Introduction to Battery Management
Maria Cortez Upal Sengupta

• 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
3

• 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

• Lead Acid
↑ 100 years of fine service!
↓ Heavy, low energy density, toxic materials
• NiCd
↑ High cycle count, low cost
↓ Toxic heavy metal, low energy density
600
500
400
300
200
100
0
0
Li-ion
Lipolymer
• NiMH
↑ Improvement in capacity over NiCd
↓ High self-discharge
Ni-Cd
Lead Acid
50
Ni-MH
100 150
Gravimetric energy density (Wh/kg)
200
• Lithium Ion / Polymer
↑ High Energy density, low self-discharge
↓ Cost, external electronics required for battery management

Lead Acid
• High Power, standby apps
• Car battery, EV, UPS
(+)
Simple & low cost
(lowest
$/kWH)
Mature & well understood
NiCd
• High Power, long usage
• Consumer electronics, electronic bikes, power tools
(-) +
Low Specific
Energy – poor energy / weight ratio
Slow charge
(up to 14 hours)
Fast & simple charging
Good cycle life & shelf life
(-)
Low specific energy
Memory effect
NiMH
• High Capacity, long usage
• Consumer electronics, cameras, some industrial applications
Li-Ion
• High Capacity, long usage
• Video cameras, digital cameras, mobile phones, PC / tablets
(+) (-) (+)
30-40% more capacity vs.
NiCd
Simple storage & transport
Limited cycle life – reduced w/ deep discharge
Complex charge algorithm
High Energy density
Relatively low selfdisch.
(-)
Requires protection circuit
Subject to aging (even if not used)
High
Discharge
Currents
Low Self
Discharge
Low temp performance
Toxic materials
Limited cycle life, reduced w/ deep cycling
Good load performance over temperature
Toxic - not environment ally friendly
Economical High self discharge
Environmentally
“friendlier”
Nickel content makes recycling profitable
Poor overcharge tolerance
Generates heat at high discharge
Low maintenance
Transport regulations when shipping in high volumes
No memory
Simple storage & transport
Toxic materials
High self discharge – worse @ high temp
No periodic discharge needed

• 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
3000
2500
2000
1500
1000
500
8%
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
18650 Cell
18mm

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

• 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
+7.2V
Series
+3.6V
+3.6V
1s3p
3.6V
2s3p
7.2V
1s2p configuration
2s1p
Parallel

SPI or I2C
System Host
PMIC
Multi-Rail
System Rails
SMPS
3 V / 5V
DC +
AC Adapter
DC-
Charger IC
1-4 Cells
Host Controlled
SMBus or
Stand Alone
Pack+
CLK
I2C / SMBus
DATA
Chemical Fuse
Chg Dsg
Temp
Fuel Gauge IC
Protection
Over /Under Voltage
Temp Sensing
Gauging
Charge Control
Authentication
AFE IC
Analog Interface
Over Current
Cell Balancing
Voltage ADC
Current ADC
Pack-
V
CELL1
V
CELL2
Secondary
Safety
Over Voltage
Protection IC
(Optional)
Sense
Resistor
Li-Ion Battery Pack

 V1
 V2
Self-heating Effect Lowers the Internal Impedance
 V =  I × R
BAT

• Organic electrolyte makes internal resistance of Li-Ion battery more temperature dependent than other batteries
Self-heating Effect Lowers the Internal Impedance

4.5
4.0
Fresh Battery
Aged Battery with 100 Cycles
3.5
3.0
0 0.5 1.0 1.5 2.0
Time (h)
• Change of no-load capacity during 100 cycles < 1%
• Also, after 100 cycles, impedance doubles
• Double impedance results in 7% decrease in runtime

1100
1000
900
800
4.2 V
700
600
4.3 V
500
4.35 V
4.25 V
400
0 100 200 300 400 500 600
Number of Cycles
• 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 LiCoO
2
,” Journal of Power Sources 111 (2002) 130-136

100
80
20 0 C at 4.1 V
20 0 C at 4.2 V
60
40
60 0 C at 4.1 V
60 0 C at 4.2 V
20
0
1 3 5 7 9 11 13
Months
• 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)

900
1.0C
800
700
1.1C
1.3C
1.5C
600
2.0C
500
0 100 200 300 400 500 600
Number of Cycles
• 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 LiCoO
2
,” Journal of Power Sources 111 (2002) 130-136

Cathode
Material
(LI+)
Anode
Material
V max
V mid
V min
Li -
CoO
2
Li -
Mn
2
O
4
Li -
FePO
4
Li -
NMC
Li -
NCA
Li -
CoO
2
-
NMC
Li -
MnO -
NMC
Li -CoO
2
Li -CoO
2
Graphite
Hard
Carbon
LTO
(“Titanate”)
4.20
4.20
3.60
4.20
4.20
4.35
4.20
4.20
2.70
3.60
3.00
3.80
2.50
3.30
2.00
3.65
2.50
3.60
2.50
3.70
3.00
3.75
2.00
3.75
2.50
2.20
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

• 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

Cell Voltage
4.2
4.03
3.86
3.69
3.52
3.35
Li-Iron-Phosphate
3.18
3.01
2.84
2.67
Depth of Discharge
2.5
0 20 40
Li-Cobalt
DOD, %
60 80 100

• 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

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
– 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
63%

CHG DSG
V
PACK
I
CHG
V
CHG
V
DSG comm Gas
Gauge T batt
V batt
I
DSG
I batt
Rs q k
 q
0
  t
  k
I k

Battery Chemical Capacity (Qmax)
4.5
4.0
3.5
3.0
0 1
Li-Ion Discharge Profile
Load current: < 0.1C
2
3.6V (Battery rated voltage)
EDV = 3.0V/cell
3
Capacity, Ah
4
Battery Model
C
BAT
R
BAT
5
Q max
6
Definitions:
•
Battery Capacity =
“1C”
1C Discharge rate is c urrent to completely discharge a battery in one hour
Example:
• 2200mAh battery
• 1C discharge rate = 2200mA @ 1 hr
• 0.5C rate: 1100mA @ 2hrs
• 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

Useable Capacity “Q
USE
”
4.2
3.6
Open Circuit Voltage (OCV)
I•R
BAT
3.0
Cell voltage under load
EDV
Q use
Q max
2.4
Quse
• EDV will be reached earlier for higher discharge current.
• Useable capacity Q use
< Q max
Qmax
+
+ -
OCV
I
R
BAT
V = OCV - I*R
BAT
-

Types of Gauging Algorithms…
Cell Voltage Measurement
• Measures cell voltage
• Advantage:
Simple
• Not accurate over load conditions
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
Impedance Track™
• Directly measures effect of discharge rate, temp, age and other factors by learning cell impedance
• Calculates effect on remaining capacity and full charge capacity
• No learning cycles needed
• No host algorithms or calculations

Simple Measured Cell Voltage - Effect of IR drop
4.2
Open-Circuit Voltage (OCV)
I × R
BAT
3.9
3.6
3.3
3.0
EDV
ISSUES:
• 25% granularity
Battery Capacity
Q
USE
Q
MAX
• First bar lasts many times longer then subsequent bars
• No compensation for cell age
• Less run time
• Two bars represents over 50% capacity between 3.8 and 3.4 V
• Pulsating load varies capacity bar up and down
• Accurate ONLY at very low current V

V
OCV
+
+
R
BAT
V =OCV - I*R
BAT
?

4.5
Li-Ion Battery Cell Voltage
0.2C Discharge Rate
4.0
Q
3.5
• Measure and Integrates Charge
• Current sensed w/ resistive shunt
EDV
3.0
Q
  i dt
0 1
• Use this combined with OCV to relate battery Charge
(mAH) to battery DOD (%) and battery Voltage (V)
EDV = End of Discharge Voltage
2 3
Capacity, Ah
4
ISSUE: Internal battery resistance is variable cell impedance changes with…
• Current • Voltage • Time • Temperature
5
Q max
6

Coulomb Counting - Learning before Fully Discharged
4.5
4
3.5
3
EDV2
EDV1
EDV0
7%
3%
0%
0 1 2 3 4 5 6
Capacity Q (Ah)
ISSUES:
– 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
• 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

4.5
4
3.5
3
2.5
20 0 20 40
SOC, %
60 80 100 120
Impedance Track™ gas gauge
• Incorporates
–
Voltage-based gauge: Accurate gauging under no load
– Coulomb counting: Accurate gauging under load
–
Real-time impedance update 13
–
Remaining run-time calculation
12
– Safety and State of Health
• Updates impedance at every cycle
R
BAT

OCV

V
BAT
11
I
AVG
IR
OCV
Voltage under load
• Uses impedance, discharge rate
, and
10 temperature
(a) rate/temperature adjusted FCC (Full Charge Capacity)
0
V = OCV(SOC, T) –
20 40 60
I × R (SOC, T)
10 0
SOC, %
I

R
10
10
9
9
0
OCV OCV
IR
Voltage under load
4.5
Voltage Under Load
4
20
3.5
40
SOC, %
60
0.15
3
0.13
2.5
20 0 20
(a)
80
40
SOC, %
(b)
60 80
10 0
100 120
0.13
0.13
(b)
0.1
0
0 20 40 60 80 100
SOC, %
SOC (%)
0.1
0 20 40
SOC, %
60 80 100

OCV - Voltage lookup
• One can tell how much water is in a glass by reading the water level
– 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 mL marks

+10
0
-10
100%
4.20
3.93
3.67
Comparison of OCV Profiles for 5 Manufacturers
3.40
100%
50%
State of Charge (SOC)
+2.6
+1.3
0
-1.3
-2.6
100%
0%
• OCV profiles similar for all tested manufacturers, using same chemistry
• Average SOC prediction error based on OCV – SOC correlation is about 1%
• Same database can be used with batteries produced by different manufacturers as long as cell chemistry is the same
• TI maintains OCV profile data for many different cell types (“Chemical ID” table)
50% 0% 50% 0%

Learning Q max without Full Discharge
4.2
4.1
Start of Charge
P2
4.0
P1
 Q
3.9
3.8
OCV
Measurement Points
0 0.5 1.0 1.5 2.0 2.5 3.0
Time (hour)
Start of Discharge
P1
OCV
Measurement Points
 Q
P2
0 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 max

SOC 1

Q

SOC 2

1
0.5
0
0.5
1
1.5
0
• Error is shown at 10%,
5% and 3% points of discharge curve
50 100 150 200 250 300
Cycle Number error at 10% error at 5% error at 3%
• For all 3 cases, error stays below 1% during entire 250 cycles

Introduction to Battery Management

Introduction to Battery Management

• 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

“CV”
“CC”

V
OREG
Pre-charge
(Trickle Charge)
Fast-charge
(Constant Current)
I
CHARGE
Constant Voltage
Battery Pack Voltage
V
Precharge
~3.0V
V
Short
~2.0V
I
Short
I
Precharge
Fast Charge
(PWM charge)
Taper Current
I
TERM

• +/- 50mV on each charge cycle  +/- 100 cycles to the same point of degradation!
4.2V
4.35V
4.3V
4.25V

• 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

• USB port limited to 500mA
• But… w/ Switching charger, can charge > 500 mA
1
500 mA I
CHG
=?
0.9
USB V
BAT Switching Charger
0.8
+
V
IN
Charge
Controller
0.7
0.6
Linear Charger
Charger 0.5
• 500-mA Current Limit
• 40% more charge current with switcher
• Full use of USB Power
• Shorter charging time
• Higher efficiency, lower temperature
0.4
2.4
2.6
2.8
3 3.2
3.4
3.6
3.8
4 4.2
Battery Voltage (V)
I
CHG_sw
=
V
IN
V
BAT
• η • 500mA

• 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 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

• Standalone charger:
– HW controlled
– Set critical parameters with resistors or pullup/pulldowns
– Fixed functionality

• 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.)

• 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

• 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

• 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, E-
Bikes, Electric & Hybrid Vehicles, UPS Systems, etc.

• 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

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
... More difficult to connect cells in order
... Cell connections more fragile (likely to break)
• Higher cell count = Higher voltage = More catastrophic potential
... 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)

• 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)

• 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

• 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

• 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)
5
4
3
Voltage
(mV)
4017
4018
4017
-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
4.4
4.2
4.0
3.8
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
Overcharge
Undercharge
3.6
High Cell Count battery systems are more likely to see imbalance due to temperature gradients and cell selfheating at high discharge rates.
3.4
3.2
3.0
Time 2
Capacity
Under-used
4 6 8 10

Cell Balancing Circuits
• Linear Cell Bypass / “Bleed Balancing”
– Linear / dissipative – low current
– Most common implementation for small and medium size battery packs
• 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
PACK +
V3
Battery
Monitor /
Balance
Controller
Rdson
R4, R5 << Rdson
1 k

R1
R5
Q2
1 k 
R2
R4
Q1
1 k

R3
CELL 2
CELL 1
R
Q1
Q2
Active / Switch Mode
1
V2
2
V1

Cathode
Material
(LI+)
Anode
Material
V max
V mid
V min
Li -
CoO
2
Li -
MnO
Li -
FePO
4
Li -
NMC
Li -
NCA
Li -
CoO
2
NMC
-
Li -
MnO -
NMC
Li -
CoO
2
Graphite
Hard
Carbon
4.20
4.20
3.60
4.20
4.20
4.35
4.20
4.20
Li -CoO
2
LTO
(“Titanate”)
2.70
3.60
3.80
3.30
3.65
3.60
3.70
3.75
3.75
3.00
2.50
2.00
2.50
2.50
3.00
2.00
2.50
2.20
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

Example of typical implementation bq34z100
Li-Ion Pack Monitor bq78PL900 bq78PL910A bq77908A
Protection and
Balancing not required for NiXX,
PbSO4, etc.

• 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

TM

 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…

• 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
D z < D

• Coil Geometry
– Distance (z) between coils
– Ratio of diameters (D two coils
2
/ D) of the
 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)

Optimal operating distance
40% at 1 diameter
1% at 2.5 diameter
0.1% at 4 diameters
0.01% at 6 diameters

Power
Communication / Feedback

• 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 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

• 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

Measured from DC input of Transmitter to DC output of Receiver
Tx Eff.
Magnetics Eff.
Rx Eff.
System Efficiency
100%
RX Eff TX Eff
Magnetics Eff
90%
80%
70%
60%
50%
40%
30%
20%
0 0,5 1 1,5 2 2,5
Output Power (W)
3
System Eff
3,5 4 4,5 5

bqTESLA Evaluation Modules
New bq51013EVM-725

40-mm x 30-mm x 0.75mm
WPC Compliant Receiver Coil
WPC Compliant Transmitter Coil

(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.

• 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 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

http://www.wirelesspowerconsortium.com