PowerPoint

Report
Introduction to Battery Management
Part 1: Battery Technology Overview
Maria Cortez
Upal Sengupta
INTRODUCTION TO BATTERY
MANAGEMENT
• Part 1: Battery Technology Overview
• Part 2: Battery Gauging, Cell Balancing, and Protection
• Part 3: Li-Ion Battery Charging
• Part 4: Special Considerations for Large Battery Packs
• Part 5: Intro to Wireless Power and bqTESLA™
Introduction to Battery Management
Part 1: Battery Technology Overview
3
Intro to Battery Management – Part 1
• Basic comparison of rechargeable batteries
• Li-Ion Battery Performance Characteristics
• New types of Li-Ion batteries optimized for diverse
applications
• Basic requirements for / functions of battery management
Rechargeable Battery Options
• Lead Acid
• NiCd
Volumetric energy density (Wh/L)
↑ 100 years of fine service!
↓ Heavy, low energy density,
toxic materials
↑ High cycle count, low cost
↓ Toxic heavy metal, low
energy density
600
• NiMH
↑ Improvement in capacity
over NiCd
↓ High self-discharge
500
Li-ion
400
Lipolymer
300
• Lithium Ion / Polymer
Ni-MH
200
Ni-Cd
100
Lead Acid
0
0
50
100
150
Gravimetric energy density (Wh/kg)
200
↑ High Energy density, low
self-discharge
↓ Cost, external electronics
required for battery
management
Battery Chemistry Comparison
Lead Acid
NiCd
• High Power, standby apps
• Car battery, EV, UPS
(+)
(-)
NiMH
• High Power, long usage
• Consumer electronics, electronic
bikes, power tools
+
Li-Ion
• High Capacity, long usage
• Consumer electronics, cameras,
some industrial applications
• High Capacity, long usage
• Video cameras, digital cameras,
mobile phones, PC / tablets
(-)
(+)
(-)
(+)
(-)
Simple &
low cost
(lowest
$/kWH)
Low Specific
Energy – poor
energy /
weight ratio
Fast &
simple
charging
Low specific
energy
30-40% more
capacity vs.
NiCd
Limited cycle
life – reduced
w/ deep
discharge
High Energy
density
Requires
protection
circuit
Mature &
well
understood
Slow charge
(up to 14
hours)
Good cycle
life & shelf
life
Memory
effect
Simple
storage &
transport
Complex
charge
algorithm
Relatively
low selfdisch.
Subject to
aging (even if
not used)
High
Discharge
Currents
Toxic
materials
Good load
performance
over
temperature
Toxic - not
environment
ally friendly
Environmentally
“friendlier”
Poor
overcharge
tolerance
Low
maintenance
Transport
regulations
when
shipping in
high volumes
Low Self
Discharge
Limited cycle
life, reduced
w/ deep
cycling
Economical
High self
discharge
Nickel
content
makes
recycling
profitable
Generates
heat at high
discharge
No memory
Simple
storage &
transport
Toxic
materials
High self
discharge –
worse @
high temp
No periodic
discharge
needed
Low temp
performance
Why is Li-Ion popular?
• A high performance battery for high performance devices!
– Gravimetric energy density  High Capacity, Light weight battery
– Volumetric density energy  High Capacity, Thin battery
– Low self-discharge  Stays charged when not in use
18650 Li-Ion Cell Capacity Development Trend
3500
18650 Cell
8%
2500
2000
65mm
Cell Capacity mAh
3000
1500
1000
500
0
1990
1994
1998
2002
2006
2010
• 18650: Cylindrical, 65mm length, 18mm diameter
• 8% yearly capacity increase over last 15 years
• Capacity increase has been delayed from 2010
• Li-Ion Battery Tutorial, Florida battery seminar
18mm
Cell Construction and Safety
Safety Elements
• Aluminum or steel case
• Pressure relief valve
• PTC element
• Polyolefin separator
– Low melting point
(135 to 165°C)
– Porosity is lost as melting
point is approached
– Stops Li-Ion flow and shuts
down the cell
• Recent incidents traced to
metal particles that pollutes
the cells and creates
microshorts
What is Battery Management?
• Battery management circuits
–
–
–
–
Control charge flow into battery
Prevent abuse conditions
Monitor critical parameters
Communicate information
• Systems require battery management to extend run-times,
maximize safety, and maximize battery-life
• Degrees of implementation vary, but sophisticated battery
management consists of three components : Battery Charge Mgmt,
Battery Gauge / Mgmt, and Protection.
Pack Configurations
+7.2V
Series
+3.6V
+7.2V
+3.6V
2s3p
7.2V
1s3p
3.6V
1s2p configuration
Parallel
2s1p
Li-Ion Battery Management Components
SPI or I2C
PMIC
System Host
SMPS
AC Adapter
DC-
3 V / 5V
Chemical Fuse
Pack+
DC+
Charger IC
1-4 Cells
Host Controlled
SMBus
or
Stand Alone
System Rails
Multi-Rail
Fuel Gauge IC
CLK
I2C / SMBus
DATA
Temp
Protection
Over /Under Voltage
Temp Sensing
Gauging
Charge Control
Authentication
Chg
Dsg
AFE IC
VCELL1
Analog Interface
Over Current
Cell Balancing
VCELL2
Voltage ADC
Current ADC
Pack-
Secondary
Safety
Over Voltage
Protection IC
(Optional)
Sense
Resistor
Li-Ion Battery Pack
Li-Ion 18650 Discharge at Various Rates
V1
V2
Self-heating Effect Lowers
the Internal Impedance
V = I × RBAT
Li-Ion 18650 Discharge
at Various Temperatures
• Organic electrolyte makes internal resistance of Li-Ion
battery more temperature dependent than other batteries
Self-heating Effect Lowers the Internal Impedance
Effect of Impedance Increase on Runtime
Battery Voltage (V)
4.5
Fresh Battery
Aged Battery with 100 Cycles
4.0
3.5
3.0
0
0.5
1.0
Time (h)
1.5
2.0
• Change of no-load capacity during 100 cycles < 1%
• Also, after 100 cycles, impedance doubles
• Double impedance results in 7% decrease in runtime
Charge Voltage Affects Battery Service Life
1100
Cell Capacity (mAh)
1000
900
4.2 V
800
700
600
4.3 V
500
400
4.35 V
0
100
200
300
400
Number of Cycles
4.25 V
500
600
• The higher the voltage, the higher the initial capacity
• Overcharging shortens battery cycle life
Source: “Factors that affect cycle-life and possible degradation mechanisms of
a Li-Ion cell based on LiCoO2,” Journal of Power Sources 111 (2002) 130-136
Recoverable Capacity (%)
Shelf-life, Degradation without Cycling
200C at 4.1 V
100
200C at 4.2 V
80
60
600C at 4.1 V
40
600C at 4.2 V
20
0
1
3
5
7
Months
9
11
13
• If battery sits on the shelf too long, capacity will decrease
• Degradation accelerates at higher temperatures and voltages
• Depending on chemistry, there are specific recommendations for
best storage conditions
Source: M. Broussely et al at Journal of Power Sources 97-98 (2001)
Charge Current versus Battery Degradation
Cell Capacity (mAh)
900
1.0C
800
1.1C
1.3C
1.5C
700
600
500
2.0C
0
100
200
300
400
Number of Cycles
500
600
• Charge Current: Limited to 1C rate to prevent overheating that can
accelerate degradation
• Some new cells can handle higher-rate
Source: “Factors that affect cycle-life and possible degradation mechanisms of
a Li-Ion cell based on LiCoO2,” Journal of Power Sources 111 (2002) 130-136
There are many variations of “Li-Ion” Batteries!
Cathode
Material
(LI+)
Li CoO2
Li Mn2O4
Li FePO4
Anode
Material
Li NMC
Li NCA
Li CoO2 NMC
Li MnO NMC
Graphite
Li -CoO2
Li -CoO2
Hard
Carbon
LTO
(“Titanate”)
Vmax
4.20
4.20
3.60
4.20
4.20
4.35
4.20
4.20
2.70
Vmid
3.60
3.80
3.30
3.65
3.60
3.70
3.75
3.75
2.20
Vmin
3.00
2.50
2.00
2.50
2.50
3.00
2.00
2.50
1.50
Typical Anode and Cathode Materials used for Li-Ion Cells
•
•
•
All the above cells are considered “Li-Ion”
In addition to the different voltage ranges shown, they will also have different capacity, cycle life, and
charge/discharge rate performance (not shown)
Specific performance parameters can be optimized based on chemistry and physical design of a cell –
the “important” parameters depend on the application
Energy Cells, Power Cells, and “Mid-Rate”
Cells for different applications…
“What’s Important” in various applications
• Vehicle Starter Application:
–
–
–
–
Extremely high surge current  need very low cell internal resistance
Must work at all extremes of temperature
Battery discharge is very brief – spend > 99% of time being recharged
Battery is rarely (ideally, “never”) fully discharged  cycle life is not important
• Power Tool Application:
– Frequent, fast discharge use  need low resistance and fast recharge
– Rugged, durable cells are desirable
– “Green” trend replaces NiCd with Li-Ion  DIFFERENT TYPE OF LI-ION than in phone &
notebook PC applications, optimized for high current usage
– requires different type of charging & monitoring circuits!
• Small Handheld Devices:
– Light weight and small size are critical
– High energy capacity for longest possible run time
– Accurate capacity monitoring is important – especially for smartphones
“Standard” LiCoO2 vs. LiFePO4
Cell Voltage
4.2
4.03
3.86
Li-Cobalt
Voltage, V
3.69
3.52
3.35
3.18
Li-Iron-Phosphate
3.01
2.84
2.67 Depth of Discharge
2.5
0
20
40
60
DOD, %
80
100
Summary – Battery Management
• The basic functions of all Li-Ion pack monitoring circuits are
the same regardless of the application
– Maintain safe operation of the battery pack under all conditions
– Extend / maintain service life of the battery as much as possible
– Ensure complete charging and utilization of the pack without violating safety
thresholds
• The level of sophistication / features implemented will vary
depending on the application
– Performance / Cost tradeoffs
– Criticality of safety & accuracy requirements
– Physical size and energy capability
Introduction to Battery Management
Introduction to Battery Management
Maria Cortez
Upal Sengupta
Introduction to Battery Management
Part 2: Battery Monitoring / Gauging
Battery Monitoring / Gauging
What is Fuel Gauging Technology?
Fuel Gauging = technology to report battery operational status and
predict battery capacity under all system active and inactive conditions.
Key benefits are providing extended RUN TIME and LIFE TIME!
The Gas Gauge function autonomously reports and calculates:
– Voltage
– Charging or Discharging Current
63%
– Temperature
– Remaining battery capacity information
• Capacity percentage
• Run time to empty/full
• Talk time, idle time, etc.
– Battery State of Health
– Battery safety diagnostics
Run Time 6:27
Basic Smart Battery System
CHG
DSG
VPACK
Vbatt
ICHG
comm
Gas
Gauge
Tbatt
Battery Model
VDSG
Load
Charger
VCHG
IDSG
Rs
qk q0 t  k I k
Ibatt
Gauging
Battery Chemical Capacity (Qmax)
Definitions:
Li-Ion Discharge Profile
• Battery Capacity = “1C”
1C Discharge rate is current to
4.5
completely discharge a battery in one
hour
Example:
• 2200mAh battery
• 1C discharge rate = 2200mA @ 1 hr
• 0.5C rate: 1100mA @ 2hrs
Voltage, V
Load current: < 0.1C
4.0
3.6V (Battery rated voltage)
3.5
EDV = 3.0V/cell
3.0
0
1
2
3
4
Capacity, Ah
Battery Model
CBAT
RBAT
5
Qmax
6
• Battery Capacity (Qmax):
Amount of charge can be
extracted from the fully charged
cell to the end of discharge
voltage (EDV).
• EDV (End of Discharge Voltage):
Minimum battery voltage
acceptable for application or for
battery chemistry
Gauging
Useable Capacity “QUSE”
4.2
Open Circuit Voltage (OCV)
I•RBAT
3.6
Battery Voltage (V)
Qmax
Quse
Cell voltage under
load
3.0
EDV
2.4
Quse
•
EDV will be reached earlier for higher
discharge current.
•
Useable capacity Quse < Qmax
Qmax
+
+
-
I
RBAT
OCV
V = OCV - I*RBAT
-
Gauging
Types of Gauging Algorithms…
Impedance Track™
Cell Voltage Measurement
• Measures cell voltage
• Advantage: Simple
• Not accurate over load conditions
• Directly measures effect of
discharge rate, temp, age and other
factors by learning cell impedance
• Calculates effect on remaining
capacity and full charge capacity
Coulomb Counting
•
•
•
•
•
•
•
Measures and integrates current over time
Affected by cell impedance
Affected by cell self discharge
Standby current
Cell Aging
Must have full to empty learning cycles
Must develop cell models that will vary with
cell maker
• Can count the charge leaving the battery,
but won’t know remaining charge without
complex models
• Models will become less accurate with age
• No learning cycles needed
• No host algorithms or calculations
Gauging
Simple Measured Cell Voltage - Effect of IR drop
4.2
Battery Voltage (V)
Open-Circuit Voltage (OCV)
3.9
I × RBAT
3.6
3.3
3.0
EDV
Battery Capacity
QUSE
QMAX
ISSUES:
• 25% granularity
• First bar lasts many times longer then subsequent bars
• No compensation for cell age
• Less run time
- I
+
OCV R
• Two bars represents over 50% capacity between 3.8 and 3.4 V
V=OCV - I*R +
• Pulsating load varies capacity bar up and down
V  VOCV  I  R BAT ?
• Accurate ONLY at very low current
BAT
BAT
Gauging - Coulomb Counting
Li-Ion Battery Cell Voltage
4.5
0.2C Discharge Rate
4.0
• Measure and Integrates Charge
• Current sensed w/ resistive shunt
Q
3.5
3.0
Q   i dt
EDV = End of Discharge Voltage
EDV
0
1
• Use this combined with OCV to relate battery Charge (mAH)
to battery DOD (%) and battery Voltage (V)
2
3
Capacity, Ah
4
5
Qmax
ISSUE: Internal battery resistance is variable - cell impedance changes with…
• Current
• Voltage
• Time
• Temperature
6
Gauging
Coulomb Counting - Learning before Fully Discharged
ISSUES:
4.5
– Too late to learn when 0%
capacity is reached
– How to learn if EDV
thresholds aren’t reached?
– A set voltage threshold for
given percentage of
remaining capacity
– True voltage at 7%, 3% EDV
Voltage (V)
4
EDV2
3.5
7%
EDV1
3%
EDV0
3
0
0%
1
2
3
Capacity Q (Ah)
4
5
6
• Remaining capacity depends
on current, temperature, and
impedance
DISADVANTAGES:
– New full capacity must be learned over time (full chg/dsg cycle)
– 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 cycle without learning
– End of discharge points not compensated
– Counting capacity out of battery doesn’t tell how much the battery can still deliver
under all conditions, needs capacity learning.
– Not suitable for high variable load current
– Uses processor resources for gauging computations
Gauging
Impedance Track™ gas gauge
• Incorporates
Voltage-based gauge: Accurate gauging under no load
Coulomb counting: Accurate gauging under load
13
Real-time impedance update
Remaining run-time calculation
12
Safety and State of Health
• Updates impedance at every cycle
voltag e, V
–
–
–
–
–
R BAT
OCV
OCV  VBAT
 11
IR
IAVG
Voltage under load
(a)
• Uses impedance, discharge rate, and temperature information to calculate
rate/temperature adjusted FCC (Full Charge Capacity)
V = OCV(SOC, T) – I × RBAT (SOC, T)
9
10
0
80
10 0
OCV
IR
Voltage under load
10
4.5
10
Voltage Under Load
9
(a)
20
3.5
3
0.13
0.13
0.1
0
4
0
(b)
R, Ohm
11
11
Resistance ()
OCV
Voltage, V
voltag e, V
Voltage (V)
IR
12
12
0.15
40
60
SOC, %
0.15
0.15
13
13
9
20
0
40
60
SOC, %
80
10 0
20
20
40
60
40 SOC, % 60
SOC (%)
80
80
100
100
Gauging
OCV - Voltage lookup
• One can tell how much water is in a glass by reading
the water level
mL
marks
– Accurate water level reading should only be made after
the water settles (no ripple, etc)
I(t)
• One can tell how much charge is in a battery by
reading well-rested cell voltage
– Accurate voltage should only be made after the battery
is well rested (stops charging or discharging)
q(t )
V(t)
• OCV measurement allows SOC estimation
• Relaxation time varies depending on:
• SOC,
• Prior load rate
• Temperature
Open Circuit Voltage (OCV)
Comparison of OCV Profiles for 5 Manufacturers
• OCV profiles similar for all
tested manufacturers, using
same chemistry
4.20
3.93
• Average SOC prediction
error based on OCV – SOC
correlation is about 1%
3.67
3.40
100%
50%
• Same database can be
used with batteries
produced by different
manufacturers
as long as cell chemistry
is the same
0%
+10
SOC Error (%)
Voltage Deviation (mV)
State of Charge (SOC)
0
-10
100%
+2.6
• TI maintains OCV profile
data for many different cell
types (“Chemical ID” table)
+1.3
0
-1.3
-2.6
50%
0%
100%
50%
0%
Learning Qmax without Full Discharge
Cell Voltage (V)
4.2
4.1
Start of Discharge
Start of Charge
4.0
P1
P2
Q
P1 Q
3.9
OCV
Measurement Points
OCV
3.8
Measurement Points
0
0.5 1.0 1.5 2.0 2.5 3.0
Time (hour)
0
P2
0.5 1.0 1.5 2.0 2.5 3.0
Time (hour)
• Change in capacity (mAH) is determined by exact coulomb counting
• Relative SOC1 and SOC2 are correlated with OCV after rest period
• Method works for both charge or discharge
Q
Qmax 
SOC1  SOC 2
Z-track Accuracy in Battery
Cycling Test
Remaining Capacity Error, %
1
0.5
•
Error is shown at 10%,
5% and 3% points of
discharge curve
•
For all 3 cases, error
stays below 1% during
entire 250 cycles
0
0.5
1
1.5
0
50
100
150
Cycle Number
error at 10%
error at 5%
error at 3%
200
250
300
Introduction to Battery Management
Introduction to Battery Management
Part 3: Li-Ion Battery Charging
Introduction to Charging Concepts
• Numerous ICs are available for charging Li-Ion batteries
• In order to choose the best device, the system designer needs to
consider a number of factors beyond the simple power requirements
associated with a given battery pack.
• This section will review some basic issues such as:
– Li-Ion Charging Profile, and how charger accuracy can affect the service life
of the battery
– When to use a linear or switch-mode charger
– The benefit of “power path management” in a system with an internal
battery pack
– When to use a host (microcontroller)-based charging scheme
Review: “Ideal” Li-Ion CC-CV Charge Curve
“CV”
“CC”
Practical “CC-CV” – allows for fault conditions
Pre-charge
(Trickle Charge)
Fast-charge
(Constant Current)
VOREG
ICHARGE
Constant Voltage
Battery Pack Voltage
Taper Current
VPrecharge
~3.0V
VShort
~2.0V
IPrecharge
ITERM
IShort
Fast Charge (PWM charge)
CCCV - From an actual data sheet…
Charge Voltage Accuracy vs. Cycle Life
• +/- 50mV on each charge cycle  +/- 100 cycles to the same point of
degradation!
4.2V
4.3V
4.35V
4.25V
Linear or Switch-Mode Charger…
• Same type of decision as whether to use an LDO or a DC/DC converter
– Low current, simplest solution  Linear Charger
– High Current, high efficiency  Switch-Mode Charger
• General Guideline ~ 1A and higher should use switching charger… or,
if you need to maximize charge rate from a current-limited USB port
Charging from a Current-Limited Source
• USB port limited to 500mA
• But… w/ Switching charger, can charge > 500 mA
500 mA
ICHG =?
USB
VBAT
+
VIN
Charge
Controller
Charger
•
•
•
•
•
500-mA Current Limit
40% more charge current with switcher
Full use of USB Power
Shorter charging time
Higher efficiency, lower temperature
Charge Current (A)
1
0.9
Switching Charger
0.8
0.7
0.6
Linear Charger
0.5
0.4
2.4
2.6
2.8
3
ICHG_sw=
3.2 3.4 3.6 3.8
Battery Voltage (V)
4
VIN
• η • 500mA
VBAT
4.2
Simplest Charger Architecture
• Some possible concerns / issues:
– What happens when battery is very low?
– What happens if battery is missing or defective?
– If system is operating, how can charger determine if battery current has
reached a termination level?
DC Source
System
Charger IC
Power Path Management
• Power supplied from adapter through Q1; Charge current controlled by Q2
• Separates charge current path from system current path; No interaction
between charge current and system current
• Ideal topology when powering system and charging battery simultaneously is a
requirement
Simplicity vs. Flexibility, or HW vs. SW
• Standalone charger:
– HW controlled
– Set critical parameters with resistors or pullup/pulldowns
– Fixed functionality
Host – Controlled Charger
• Can adjust charge voltage, current limits, timer settings, etc. “on the fly”
with software
• Adaptable to different cell types or application needs
• Allows more detailed read-back of fault conditions (overtemp, timer
limit, input voltage, etc.)
Summary – Battery Charging
• First charger selection criteria – “volts and amps” like any power supply
requirement
• Consider additional requirements unique to the battery type you are
using
• Consider other application requirements to decide if you should use
power path system, standalone charger, or host-controlled charger
• See additional appnotes and “E2E Forum” at www.ti.com/battery
Introduction to Battery Management
Introduction to Battery Management
Part 4: Special Considerations for
larger battery packs
The basic functions of all pack monitoring
circuits are the same…
• Maintain safe operation of the battery pack
under all conditions
• Extend / maintain service life of the battery
as much as possible
• Ensure complete charging and utilization of
the pack without violating safety thresholds
However…
• There are some significant differences!
Newer Applications = More Cells
• New applications with higher series cell counts
• Typically 8- to 16-series cells for a centralized system and 17- to
200-series cells for a distributed system
• Higher voltages, and higher currents
• Additional challenges in applications such as Power Tools, EBikes, Electric & Hybrid Vehicles, UPS Systems, etc.
New Application Challenges
• Very High Currents
– Can be High on Discharge, Low on Charge
• Uneven self-heating of cells on Discharge =
Imbalance
– Load and Battery Physically Separated
• Ex: String Trimmer - Motor 3 feet away - inductive
spikes
• Separate Charge & Discharge Paths
– Allows protection pass element (MOSFET) to be
sized to requirement
• Load and Charger have unique connection to
battery
• Fast Transition between Charge &
Discharge
– Hybrid Electric Vehicles, UPS Systems
"Big" Battery Challenges
High Cell Count Battery Systems
• Weakest link (cell) rules the pack
• Physically larger battery pack so...
... Cells on the ends may see a larger temperature
swings than others
• More likely to become imbalanced
• Higher cell count = Higher voltage = More
catastrophic potential
8.5"
... More difficult to connect cells in order
... Cell connections more fragile (likely to break)
... Series cell break, remaining cells see full
reverse stack voltage
... Shorted cell (>1P) fed by parallel cells
6.5"
Source: Eagle-Picher at 27th
International Battery Seminar &
Exhibit (2010)
Issues to consider
• What level of functionality is required for
battery management in a large pack?
– Safety only?
– Safety & Cell Balancing?
– Safety, Cell Balancing, and Fuel Gauging?
• Due to the sheer number of interconnections
in a larger pack, a single-chip solution may
not be practical
• Design the a pack management system to
meet the requirements (functionality vs.
cost/complexity)
Secondary / Redundant Protection
•
•
•
•
Multicell packs may need secondary protection
Backup system in case of catastrophic faults
Simple – protect against worst case scenarios only
Second protector thresholds set higher (e.g.
overvoltage) than primary protector
• System may or may not be recoverable (usable) after
secondary fault
Why balancing?
• Failed battery - about 3 years
old, perhaps 50 - 100 cycles
• Run time was decreasing,
eventually died
• Case removed for
construction inspection
– Sturdy assembly
– Electronics at end
– Flex for temp & cell voltage
• Potted circuit board
– Unknown protection circuit (Not
from TI!)
– Discharge limiting FETs were OFF
Picture of battery
Failed battery cell voltages
• Cell voltage check
showed a single bad
cell
• What went wrong?
– Open circuit cell with bias
from electronics?
– Cell forced into reverse
voltage?
• Would balancing have
prevented this?
Cell
6 (top)
Voltage
(mV)
4017
5
4018
4
4017
3
-536
2
4022
1 (bottom)
4023
Cell Balancing
What Causes Imbalance?
• Capacity variation (1~2% cellto-cell for same model)
• Charging state difference (i.e.
SOC difference)
• Impedance variation (up to
15%) causes voltage difference
when charging/discharging
• Localized heat degrades cells
faster than others, particular
self-heating at high discharge
rate
Effects of unbalanced cells:
• Reduced run-time due to...
- Premature charge termination
- Early discharge termination
• Further cell abuse from cycling above
or below optimal cell voltage limits
4.4
Overcharge
4.2
4.0
3.8
Undercharge
3.6
3.4
High Cell Count battery systems are
more likely to see imbalance due to
temperature gradients and cell selfheating at high discharge rates.
Capacity
Under-used
3.2
3.0
Time
2
4
6
8
10
Cell Balancing Circuits
• Linear Cell Bypass / “Bleed Balancing”
• Active / Switch Mode:
– Switch mode energy transfer from cell to cell
– Allows higher balance current with less heat
– More expensive and complex to implement – used
only for very large / high capacity applications
– Linear / dissipative – low current
– Most common implementation for small and
medium size battery packs
PACK +
V3
1 k
R1
Q1
R5
Q2
Battery
Monitor /
Balance
Controller
CELL 2
V2
1 k
R2
2
CELL 1
1 k
R3
R4, R5 << Rdson
Q2
R4
Q1
Rdson
1
V1
R
Active / Switch Mode
There are many variations of “Li-Ion” Batteries!
Cathode
Material
(LI+)
Li Li CoO2 MnO
Li FePO4
Anode
Material
Li NMC
Li NCA
Li CoO2 NMC
Li MnO NMC
Graphite
Li CoO2
Li -CoO2
Hard
Carbon
LTO
(“Titanate”)
Vmax
4.20
4.20
3.60
4.20
4.20
4.35
4.20
4.20
2.70
Vmid
3.60
3.80
3.30
3.65
3.60
3.70
3.75
3.75
2.20
Vmin
3.00
2.50
2.00
2.50
2.50
3.00
2.00
2.50
1.50
Typical Anode and Cathode Materials used for Li-Ion Cells
• All the above cells are considered “Li-Ion”
• In addition to the different voltage ranges shown, they will also have different
capacity, cycle life, and charge/discharge rate performance (not shown)
• Specific performance parameters can be optimized based on chemistry and
physical design of a cell – the “important” parameters depend on the application
Can the battery management circuit
handle the different chemical variations?
• Fuel Gauge: TI Impedance Track Gauges can
be programmed with specific “Chemical ID”
values to provide accurate monitoring of
different types of batteries
• Protectors: TI Precision Protectors are
designed for a variety of OV / UV thresholds
– Either user-programmable (EEPROM) or factory
orderable options for specific levels
“Pack-Based” Gauging for large packs
• Smaller packs can have a single IC to monitor
each individual cell, balance and gauge on a cellby-cell basis
• This may not be practical for larger packs due to
number of interconnections (and memory) required
• bq2060A has been TI’s long-running solution for a
generic pack-based gauge
• bq34z100 family is the new generation of devices
for this need
o When used with Li-Ion type cells, cell balancing & individual cell
OVP must be done externally  see bq779xx family devices
bq34z100
Example of typical implementation
bq34z100
Li-Ion Pack Monitor
bq78PL900
bq78PL910A
bq77908A
Protection
and
Balancing
not required
for NiXX,
PbSO4, etc.
The basic functions of all pack monitoring
circuits are the same…
• Maintain safe operation of the battery pack
under all conditions
• Extend / maintain service life of the battery
as much as possible
• Ensure complete charging and utilization of
the pack without violating safety thresholds
Introduction to Battery Management
Introduction to Battery Management
Part 5: Introduction to Wireless
Power
bqTESLATM Wireless Power
Solutions
Wireless Power Consortium (WPC)




Industry wide standard for delivering wireless
power up to 5W
Aimed to enable interoperability between
various charging pads and portable
devices
Standard continues to gain traction with
increasing list of members (105+)
Compatible devices will be marked with a Qi
logo
and more…
Qi: Intelligent Control of
Inductively Coupled Power Transfer
• Power transmitted through shared
magnetic field
– Transmit coil creates magnetic field
– Magnetic field induces current in receiver coil
– Shielding material below TX and above RX coils
• Power transferred only when needed
– Transmitter waits until its field has been perturbed
– Transmitter sends seek energy and waits for a digital
response
– If response is valid, power transfer begins
• Power transferred only at level needed
– Receiver constantly monitors power received and
delivered
– Transmitter adjusts power sent based on receiver
feedback
– If feedback is lost, power transfer stops
I
z<D
D
Factors Affecting Coupling Efficiency
• Coil Geometry
– Distance (z) between coils
– Ratio of diameters (D2 / D) of the
two coils  ideally D2 = D
– Physical orientation
• Quality factor
– Ratio of inductance to resistance
– Geometric mean of two Q factors
• Near field allows TX to
“see” RX
• Good Efficiency when
coils displacement is
less than coil diameter
(z << D)
Factors Affecting Coupling Efficiency
Optimal operating distance
40% at 1 diameter
1% at 2.5 diameter
0.1% at 4 diameters
0.01% at 6 diameters
bq50k + bq51K: Qi-Compliant Solution
Power
Communication / Feedback
Communication - Basics
• Primary side controller must detect that an object is placed on the
charging pad.
– When a load is placed on the pad, the primary coil effective
impedance changes.
– “Analog ping” occurs to detect the device.
• After an object is detected, must validate that it is WPC-compatible
receiver device.
– “Digital Ping” – transmitter sends a longer packet which powers up
the RX side controller.
– RX side controller responds with signal strength indicator packet.
– TX controller will send multiple digital pings corresponding to each
possible primary coil to identify best positioning of the RX device.
• After object is detected and validated, Power Transfer phase begins.
– RX will send Control Error Packets to increase or decrease power
level
• WPC Compliant protocol ensures interoperability.
Analog Ping with and without object on pad
Vertical Scale: 20V/div
Horizontal Scale: 20 uS/div
(a) No object on pad
(b) RX Device placed on pad
Analog Ping / Digital Ping / Startup
Analog Pings – no
object detected
Analog Ping – object detected.
Followed by subsequent Digital
Ping and initiation of
communications functions
TX COIL
VOLTAGE
Vertical Scale: 20V/div
Horizontal Scale: 200
mS/div
COMM
Communication – How it works…
Switching Frequency Variation
•
System operates near
resonance for improved
efficiency.
•
Power control by changing
the frequency, moving along
the resonance curve.
•
Modulation using the power
transfer coils establishes the
communications.
•
Feedback is transferred to
the primary as error.
Operating Point
80 KHz
100 KHz
120 KHz
bqTesla System Efficiency Breakdown
Measured from DC input of Transmitter to DC output of Receiver
Tx Eff.
Magnetics Eff.
100%
Rx Eff.
System Efficiency
TX Eff
RX Eff
Magnetics Eff
90%
Efficiency (%)
80%
70%
System Eff
60%
50%
40%
30%
20%
0
0.5
1
1.5
2
2.5
Output Power (W)
3
3.5
4
4.5
5
bqTESLA EVMs
bqTESLA Evaluation Modules
New
bq51013EVM-725
Qi-compliant coil used w/ EVM kit
40-mm x 30-mm x 0.75-mm
WPC Compliant Receiver Coil
WPC Compliant Transmitter Coil
bq51013 “form factor” demo PCB (PR1041)
5 x 15 mm footprint for all RX side circuitry
• Represents what an OEM would design into their actual end-product to
enable wireless power from a QI-compliant charging pad.
Wireless Power Transmitter – App Note
• Title, “Building a Wireless
Power Transmitter”
• Detailed Discussions on:
– Critical Design Parameters
– Layout Requirements
– Component Requirements
– Example Waveforms
– Most Common Pitfalls for Qi
Certification
• TI Wireless Power Web Page
http://www.ti.com/wirelesspower
(http://www.ti.com/litv/pdf/slua635)
TI Wireless Power Forum
• TI E2E Community-Wireless Power
http://e2e.ti.com/support/power_management/wireless_power/default.aspx
• External Forum—available for
entire engineering community
• Ground Rules
–
Keep the questions technical in nature,
good question your peers can benefit
from at a later date.
–
One question per-post to make it
easer to search.
–
Place P/N in Topic with description
of question again to make it easier to search.
–
Refer to your TI Sales Rep for pricing & delivery questions
For technical specs & tutorials:
http://www.wirelesspowerconsortium.com

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