Ultracapacitor Applications and 

Evaluation for Hybrid Electric Vehicles

Ahmad Pesaran

(ahmad.pesaran@nrel.gov)

Jeff Gonder, Matt Keyser

National Renewable Energy Laboratory

Presented at the 7th Annual Advanced Capacitor World Summit Conference

NREL/PR-540-45596

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.
2

Discussion Points

• Discussion of batteries vs. ultracapacitors for 

advanced vehicles 

• Simulation results of HEV fuel economy impact 

from reducing the storage system’s energy 

window 

• 15%-30% HEV fuel economy improvements with 

50-100 Wh ultracapacitors 

• Evaluation of lithium ion capacitors for HEV 

applications 

• Thermal evaluation of a high-voltage 

ultracapacitor module for start-stop applications 
3

Strong Attributes of Ultracapacitors Potential Specific Use

High specific power and efficiency Engine assist

Efficient and fast charge acceptance Regen capture

Low resistance Lower cooling needs (less expensive)

Quick response (short time constant) Supporting engine transients

Long anticipated calendar and cycle life Fewer replacements (less expensive)

High specific power at low temperatures (cold starts) Smaller size and less expensive

Weak Attributes of Ultracapacitors Specific Use

Low specific energy Limited “durations” for power draw

High self-discharge Loss of functionality and balance at start 

Quick voltage variation More difficult to control

Low energy density Limited time for running auxiliaries at idle

High cost per unit energy Too expensive currently

The best use for Ucaps are strategies that make engines operate more efficiently 

(idle off, load leveling), frequent use capturing regen energy, and start-stop.

Strengths and Weaknesses of Ultracapacitors
4

A Couple of Thoughts

• Taking advantage of an ultracapacitor’s strengths 

while minimizing the impact of its weaknesses to 

make its “value” competitive with batteries

• It should be for a specific application to show 

“value” in terms of “life-cycle cost”

— Fuel economy

— Replacement cost

— Life

— Durability and reliability

— Quality

— Functionality 
5

Event How Much 

Energy Needed

Assist:20/30 kW constant power for 15/10 s 83.3 Wh

Accessory: 3 kW constant draw for 1 minute 50

Accessory: 1 kW constant draw for 1 minute 16.7

2% Grade going 35 mph for 1 minute Ŧ 70 Wh

4% Grade going 35 mph for 1 minute Ŧ 170 Wh 

US06 Driving Cycle * 155 Wh

UDDS Driving Cycle * 80 Wh

Ucap Is Energy Limited!

How Much Energy Is Needed for Various Events?

Prius has a 1.4 kWh NiMH battery but capacity is for life margin and warranty.

Vue mild hybrid has a 0.6 kWh NiMH battery.

•Total Energy (at wheels) calculated for 1520 kg vehicle (regen); 50% of energy in 

the cycle’s largest deceleration event

Cold-start capability is expected to dictate the size of batteries, but not the case for Ucap.

Ŧ Note: Engine provides propulsion up a grade, the estimate is for capturing regen 

to hold a 1520 kg vehicle speed going down a grade.
6

Potential Use of Ultracapacitors in Light-Duty 

Electric-Drive Vehicles

NiMH and Li-ion: Yes

Ucap: Likely

Ucap + VRLA: Possible

Micro Hybrids (12 V-42 V: 

Start-Stop, Launch Assist)

Li-ion: Yes

Ucaps + high energy Li-ion : Possible 

Plug-in HEV (EV)

NiMH and Li-ion: Yes 

Ucaps: Likely if Fuel Cell is not downsized

Ucaps + (NiMH or Li-Ion): Possible

Fuel Cell Hybrids

NiMH and Li-ion: Yes

Ucaps: Possible 

Ucaps + (NiMH or Li-Ion): Possible

Full Hybrids (150 V-350 V: 

Power Assist HEV)

NiMH and Li-ion: Yes

Ucaps: Likely if engine is not downsized

Ucaps + VRLA: Possible

Mild Hybrids (42 V-150 V: 

Micro HEV Function + 

Regen)

15-25 Wh

25-70 Wh

60-150 Wh

60-150 Wh

5-20 kWh

(50-90 Wh*)

Min energy 

needed

* Energy for a Ucap in combination withLi-Ion 
7

Analyzing the Impact of Energy 

Window on Power-Assist HEVs 

• Motivation: Investigate the relation between in-use energy window 

and fuel economy (a request from USABC/FreedomCAR)

• Approach: Simulate a midsize sedan with different component 

power levels and control settings for different drive cycles using 

PSAT 

Midsize Car 

Assumptions

Mass = 1675 kg

Engine = 90 kW

RESS/Motor = 30 kW

Elec accessories = 500 W

Mech accessories = 230 W

FA = 2.27 m2

CD = 0.30

Crr1 = 0.008

Crr2 = 0.00012

Simulated different ES energy content cases 

with the otherwise constant platform values

Constant 30 kW power  changing P/E ratio

Smallest ES 

energy

Largest ES 

energy

Wh

Upper threshold

Target level

Lower threshold

Constant SOC-based controls (charge sustaining)

Changing Wh control window tolerance

Source: J. Gonder, Presentation to USABC, July 19, 2007
8

Definition of ES Energy Window Use 

(for a drive cycle or event) 

Energy out for electric 

launch/assist

Cumulative RESS Wh 

to vehicle

RESS use indicated by slope of energy line

Energy return from 

charging/regen

“Energy window” defined 

by (max – min) for the 

particular cycle

(not “target window” from 

control strategy)

Charge sustaining 

over cycle

(no net energy use)

Energy Window Used ≤ Available Energy

Source: J. Gonder, Presentation to USABC, July 19, 2007
9

US06 Cycle

0 500 1000 1500 -20

0

20

40

60

80

time (s)

Vehicle speed (mph)

Cycle grade (%)

• Mean power during:

Propulsion = 21 kW

Deceleration = -17 kW

• No grade

“NREL to 

Genesee Cycle”

Aggressive driving

Up and down, 

foothills driving

0 200 400 600 0

20

40

60

80

100

time

Vehicle speed (mph)

time (s)

• Mean power during:

Propulsion = 23 kW

Deceleration = -12 kW

• Considerable grade

0 500 1000 1500 0

20

40

60

time (s)

Vehicle Speed (mph)

UDDS Cycle

Mild urban driving

• Mean power during:

Propulsion = 7 kW

Deceleration = -5 kW

• No grade

 

 p 

Constant 30 kW power  changing P/E ratio

Smallest ES 

energy

Largest ES 

energy

 

 

 

 p 

Constant 30 kW power  changing P/E ratio

Smallest ES 

energy

Largest ES 

energy

 

 

Source: J. Gonder, Presentation to USABC, July 19, 2007

Three Cycles Simulated to Observe 

Energy Window and Fuel Use
10

On City Cycle (UDDS), Large Fuel Savings 

Result from Hybridization 

6.50

7.50

8.50

9.50

10.50

11.50

12.50

0 100 200 300 400

ESS Energy Window (Wh)

Fuel Consumption (L/100 km)

(mpg reference)

18.8

20.5

22.4

24.8

27.7

31.4

36.2

Conventional 

Vehicle

Smallest 

RESS case

Largest 

RESS case

21% decrease

18% decrease

RESS Energy Window (Wh)

Source: J. Gonder, Presentation to USABC, July 19, 2007
11

6.50

7.50

8.50

9.50

10.50

11.50

12.50

0 50 100 150 200 250 300 350 400

ESS Energy Window (Wh)

Fuel Consumption (L/100 km)

(mpg reference)

18.8

20.5

22.4

24.8

27.7

31.4

36.2

Summary Results of ES Energy Window and 

Fuel Economy Simulations

UDDS Cycle

US06 Cycle

Foothills Driving

Same Vehicle 

(largest RESS)

Same Vehicle 

(smallest RESS)

Same Vehicle 

(conventional)

RESS Energy Window (Wh)

Source: J. Gonder, Presentation to USABC, July 19, 2007
12

3

4

5

6

7

8

9

10

11

0 100 200 300 400 500 600 700

Energy Window (Wh)

Fuel Consumption (L/100km)

Fuel Economy (mpg)

58.81

47.0

39.20

33.60

29.40

26.14

23.52

21.3

78.41

Charge Sustaining

Not Charge Sustaining

UDDS

US 06

HWFT

MT

SS Speeds

Prius Camry Escape Accord

All the charge sustaining (CS) 

tests use windows <200 Wh

(for these vehicles and CS cycles)

Test data analysis seems to validate 

simulation finding of significant hybridization 

benefit in the 50-150 Wh range 

Vehicle Test Results: Battery Energy Use for 

Today’s HEVs under Various Drive Cycles

Prius and Escape Test Data: Tony Markel, NREL

Camry and Accord Test Data: Mike Duoba, ANL

Test Data Analysis: Jaehun Rhee and Jeff Gonder, NREL

Source: J. Gonder, Presentation to USABC, 

July 19, 2007
13

2007 Mild Hybrid Dyno Data* Analysis 

Indicates <50 Wh Energy Use for Typical 

Driving—Already Reasonable Ucap Range

Driving Energy Analysis (UDDS cycle example)

Energy window

* Department of Energy-sponsored dynamometer testing
14

Mild and Power-Assist Hybrids with Ucaps

• It is possible to use ultracapacitors (with available 

energy of 50-150 Wh) in power-assist HEVs with 

modest fuel economy improvements

— However, acceleration and passing on grade 

performance considerations could be limiting factors

• 15%-30% HEV fuel economy improvements with 

50-100 Wh ultracapacitors 

• A project is underway on a vehicle to demonstrate 

Ucaps in mild hybrids 

— To be discussed in future meetings
15

 

 

0

2

4

6

8

10

12

14

16

18

20

0 100 200 300 400 500 600 time (s)

Voltage (V)

Hybrid Pack – UC Voltage Battery-only – Voltage

Previous NREL Tests Have Shown That Combining 

Ultracapacitors Filters High Current Transients In Batteries

Source: M. Zolot (NREL Reports and 2003 Florida Capacitor Seminar)

Ultracapacitor module of 8 cells (up to 20V) and two 6.5Ah NiMH module of 14.4V 

(18V max). Ultracap module and battery pack were arranged in parallel to share the 

current load depending on internal impedance.

0

5

10

15

20

25

30

35

40

45

-40 to -

110

-30 -20 -10 0 10 20 30 40 to

100

Current (A)

g g g 

% Freq Hyb_NiMH % Freq Bat_NiM

Discharge Charge

Battery-only - Battery Current

Hybrid Pack - Battery Current

• Overall, batteries in the hybrid pack 

experienced no currents larger than 

±40 A, while the batteries in traditional 

pack saw currents up to ±110 A.

• Up to 33% narrower battery SOC 

cycling range was observed in hybrid 

pack; this has the potential to increase 

battery life.

Parallel connection; no DC/DC converter

May not be practical to implement in vehicles.
16

Advantages/Disadvantages of

Hybridizing Energy Storage (Ucap + Battery)

Advantages

• Reduced battery currents

• Reduced battery cycling range

• Increased battery cycle/calendar life (to 

what extent?)

• Increased combined power and energy 

capabilities

• Lower cooling requirements

• Better low-temperature performance

Disadvantages

• Complex control strategy

• Larger volume & mass

• Need for electronics for each system

• Increased energy storage cost

• Unknown side effects if directly coupled

• Any need for DC/DC converters adds 

even more cost and complexity

+

Ucap

Module

+

Battery

Module

DC

DC DC

EM +

14V

Energy Storage Subsystem

EE300-20

AC

Power

DC

DC

UUCap > UBatt

DC

DC

+ UUcap > UBatt

Ucap

Module

+

Battery

Module

DC

DC DC

EM +

14V

Energy Storage Subsystem

EE300-20

AC

Power

DC

DC

UUCap > UBatt

Power

DC

DC

UUCap > UBatt

DC

DC

UUcap > UBatt

Source: Continental ISAD, “New Energy 

Storage Concept,” Proceedings of AABC-04
17

• JSR Micro contacted us to express interest in thermal 

characterization of their asymmetric capacitor

• JSR Micro claimed higher energy than C-C Ucaps with the 

same power capability

• We received 3 cells for characterization per USABC 

protocols

Thermal/Electrical Characterization of

JSR Micro Lithium Ion Capacitor (LIC)

Source: www.jmenergy.co.jp/en/product.html
18

JSR Micro LIC Cell Characteristics

Cell 1 0.205 2.669 5.5" x 4" x 0.330" 1.58

Cell 2 0.205 2.669 5.5" x 4" x 0.330" 1.62

Cell 3 0.205 2.672 5.5" x 4" x 0.330" 1.6

Impedance (mOhms)

Cell Number 

(#) Mass (kg)

Voltage 

(Volts)

Dimensions 

(inches)

Nominal 2200 F

14 Wh/kg

3.8 V – 2.2 V Source: www.jmenergy.co.jp/en/product.html
19

Infrared Thermal Imaging

+ -

+

Cell #1 Cell #2 Cell #3

-

- -

+

+

+

Temperatures: Ambient

Profiles: 50C, 100C, and Geometric Cycle
20

Thermal Image and Thermal Lines of 3 

LIC Cells – 100 A Discharge
21

Thermal Characterization in NREL Calorimeter 

Lithium Ion Capacitor 2200 F Cells 

•Temperatures: +30 ̊C

• Profiles: CC discharge cycles

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (Hours)

Calorimeter Response to Constant Current Charge/Discharge Heat Generation

Discharge - Exothermic

Charge - Endothermic

Increasing Discharge Current

Increasing Charge Current
22

Electrical Characterization: 

Lithium Ion Capacitor Cells

• C/1, 10 C, 100 C, and HPPC Testing

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

0 10 20 30 40 50 60 70 80 90 100

DOD (%)

OCV (Volts)

Energy: 14 Wh/kg 

Power: 1500 W/kg 

This asymmetric capacitor had high resistance; 

the next generation is claimed to be better.

2200 F cell 0 20 40 60 80 100 120

Depth of Discharge (%)

Discharge Regen

Power

HPPC Discharge/Regen Power

0.7

0.75

0.8

0.85

0.9

0.95

1

0 20 40 60 80 100 120

Current (Amps)

Discharge Capacity (Ah)

Cell #1 Cell #2 Cell #3
23

Expected Calendar Life of Typical Current EDLC Technology

Much Better Than Batteries if Stored at Low Voltages

70 C

60 C

50 C

40 C

30 C

Clamped Voltage 

Expected Calendar Life (Years)

Source: Anderman, 2004 Advanced 

Automotive Battery Conference
24

Thermal Evaluation: 

High-Voltage Ultracap Module

• Tested as part of USABC deliverable

• Eighteen (18) symmetric carbon-carbon ultracapacitors

• Tested under realistic conditions and operation

• Used different power profiles and chamber temperatures

5

0

5

0

5

0

0 1 2 3 4 5 6

Time (Hours)

 

 

Terminal Temperatures after One Hour Geometric Cycling

5 6 13 14

4 7 12 15

3 8 11 16

2 9 10 17

1 18

Balancing Board

5 6 13 14

4 7 12 15

3 8 11 16

2 9 10 17

1 18

Balancing Board

Balancing Board Affecting Terminal Temps

Heat from cells is conducted through 

the ends to the case and rejected 

through the top metal heat sink/fins.
25

Thermal Evaluation: 

High-Voltage Ultracap Module

• Continuous US06 cycling for two hours 

• Balancing board did a good job equalizing cells

• Energy drain for balancing could be a concern

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10 12 14 16 18 20

Time (Hours)

Balance Board Temperature (C)_

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Max to Min Cell Voltage (Volts)_

Balance Board Temp

Max to Min Voltage Difference

29

30

31

32

33

34

35

36

0 1 2 3 4 5 6

Time (Hours)

Terminal Temperature (C)

Cell Terminal #1 Cell Terminal #3 Cell Terminal #5 Cell Terminal #7 Cell Terminal #9

Cell Terminal #11 Cell Terminal #13 Cell Terminal #14 Cell Terminal #16 Cell Terminal #18

Cell #1

Interior Cells

Exterior Cells

(next to 

balancing 

board)

Temperature difference less than 1.5°C 

except for Cell #1 which heated due to 

balancing board.
26

Concluding Remarks

• Ultracapacitors provide opportunity for modest fuel savings in 

hybrid cars

— Idle-off: 5%-10% FE improvement and most likely to be implemented

— Mild and full hybrid: 15%-25% FE improvement, possible

— Plug-in hybrids: possible Ucap combined with batteries; cost??

• Competition from Li-ion is strong; ultracapacitors should 

provide “added value” to compete

— Low-temp performance

— Longer cycle and calendar life

• Asymmetric capacitors such as lithium ion capacitors have 

potential if power and cost are improved

• Thermal issues are important and must be taken into account to 

achieve the desired performance and life

• Lower cost is the key for increased market growth in automotive

• Micro and mild hybrids provide biggest opportunity for Ucaps in 

the short term; will be accelerated by new CAFÉ mandates
27

Acknowledgements

• Support provided by FreedomCAR and Fuel Partnership in 

the Vehicle Technologies Program of the U.S. Department 

of Energy 

— David Howell, Energy Storage Technology Manager

• Technical insight and support

— Harshad Tataria, USABC/GM

— Jim Banas, JSR Micro Inc. 

nrel.gov/vehiclesandfuels/energystorage/publications.html