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) - IR(T,SOC, Aging)
• Current integration gas gauge: CEDV = OCV(T,SOC) - IR(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