based Gauging
• Battery is fully charged
• During discharge
capacity is integrated
• Qmax is updated every
time full discharge
occurs
Q i dt
4.5
Li-Ion Battery Cell Voltage
0.2C Discharge Rate
4.0
Q
3.5
3.0
EDV: End of Discharge Voltage
0
EDV
1
2
3
4
Capacity, Ah
5
6
Qmax
Example: bq27010, bq27210
18
Learning before Fully Discharged
Voltage (V)
4.5
4
3.5
3
0
EDV2
7%
EDV1
EDV0
1
3%
0%
2
3
4
Capacity Q (Ah)
5
6
• Too late to learn when 0% capacity is reached
• Set voltage threshold for given percentage of remaining capacity
• True voltage at 7%, 3% remaining capacity depends on current,
temperature, and impedance
19
Compensated End of Discharge Voltage (CEDV)
Voltage (V)
4.5
4
3.5
7%
CEDV2 (I1)
CEDV2 (I2)
30%
7%
3
0
1
2
3
4
Capacity Q (Ah)
5
6
CEDV = OCV(T,SOC) - I*R(T,SOC)
• Modeling: R(SOC,T), good for new battery
• Calculate CEDV2 (7%) and CEDV1 (3%) threshold at any I and T.
• Not Accurate for Aged battery
20
Coulomb Counting Based Gauging
Advantages
• Not influenced by distortions of voltage measurement
• Accuracy is defined by current integration hardware
• Gauging error: 3-10% depending on operation conditions and usage
Disadvantages
• Learning cycle needed to update Qmax
– Battery capacity degradation with aging
• Qmax Reduction: 3-5% with 100-cycles
– Gauging error increases 1% for every 10-cycles without
learning
• Self-discharge has to be modeled: Not accurate
Key Parameter related to Aging: Impedance
V = VOCV - I •RBAT ?
21
Advantages for typical gas gauges
Battery Voltage (V)
4.2
I•RBAT
3.6
3.0
Open Circuit Voltage
(OCV)
EDV
2.4
Battery Capacity Quse Qmax
• Very accurate gauging from OCV without load (relaxation)
• Very accurate gauging with Coulomb Counting with load
22
Issue Review
Battery Voltage (V)
4.2
I•RBAT
3.6
3.0
Open Circuit Voltage
(OCV)
EDV
2.4
Battery Capacity Quse Qmax
• Voltage Based gas Gauge: V = OCV(T,SOC) - IR(T,SOC, Aging)
• Current integration gas gauge: CEDV = OCV(T,SOC) - IR(T,SOC, Aging)
Problem: Battery Impedance
23
Finish
Back Up Slides:
Impedance Track Reference
25
Single Cell Impedance Track (IT)
Basic Terminology and Relationships
• OCV – Open Circuit Voltage
• Qmax – Maximum battery chemical capacity
Qmax
PassedQ
=
|SOC1 - SOC2 |
(SOC1/SOC2 is correlated from OCV table after OCV1/OCV2 measurement)
• SOC – State of Charge
PassedQ*
SOC = 1 Qmax
•
(* From Full Charge State)
• RM – Remaining Capacity
RM = ( SOCstart - SOCfinal ) × Qmax
•
(SOC start is present SOC, SOC final is SOC at system terminate voltage)
26
Single Cell Impedance Track (IT)
Basic Terminology and Relationships
• 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
RSOC
RM 100
FCC
27
Single Cell Impedance Track (IT)
Fuel Gauge Introduction
28
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
4.5
Voltage, V
4
Q
15%
3.5
3%
EDV
3
0
1
2
3
Capacity, Ah
4
5
0%
6
Q
FCC
max
Error
checkpoints
Check point at 0% is not meaningful – EDV
is the voltage where system crashes!
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.
29
Single Cell Impedance Track (IT)
Error Definition and Calculation
• Relative State of Charge (RSOC) Error
RSOC Error = RSOC calculated - RSOC reported
RSOC calculated
FCC Qstart PassedQ
100
FCC
(RSOC reported is the RSOC reported by bq275xx Impedance Track TM algorithm)
30
Single Cell Impedance Track (IT)
Error Definition and Calculation
• Remaining Capacity (RM) Error
RM Error
RMcalculated RMreported
FCC
RM calculated = FCC - Qstart - PassedQ
(RM reported is the RM reported by bq275xx Impedance Track TM algorithm)
31
Example error plots
True vs reported RSOC
RSOC error
12
2
11.5
1
11
RSOC error
Voltage
0
10.5
10
1
9.5
2
9
3
8.5
0
20
40
60
80
100
4
0
10
20
30
40
RSOC
60
70
80
90
100
RSOC
SMB RSOC
true RSOC
SMB RSOC
Remaining capacity test
Relative RemCap error
12
4
11.5
3.13
11
2.25
Relative RemCap error, %
Voltage
50
10.5
10
9.5
1.38
0.5
0.38
1.25
9
2.13
8.5
0
1
2
3
4
5
3
0
12.5
Capacity, mAh
SMB remaining capacity
true remaining capacity
25
37.5
50
62.5
75
87.5
100
SOC
SMB RSOC
32