Electric Vehicle Battery Thermal 

Issues and Thermal Management 

Techniques

John P. Rugh, NREL

Ahmad Pesaran, NREL

Kandler Smith, NREL

NREL/PR-5400-52818

Presented at the 

SAE 2011 Alternative Refrigerant and System Efficiency Symposium 

September 27-29, 2011 

Scottsdale, Arizona USA
2 

Outline

• Introduction

• Importance of battery temperature

• Review of electric drive vehicle (EDV) battery 

thermal management options

• Techniques to improve battery life

– Standby thermal management

– Preconditioning

• Tradeoff with thermal comfort

• Summary
3 

Battery is The Critical Technology for EDVs

 Enables hybridization and electrification 

 Provides power to motor for acceleration

 Provides energy for electric range and other auxiliaries

 Helps downsizing or eliminating the engine

 Enables regenerative braking

× Adds cost, weight, and volume

× Could decrease reliability and durability

× Decreased performance with aging

× Raises safety concerns 

3 

Lithium-ion battery cells, module, and battery pack for 

the Mitsubishi iMiEV (Courtesy of Mitsubishi)
4 

As The Size of The Engine Is Reduced, The 

Battery Size Increases

Size of Electric Motor (and associated energy storage system)

Size of Fueled Engine

Conventional internal combustion engine (ICE) vehicles

Electric vehicles (EVs) (battery or fuel cell)

Micro hybrids (start/stop)

Mild hybrids (start/stop + kinetic energy recovery)

Full hybrids (medium hybrid capabilities + 

electric launch)

Plug-in hybrids (full hybrid capabilities + 

electric range)

Medium hybrids (mild hybrid + engine assist )

Axes not to scale
5 

Battery Requirements for Different EDVs

Vehicle Power (kW) Energy (kW/h) Cycles

Micro and Mild 

Hybrid Electric 

Vehicles (HEVs)

Very high power Low energy Many (400K) shallow 

charge/discharge cycles 

(±5% change)

Medium and 

Full HEVs 

High power Moderate energy Many (300K) shallow 

charge/discharge cycles 

(±10% change)

Plug-in HEVs 

(PHEVs)

High power High energy Many (200K) shallow 

charge/discharge cycles 

(±5% change) 

Many (3-5K)deep discharge 

cycles (50% change)

Battery EVs Moderate 

power

Very high energy Many (3-5K) deep 

discharges (70% change)

Calendar life of 10+ years Safety: the same as ICE vehicles
6 

Lithium ion technology comes close to meeting most of the required technical and 

cost targets in the next 10 years.

Energy and Power by Battery Type

www1.eere.energy.gov/vehiclesandfuels/facts/2010_fotw609.html
7 

Battery Cycle Life Depends on State-of-Charge Swing 

4,000 

50% 

70% 

• PHEV battery likely to deep-cycle each day driven: 15 yrs equates to 4,000–5,000 deep cycles

Source: Christian Rosenkranz (Johnson Controls) at EVS 20, Long Beach, CA, November 15-19, 2003 

Potential

Potential

Potential

Potential
8 

Outline

• Introduction

• Importance of battery temperature

• Review of EDV battery thermal management 

options

• Techniques to improve battery life

– Standby thermal management

– Preconditioning

• Tradeoff with thermal comfort

• Summary
9 

Phoenix

44oC max, 24oC avg

0oC

Houston

39oC max, 20oC avg

Minneapolis

37oC max, 8oC avg

10oC

20oC

30oC

Impact of Geography and Temperature on 

Battery Life

Li-ion technology 

must be sized 

with significant 

excess power to 

last 15 years in 

hot climates
10

Li-Ion Battery Resistance Increases with 

Decreasing Temperature

• Power decreases 

with decrease in 

temperature

• Impacts power 

capability of 

motor and 

vehicle 

acceleration 
11

• Useful energy from 

the battery 

decreases with 

decrease in 

temperature

• Impacts driving 

range and 

performance of 

vehicle 

Li-Ion Battery Capacity Decreases with 

Decreasing Temperature
12

Battery Temperature is Important 

Temperature affects battery:

 Operation of the electrochemical system

 Round trip efficiency 

 Charge acceptance

 Power and energy availability

 Safety and reliability

 Life and life-cycle cost

Battery temperature affects vehicle 

performance, reliability, safety, and 

life-cycle cost

http://autogreenmag.com/tag/chevroletvolt/page/2/
13

Temperature Impacts Battery Sizing & 

Life and Thus Cost

Power

Limits

15°C 35°C

discharge

charge

Rated

Power

T

Degradation Sluggish

Electrochemistry

Power

Limits

15°C 35°C

discharge

charge

Rated

Power

T

Degradation Sluggish

Electrochemistry

Dictates power capability

Also limits the electric driving 

range

Power and energy fade 

rates determine the 

original battery size 

Power limited to 

minimize T increase 

and degradation 

Kandler Smith, NREL Milestone Report, 2008

Desired 

Operating 

Temperature
14

Battery High-Temperature Summary

• Primary considerations

– Life

– Safety

– Non-uniform aging due to thermal gradients 

• Cooling typically required

– In hot environments (could be 24 hr)

– During moderate to large current demands during 

drive

– During fast charging

Photo Credit: John Rugh, NREL
15

Battery Low-Temperature Summary

• Primary considerations

– Performance

– Damage due to charging too fast

• Heating typically required

– In cold environments during charging and 

discharging

Photo Credit: Mike Simpson, NREL
16

Outline

• Introduction

• Importance of battery temperature

• Review of EDV battery thermal management 

options

• Techniques to improve battery life

– Standby thermal management

– Preconditioning

• Tradeoff with thermal comfort

• Summary
17

• Regulate pack to operate in the desired temperature 

range for optimum performance/life

 15oC – 35oC 

• Reduce uneven temperature distribution

 Less than 3oC – 4oC 

• Eliminate potential hazards related to uncontrolled 

temperatures – thermal runaway

Battery Pack Thermal Management Is Needed 
18

Battery Thermal Management System Requirements

• Compact 

• Lightweight

• Easily packaged

• Reliable 

• Serviceable

• Low-cost 

• Low parasitic power

• Optimum temperature 

range 

• Small temperature 

variation

http://www.toyota.com/esq/articles/2010/Lithium_Ion_Battery.html
19

Outside Air Battery Pack Exhaust 

Fan

Battery Pack

Cabin Air

Exhaust 

Fan

Outside Air

Vehicle Return 

heater and 

evaporator 

cores 

Battery Pack Exhaust 

Fan

Outside Air

Return Auxiliary or vehicle

heater and evaporator 

cores 

Outside Air Ventilation

Cabin Air Ventilation

Heating/cooling of Air to Battery – Outside or Cabin Air 

Prius & Insight

Thermal Control Using Air

i-MiEV (fast charge)
20

Battery Heating and Cooling Using Air

Pro Con

All waste heat eventually has to go to 

air

Low heat transport capacity

Separate cooling loop not required More temperature variation in pack

Low mass of air and distribution system Connected to cabin temperature control

No leakage concern Potential of venting battery gas into 

cabin

No electrical short due to fluid concern High blower power

Simple design Blower noise

Lower cost

Easier maintenance
21

Thermal Control Using Liquid

Liquid

Pump 

Liquid/air heat 

exchanger

Fan

Outside Air Exhaust 

Battery Pack

Pump

Liquid/air heat 

exchanger

Fan

Outside air Exhaust

Battery Pack

Liquid direct -contact or jacketed

Refrigerant

Liquid

Pump

Liquid/liquid heat 

exchanger or electric 

heater

Pump

Vehicle engine

coolant Return

Battery Pack

A/C heat 

exchanger

Liquid direct-contact or jacketed

Ambient

cooling

Active dedicated 

cooling/heating Volt, Tesla
22

Battery Heating and Cooling Using Liquid

Pro Con

Pack temperature is more uniform - 

thermally stable

Additional components

Good heat transport capacity Weight

Better thermal control Liquid conductivity – electrical isolation

Lower pumping power Leakage potential

Lower volume, compact design Higher maintenance

Higher viscosity at cold temperatures

Higher cost
23

Outline

• Introduction

• Importance of battery temperature

• Review of EDV battery thermal management 

options

• Techniques to improve battery life

– Standby thermal management

– Preconditioning

• Tradeoff with thermal comfort

• Summary
24

Standby Thermal Cooling in Hot Climates

• Battery life can greatly benefit from cooling the battery during standby, 

i.e., while vehicle is plugged in to the grid

• Slower battery degradation rate enables smaller, lower cost battery

• NREL study investigated 

– Insulation

– Insulation and air cooling

– Insulation and small vapor compression system (VC)

– Insulation, small VC system, and phase change material (PCM)
25

Phoenix Houston Minneapolis Phoenix Houston Minneapolis

Saft HP-12LC Cell

(Belt/INL, ECS Mtg. 2008)

• low fade rate, high cost

DOE/TLVT Cell

(Christopersen/INL, Battaglia/LBL, 2007 Merit Review)

• moderate fade rate, lower cost

5%-10% less power 

fade with Ins. + VC

9%-22% less power 

fade with Ins. + VC

Lower cost cell preferred, 

provided it can meet life.

Next slide compares Δcosts of 

DOE/TLVT battery sized for 15 

years in Phoenix, w/ and w/o 

insulation + VC system.

Battery Life for Various Standby Systems

can differ widely depending on cell chemistry, materials, and manufacturer
26

Savings from Downsized Battery Expected to 

Significantly Outweigh Cost of Added Components

DOE/TLVT cell 

sized for 15 years; 

in Phoenix, AZ, 

charged nightly

($360) ($320) ($250) 

PHEV10

ΔkWh

ΔkW 

VC

Fan

Insulation

ΔkWh

ΔkW 

VC

Fan

Insulation

ΔkWh

ΔkW 

VC

Fan

Insulation

PHEV20 PHEV40

Total 

Savings ($)

Total savings assuming components 

represent additional cost

PHEV10 PHEV20 PHEV40
27

Standby Thermal Management – Passive 

Techniques to Reduce Battery Temperatures

• Installed metalized solar reflective film on the glazings 

of a Toyota Prius in Phoenix

• Cabin air temperature reduced ~6o

C 

• Before: Battery daily max temp 1.5o

C above ambient

• After: Battery daily max temp 2o

C below ambient

Photo Credit: John Rugh, NREL
28

Thermal Preconditioning

Issues: 

• For conventional vehicle and HEV platforms, A/C use leads 

to increased fuel consumption

• For PHEV and EV platforms, climate control energy is 

supplied by the traction battery

 

 Charge depletion (CD) range reduction 

• Batteries degrade rapidly at high temperatures and benefit 

from active cooling

• Batteries suffer from reduced power and energy at cold 

temperatures; their performance can be improved by 

preheating

 

 Battery wear and life impacts 

Potential Solution: 

• Use grid power to thermally precondition cabin and battery 

• Save valuable onboard stored energy for propulsion
29

Preconditioning, Driving & Charging Patterns 

Affect Battery Temperature and Duty-Cycle

24-hour profiles created to estimate impact of preconditioning on battery life

6 am 10 am 3 pm 8 pm 1 am 6 am

Rest

20 minute preconditioning

8:00 am: 26.6 km trip

Rest

20 minute preconditioning

5:00 pm: 26.6 km trip

10:00 pm: Charge at 6.6 kW

Rest

6 am 10 am 3 pm 8 pm 1 am 6 am

PHEV40s, hwy cycle, 95°F (35°C) ambient.

Battery heat generation rates and SOC extracted from PSAT vehicle simulations of charge-depleting and charge-sustaining operation.
30

Thermal Preconditioning can Regain CD Range 

as well as Improve Thermal Comfort

*Compared to no thermal preconditioning

EDV Platform 

(Climate 

Control)

Fuel

Consumption 

Impact*

CD Range 

Impact*

PHEV15 (heat) -1.4% +19.2%

PHEV15 (AC) -0.6% +5.2%

PHEV40 (heat) -2.7% +5.7%

PHEV40 (AC) -1.5% +4.3%

EV (heat) NA +3.9%

EV (AC) NA +1.7%
31

Thermal Preconditioning Can Also Improve 

Battery Life

• Battery capacity loss over time is driven by ambient temperature

• Thermal preconditioning has a small benefit in reducing battery 

capacity loss (2%–7%), primarily by reducing pack temperature

(2%–6%) in the high ambient temperature (35o

C/95oF) scenario

 

EDV Platform 

(Climate Control)

Capacity Loss 

Reduction* 

PHEV15(A/C) +2.1%

PHEV40 (A/C) +4.1%

EV (A/C) +7.1%

*Compared to no thermal preconditioning
32

Thermal Preconditioning Considerations

• Timing

– avoid cooling or heating too early

– does the heating/cooling coincide with peak 

demand on the grid?

• Can the charge circuit provide power for 

simultaneous heating/cooling and charging?

• When not plugged in, is it worth using onboard 

stored energy for preconditioning?

– Trade stored energy (range) for battery life
33

Systems Approach - Options for Improving 

Electric Range with Climate Control 

• Incorporate thermal preconditioning strategies

• Reduced heat transfer into/out of the cabin

• Use efficient HVAC equipment

• Reduce cooling capacity or heat load

– Zonal climate control

– Focus on occupant comfort

• HVAC controls

– Eco mode (temporarily minimize energy use)

– Eliminate inefficient HVAC control practices
34

Outline

• Introduction

• Importance of battery temperature

• Review of EDV battery thermal management 

options

• Techniques to improve battery life

– Standby thermal management

– Preconditioning

• Tradeoff with thermal comfort

• Summary
35

Tradeoff of Battery Cooling with Thermal 

Comfort 

• NREL Integrated Vehicle 

Thermal Management task

• KULI thermal model

– A/C and cabin 

– Battery cooling loop

– Motor and power electronics 

cooling loop

• Nissan Leaf size EV

• Environment

– 35 oC 

– 40% RH

• 0% recirc

• US06 drive cycle

• Cooldown simulation from a 

hot soak

Source: David Howell, DOE Vehicle Technologies Annual Merit Review 
36

After 10 Minutes, the Battery Cools to Control 

Setpoint While the Cabin is Still Warm

Cabin Air

Battery Cells
37

Initially Less Than 50% of the A/C System 

Capacity is Going to the Cabin

Evaporator

Chiller
38

Outline

• Introduction

• Importance of battery temperature

• Review of EDV battery thermal management 

options

• Techniques to improve battery life

– Standby thermal management

– Preconditioning

• Tradeoff with thermal comfort

• Summary
39

Summary

• Temperature impacts the life, performance, and cost of 

batteries in HEVs, PHEVs, and EVs

• Battery life and performance are extremely sensitive to 

temperature exposure 

• Thermal management is a must for batteries

• Thermal control of PHEVs and EVs (when parked or 

driving) could be a cost-effective method to reduce 

over-sizing of battery for the beginning of life

• Future trends

– Some variation of today’s Li-ion chemistries

– Same sized packs – larger range

– Improved cell designs to solve life issues
40

Special thanks to:

David Anderson

David Howell 

Susan Rogers

Lee Slezak

U.S. Department of Energy 

Vehicle Technologies Program

For more information:

John P. Rugh

National Renewable Energy Laboratory

john.rugh@nrel.gov 

303-275-4413 

Acknowledgments, Contacts, and Team Members

NREL: 

Robb Barnitt

Laurie Ramroth