NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Tools for Designing Thermal
Management of Batteries in
Electric Drive Vehicles
Ahmad Pesaran, Ph.D.
Matt Keyser, Gi-Heon Kim, Shriram Santhanagopalan, Kandler Smith
National Renewable Energy Laboratory
Golden, Colorado
Presented at the
Large Lithium Ion Battery Technology & Application Symposia
Advanced Automotive Battery Conference
Pasadena, CA • February 4-8, 2013
NREL/PR-5400-57747
2
Battery Temperature in xEVs
• Lithium-ion battery (LIB) technology is expected to be the
energy storage of choice for electric drive vehicles (xEVs) in
the coming years
• Temperature has a significant impact on life, performance,
safety, and cost of LIBs
Dictates power capability through
cold cranking
Also limits the electric driving range
Dictates the size depending
on the power and energy
fade rate
Limiting power to
reduced T increase and
degradation
Kandler Smith, NREL Milestone Report, 2008
Desired
Operating
Temperature
3
Battery Thermal Management for xEVs
• Higher temperatures degrade LIBs more quickly, while low
temperatures reduce power and energy capabilities, resulting in
cost, reliability, safety, range, or drivability implications
• Therefore, battery thermal management is needed for xEVs to:
o Keep the cells in the desired temperature range
o Minimize cell-to-cell temperature variations
o Prevent the battery from going above or below acceptable limits
o Maximize useful energy from cells and pack
o Use little energy for operation
• However, a battery thermal management systems (BTMS) could:
o Increase complexity
o Add cost
o Reduce reliability
o Consume energy for operation
o …
4
Battery Thermal Management System
• Most in the xEV battery community agree that the value that
a BTMS provides in increasing battery life and improving
performance outweighs its additional cost and complexity
• However, the BTMS needs to be designed appropriately with
the right tools
• The National Renewable Energy Laboratory has been a
leader in battery thermal analysis and characterization for
aiding industry to design improved BTMSs
• This presentation describes the tools that NREL has used and
that we believe are needed to design properly sized BTMSs
5
Energy Balance in a Battery
outinlossgen
EEEE
dt
dE
−+−=
Energy
Accumulation
Rate
Energy
Generation
Rate
Energy
Loss
Rate
Input
Energy
Rate
Output
Energy
Rate
in
E
+
gen
E
dt
dE
loss
E
−
6
Heat Transfer in a Battery
(Assumption: isothermal ~ very high thermal conductivity)
• Electrochemical reactions
• Phase changes
• Mixing effects
• Joule heating
conductionExtas
asgen
s
p Q
TTAe
TThAHeat
dt
dT
mC
_
44
)()( −−−−−=
δ
Rate of
Temp
Change
Rate of
Internal Heat
Generation
Convection
Heat Rate
Radiation
Heat Rate
Conduction
Heat Rate
conductionExtasas
QTTAeTThA
_
44
)()( +−+−
δ
gen
Heat
Heat generated ( )
in a battery consists of:
Method of heat rejection/addition for thermal control
a
s
T
T
= Battery Temp
= Ambient Temp
gen
Heat
7
Heat Generation Rate and Specific Heat Impact
Battery Temperature Rise
0
2
4
6
8
10
12
14
16
0 400 800 1200 1600 2000
Time (seconds)
Temperature Rise (Degree C)
2C Rate (4.45 W/cell) Cp=1019 J/kg/C
C/1Rate (1.33 W/Cell) Cp = 1019 J/kg/C
2C Rate (4.45 W/Cell) Cp = 707 J/kg/C
Slow discharge
Fast discharge
Assuming uniform battery temperature and
the same heat transfer coefficient for three cases
8
Ln
t
T
HCTTeTTh
n
n
k
.0
B
BBp,B
44
ss
n
)()(
)T(
=
∞∞
∂
∂
+−+−=
∂
∂
−
ρδ
Heat Transfer in a Battery
(Non-isothermal; case and core regions)
Core region
Case or boundary region
T
kHeat
t
T
C
gen
p ∇
⋅∇+=
∂
∂
ρ
ty
conductivithermalk :
Heat flux
from the
core
Convection
from various
case surfaces
Radiation
from various
case surfaces
Heat
accumulation
in the case
9
Case + Core Example: T Distribution in a Module
Air Cooled
Q = 5 W/cell
T
t=0
= 30
o
C
T
air
= 25
o
C
h
air
= 18 W/m
2
K
30
35
40
45
50
55
Max. Cell Temperature [
o
C]
0 10
20 30 40 50 60
Time [min]
Top
Middle
Bottom
T
top,SS
= 54
o
C
Air-cooled
5 W/cell
h = 18 W/Km
2
T
SS
= 54°C
A. Pesaran, A. Vlahinos, S. Burch. Proceedings of the 14th Electric
Vehicle Symposium, December 1997
10
What Information is Needed to Design a BTMS?
• Acceptable temperature range for cell components at all times, i.e.,
active material, binders, separators, electrolyte, etc.
• Acceptable temperature difference within cells and from cell to cell,
depending on the chemistry and management system
• Maximum and minimum temperature limits for life specifications,
performance ratings, and safety considerations
• Thermo-physical properties of cells or components (density, specific
heat, directional thermal conductivities)
• Heat generation rate under average and aggressive drive profiles and
loads for the specific electric drive
• Heat rejection rate depending on thermal management strategy
o Fluid heat transfer coefficients or sink conductance
o Cooling fluid flow rate and sink temperature
• Configurations and dimensions of cells and proposed BTMS
• Parasitic power needed to push fluids/cooling through BTMS
11
Tools for Designing BTMS
• Experimental Tools
o Isothermal calorimeters and battery testers
o Infrared thermal imaging
o Thermal conductivity meters
o Heat transfer characterization setup
o Battery thermal testing loop
• Modeling Tools
o First-order/lumped capacitance thermal and fluid models
o 1-D and 2-D thermal and fluid-flow performance models
o 1-D vehicle integrated thermal-flow models
o 3-D electro-thermal models
o 3-D electrochemical-thermal model
o Computer-aided engineering software
12
Isothermal Battery Calorimeters
• We use a single-ended (one test chamber) conduction
calorimeter to measure specific heat and heat generation at
various current rates, temperatures, and states of charge (SOCs)
Sample/Module
Battery Tester
Initially fabricated by Calorimetry Sciences Corporation; later improved by NREL.
A. Pesaran, M. Keyser, D. Russell, J. Crawford, E. Lewis. Presented at the Long Beach Battery Conference,
January 1998
13
Heat Rate (Watts)
Time (Hours)
NREL’s First Isothermal Battery Calorimeter
• Heat flux measured between the sample and a heat sink using
heat flux gauges
• The heat sink is kept at a constant temperature with a precise
isothermal bath
• Max module that could be tested: 21 cm x 20 cm x 32 cm
• Heat rate detection: 0.015 W to 100 W
• Minimum detectable heat effect: 15 J (at 25°C)
• Baseline stability: ±10 mW
• Temperature range: -30°C to 60°C (±0.001°C)
• Accuracy of better than ±3%
Calorimeter response
Total heat generation =
Area under each curve
Calorimeter
Calorimeter Cavity
Photo Credits: David Parson & Matt Keyser, NREL
14
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 20.0 40.0 60.0 80.0 100.0 120.0
Heat Generation (Watts)
RMS Discharge Current (Amps)
Initial Temp = -15 C
Initial Temp = 0 C
Initial Temp = 30 C
° ° °
Example Heat Generation Data for CC Discharge
(from max to min allowable capacity-SOC)
22-Ah Li-Ion Cell
15
Specific Heat (Heat Capacity)
• Can be estimated from constituents of cell/module
• Can be estimated using a calorimeter by measuring heat
lost/gained (Q) from the battery going from T
initial
to T
final
o Heat capacity is calculated by
∑∑
•=
n
ii
n
ipavep
mmCC
11
,,
/)(
))(/(
, finalinitialtotalavep
TTmQC −•=
Cell/Module T
average
(°C)
Heat Capacity
J/kg/°C
NiMH – 18 Ah 33.2 677
Li-Ion 18650 33.1 1,105
Li-Ion Pouch – 4 Ah 18 1,012
VRLA – 16.5 Ah 32 660
Ni Zn – 22 Ah 20 1,167
Thermal Characteristics of Selected EV and HEV Batteries, A. Pesaran,
M. Keyser. Presented at the 16th Annual Battery Conference; Long
Beach California, January 2001
16
NREL’s Large Volume Battery Calorimeter
• Single chamber, conduction, isothermal
• Includes several patent-pending
concepts
• Test chamber submerged
• Capability to test liquid-cooled batteries
• Safety features in case of events
• Test chamber 6 times larger than the
NREL module calorimeter
o 2 ft x 2 ft x 4 ft
• Heat Rate: 0.05 W to 4 kW
• Accuracy of heat meas. ±3%
Test Chamber in Isothermal Bath
Test Chamber
Flux Gauges in Test Chamber
Completed System with Heating/Cooling Unit
Photo Credits: Dennis Schroder & Ahmad Pesaran, NREL
17
NREL’s New Isothermal Cell Calorimeter
• Single chamber, conduction,
isothermal
• Test chamber submerged
under isothermal bath
• Testing chamber: 15 cm W x
10 cm L x 6 cm H
• Heat detection limit: 1 mW
and 10 J
• Initial testing shows excellent
baseline stability and an error
of less than ±1.6%
• CRADA and license agreement
signed with NETZSCH to
commercialize NREL’s battery
calorimeter design
http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/50558.pdf
Photo Credits: Dennis Schroder & Dirk Long, NREL
18
• Quickly finds thermal signature of the whole
cell under electrical loads
• Helps understand thermal behavior, creates
diagnostics, and improves designs
• Could be used as a validation of thermal
models
• Thermal signature depends on several factors
o Geometry, thermal conductivity of case and core,
location of terminals, design of interconnects,
current density, current profile, chemistry,
environment
• We spray a thin layer of boron nitride on all the
surfaces of the face that needs to be imaged
• We minimize reflections from other objects by
placing the cells in a non-reflective
environment
• We usually test three cells to see the impact of
power cable connected to the two end cells
Infrared Thermal Imaging
Thermal image of a 6.5-Ah NiMH module from
a MY 2002 Prius under 100A CC discharge
Photo Credits: Matt Keyser, NREL
19
Examples of Battery Infrared Thermal Imaging
25°C 45°C
http://www.nrel.gov/vehiclesandfuels/energystorage/publications.html
Photo Credits: Matt Keyser & Dirk Long, NREL
20
Thermal Conductivity Estimation & Measurement
Provided by Peter Ralbovsky - Netzsch Instruments
Measurement Techniques
• The core material (electrochemically active
par
t) is assumed to consist of a homogenous
material with average properties for resistivity
and thermal conductivity, but with different
properties in different directions
(orthotropic xyz or rθZ)
• Usually case and core of a cell
are considered two different
regions with different thermal
conductivity
Can use finite
element analysis
to calculate the
effective thermal
conductivity in
each direction
k
x
= q *∆x /∆T
k
y
= q *∆y /∆T
or
k
z
= q *∆z /∆T
k
r
= q *∆r /∆T
21
Measuring Thermal Conductivity of LIB Components
Flash Diffusivity Method:
• Thermal diffusivity (α) is a measure of how quickly a material can
change its temperature when heat is applied
• The temperature rise on the rear surface is measured in time using an
infrared detector
( ) ( ) ( ) ( )
TTcTTK
p
ρα
⋅⋅=
Provided by Peter Ralbovsky, Netzsch Instruments
• Measurements have shown that generally the thermal conductivity of LIB is much
lower in-plane than cross-plane
Cross plane ~ 0.8 to 1.1 W/m/K In plane ~ 28 to 35 W/m/K
Netzsch LFA 447 Unit
Photo Credit: John Ireland, NREL
22
Battery Thermal Testing Loops
• Measuring heat transfer coefficients or conductance
• Hardware in the loop thermal testing
Temp dist. in a USABC module
Photo Credits: Ahmad Pesaran
Photo Credits: Kandler Smith, NREL
23
Battery Thermal Responses
3D
Component
Analysis
System
Analysis
Cell Characteristics
• Shape and size : Prismatic/Cylinder/Oval, etc.
• Materials/Chemistries
• Voltage/current & heat gen data
• Thermal/Current Paths inside a Cell
Module Cooling Strategy
• Coolant Type: Air/Liquid
• Direct Contact/Jacket Cooling
• Serial/Parallel Cooling
• Terminal/Side Cooling
• Module Shape/Dimensions
• Coolant Path inside a Module
• Coolant Flow Rate
• Passive with phase change
• etc.
• Temperature History Cells/Module/Pack
• Temperature Distribution in a Cell
• Cell-to-Cell Temperature Imbalance in a Module
• Battery Performance Prediction
• Pressure Prop and Parasitic Power
• etc.
• Vehicle Driving Cycles
• Control Strategy
• Ambient Temperature
• etc.
Operating Conditions
Design Process
Process for Battery Thermal Modeling
24
Lumped Capacitance Thermal Model for
Vehicle Simulations
• For vehicle simulation, the thermal model needs to be linked to
the battery model for temperature dependency
• A 2-node lumped thermal model (case + homogenous core) with
simple heat convection is developed for ADVISOR vehicle
simulator
25
Example of 2-D Module Thermal Modeling
T
max
= 44°C
Delta T
core
= 9°C
Case 1. No holes and no air flow between cells
T
max
= 53°C
Delta T
core
= 13°C
Cell core
Plastic
case
Air
gap
Case 2. With holes and air flow between cells
Photo Credits: David Parsons, NREL
26
Electro-Thermal Analysis Approach
Current Density
Temperature
• Capture details of a cell including
non-electrochemical hardware with
finite element analysis
• Estimate component resistances
using geometry and materials
• Apply voltage drop to calculate
current density in components
• Estimate resistive heating (I
2
R) in
each component
• Apply electrochemical heat of
reactions in the core (active parts)
• Apply heat transfer boundary
conditions on cell exterior
• Predict temperature distribution in
the cell from current density and
related heat generation distribution
27
Example of 3-D Electro-Thermal Modeling
Design A
Terminals on
each side
Design B
Terminals on the
same side
16 Ah
Power Cell
Temp Dist Design A
Design A
Thermal Response
Design B
Thermal Response
Under 110 A RMS load
• The overall resistance of Cell
Design B is less than Cell Design A
• Under the same current profile,
Cell Design B generates less heat
and thus performs better thermally
Electro-thermal Analysis of Lithium Ion Batteries, Pesaran, A.; Vlahinos, A.; Bharathan, D. Proceedings of
the 23rd International Battery Seminar, Fort Lauderdale, Florida. March 13-16, 2006.
Photo Credit: Ahmad Pesaran
28
Combined 3D Electrochemical-Thermal Models
Comparison of two 40-Ah Li-ion
prismatic cell designs
This cell is cycled more
uniformly, can
therefore use less
active material ($) and
has longer life.
2 min 5C discharge
working potential working potential
electrochemical
current production
temperature
temperature
SOC
SOC
electrochemical
current production
High temperature
promotes faster
electrochemical reaction
Higher localized reaction
causes more heat generation
Larger over-potential
promotes faster discharge
reaction
Converging current causes
higher potential drop along
the collectors
Current Collector (Cu)
Current Collector (Al)
p
Negative
Electrode
Separator
Positive
Electrode
29
Computer-Aided Engineering of Batteries (CAEBAT)
• U.S. Department of Energy is
supporting development of
electrochemical-thermal models
and software design
• The objective is to shorten time
and reduce cost for design and
development of battery systems,
including the design and analysis
of BTMSs
• Other software design and analysis
tools dealing with other physics
may be incorporated in CAEBAT
Physics of Li-Ion Battery Systems in
Different Length Scales
Li diffusion in solid phase
Interface physics
Particle deformation &
fatigue
Structural stability
Charge balance and
transport
Electrical network in
composite electrodes
Li transport in electrolyte
phase
Electronic potential &
current distribution
Heat generation and
transfe r
Electrolyte wetting
Pressure distribution
Atomic Scale
Particle Scale
Electrode Scale
Cell Scale
System Scale
System operating
conditions
Environmental
conditions
Control strategy
Module Scale
Thermal/electrical
inte r-cell
configuration
Thermal
management
Safety control
Thermodynamic properties
Lattice stability
Material-level kinetic barrier
Transport properties
Thermal-electrochemical response of a pack
Courtesy of Christian Shaffer, EC Power-CAEBAT
30
Summary
• Battery thermal management needed for xEVs
• Battery thermal management system needs to be optimized
with right tools for lowest cost
• NREL has state-of-the art experimental and analytical tools for
analysis and design of battery thermal management systems
• Experimental tools, such as the isothermal calorimeter, are
essential for obtaining data for generating input to design
tools and eventually verifying the performance of the battery
thermal management system
• Computer-aided engineering tools for the design of battery
electrical and thermal management systems are now
accessible to automotive and battery engineers
31
Acknowledgments
• Support provided by the DOE Vehicle Technologies Program
o Dave Howell, Hybrid and Electric Systems Team Lead
o Brian Cunningham, Energy Storage Technology Manager
• Feedback from CAEBAT Subcontract Technical Leads
o Taeyoung Han (General Motors)
o Steve Hartridge (CD-adapco)
o Christian Shaffer (EC Power)
• Support from NREL Staff
o John Ireland
o Dirk Long
o Mark Mihalic
o Marissa Rusinek
nrel.gov/vehiclesandfuels/energystorage
Contact Information:
Ahmad Pesaran
ahmad.pesaran@nrel.gov
303-275-4441