NREL is operated by the Alliance for Sustainable Energy, LLC

Fuel Economy and Performance of Mild 

Hybrids with Ultracapacitors

Simulations and Vehicle Test Results

The 5th International Symposium on

Large EC Capacitor Technology and Application (ECCAP)

Long Beach, California

June 9-10, 2009

Jeff Gonder, Ahmad Pesaran, Jason Lustbader

National Renewable Energy Laboratory (NREL)

NREL/PR-540-45835

Harshad Tataria

General Motors Corporation

Funding for vehicle conversion and testing provided by General Motors Corporation 

via a Funds-In Cooperative Research and Development Agreement (CRADA)

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.
National Renewable Energy Laboratory 2 Innovation for Our Energy Future

Presentation Outline

• Background

• Project Overview and Objectives

• Details of Project Phases

– System design

– Hardware bench-top evaluation

– Vehicle conversion

– Vehicle test results 

– Comparison with NiMH vehicle

• Summary

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National Renewable Energy Laboratory 3 Innovation for Our Energy Future

Background:

In 2007-2008, NREL performed analysis in support of USABC*/ 

DOE for revisiting the energy storage requirements for HEVs 

Simulate midsize 

HEV platform

Use a range of ESS** sizes

(different energy content cases)

Observe fuel and ESS energy usage for each case:

Energy out for electric 

launch/assist

Cumulative ESS 

Wh to vehicle

Energy return from 

charging/regen.

In-use “Energy Window” defined by 

(max – min) for the particular cycle

Charge sustaining 

over cycle

(no net energy use)

* USABC = United States Advanced Battery Consortium; DOE = U.S. Department of Energy 

** ESS = Energy Storage System

Approach:

Total 

energy

“Available”

energy

Energy

window
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Most additional savings with 

expansion out to ≈150 Wh

Background: 

Simulation results for USABC showed similar fuel consumption 

vs. energy window trends for various drive cycles

Sizeable fuel savings (≈half) with 

window ≤50 Wh

ESS Energy Window (Wh)
National Renewable Energy Laboratory 5 Innovation for Our Energy Future

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

11.00

0 100 200 300 400 500 600 700

Energy Window (Wh)

Fuel Consumption (L/100km)

Fuel Economy (mpg)

58.81

47.04

39.20

33.60

29.40

26.14

23.52

21.38

78.41

• Data analysis confirmed in-use energy window <200 Wh in all charge 

sustaining tests for these vehicles and drive cycles

Charge Sustaining (CS)

Not Charge Sustaining

UDDS

US 06

HWFT

MT

SS Speeds

Prius Camry Escape Accord

Background: 

Results consistent with production HEV dyno test data*

* Mike Duoba, ANL provided access to some of the raw dynamometer test data

** SOC = State of Charge

Larger total (“nominal”) energy in 

these vehicles’ batteries used for:

• Extreme cycle requirements (e.g., 

mountain driving)

• Achieving longer cycle life from 

reduced SOC** swings)

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8

9

10

11

5

4

3

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National Renewable Energy Laboratory 6 Innovation for Our Energy Future

• Hybridization can result in sizable fuel economy improvement even with 

a small energy window ESS

• Significant fuel savings could be achieved with a 150 Wh high power 

ESS, with fuel savings tapering off at energy windows >200 Wh

• Reasons for large total “nominal” energy in present production HEVs 

– Infrequent drive cycle use (e.g., long up/downhill grades)

– Achieving longer cycle life from reduced SOC swings

– Energy comes along with sizing for power requirements (particularly at cold 

temperatures)

• Required over-sizing to achieve cycle life and power capability 

contributes to battery cost

– Power dominates cost in HEV (high P/E ratio) batteries

• Ultracapacitors should be considered (acceptable energy, low-temp. 

performance, long cycle and calendar life and potential of lower $/kW)

Background: 

Observations from the USABC/DOE HEV energy window study
National Renewable Energy Laboratory 7 Innovation for Our Energy Future

Ultracapacitor Conversion and Vehicle Testing Project

• NREL discussed with GM the rationale of demonstrating a mild 

hybrid with Ucaps instead of batteries

– Reasonable fuel economy

– Lower long-term projected costs

– Superior cycle life

– Better cold temperature performance

• A project plan was formulated to replace batteries with Ucaps in a 

mild hybrid vehicle and evaluate its fuel economy and performance

• GM supported the project and provided funding, a vehicle, and 

technical support beginning in summer 2008

• Objective

– Evaluate use of ultracapacitors instead of batteries in a 

Saturn Vue BAS (belt alternator starter) Hybrid
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Could Ucaps provide similar fuel economy benefit?

8

Production “Mild” BAS HEV System with NiMH Batteries 

Provides Significant Fuel Economy Benefit

Conventional HEV

* Caveat: Window sticker difference does not necessarily equate to hybridization improvement.

Data from www.fueleconomy.gov (using updated EPA numbers), accessed April 23, 2009.

2009 Model 2009 Model

≈ +25% mpg*

Could Ucaps provide similar fuel economy benefit? – YES!
National Renewable Energy Laboratory 9 Innovation for Our Energy Future

Project Approach

System Design Ucap Energy Storage System Design Study

Hardware Bench-top 

Evaluation Hardware Acquisition and Bench-top Verification

Vehicle Conversion Acquiring Vehicle and Integration of Ucap System 

into Vehicle

Vehicle Test Results & 

NiMH Comparison

Baseline Testing; Ucap System In-Vehicle 

Performance Testing; Modeling; Trade-Off Analysis 

of Different System Designs

Project Phase Related Activities
National Renewable Energy Laboratory 10 Innovation for Our Energy Future

Analysis of Dyno Data* on a 2007 Vue Hybrid Indicated 

Energy Use ≈50 Wh or Less

Driving Energy Analysis (UDDS cycle example)

Energy window

* From the aforementioned DOE-sponsored testing at ANL
National Renewable Energy Laboratory 11 Innovation for Our Energy Future

• Direct NiMH replacement

– No additional DC/DC converter (surrounding components rated ≈25-48 V)

– Ability to test single and two (in parallel) module configurations

– Paired with a spare Energy Storage Control Module (ESCM) – stock NiMH 

remains in vehicle; can toggle between it and the Ucaps

• Vehicle interface via bypass Rapid Control Prototyping (RCP)

– Custom Ucap state estimator bypasses code in ECU for stock NiMH

System Design: Selected off-the-shelf Maxwell 48 V, 

165 F modules (each ≈35 Wh usable)

* Electronics, mounting brackets, etc. excluded from volume, but included in this mass comparison.

NiMH*: 15.4 L, 24.7 kg

Ucap*:

11.2 L, 14.8 kg
National Renewable Energy Laboratory 12 Innovation for Our Energy Future

• Confirmed electrical performance

– Detailed characterization testing on first module (capacity, voltage)

• Characterized thermal behavior of the passively cooled module

• Obtained data set for vehicle Ucap state estimator validation

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Performed Ultracapacitor Bench-top Evaluation
National Renewable Energy Laboratory 13 Innovation for Our Energy Future

Ucap Module Testing and Instrumentation

Cooling mostly 

by heat 

conduction to 

ambient 

• Equipment

– ABC-1000: 

420 V, 1000 A, 125 kW

– Environmental Chamber: 

-45°C – 190°C, 64 ft3

– Independent DAQ system: 

National Instruments

• Instrumentation

• K-type thermocouples

• Voltage on every cell (fused)

• Tests

• Voltage range chosen for 

application: 24 V – 47 V

• Multiple cycles and 

temperatures evaluated

• Based on FreedomCAR 

Ultracapacitor Test Manual
National Renewable Energy Laboratory 14 Innovation for Our Energy Future

Module Electrical Characterization:

Performed as expected

 

Module Capacity [Ah] Capacity [Wh]

1 1.047 ± 0.005 37.2 ± 0.2

2 1.042 ± 0.005 37.3 ± 0.2

3 1.035 ± 0.005 36.7 ± 0.2

• Break-in cycling did not have a 

measurable effect over the first 615 

cycles

• Capacity was stable at 1.045 Ah 

from 24 V–47 V for the first two 

modules (module 3 was slightly 

lower)

• ESR of 6.1 mΩ ± 0.4 mΩ measured 

at 25°C on a 100 A pulse

• Good cold temperature performance 

measured

• Cell voltage range stayed under 0.1 

V during US06 bench top cycle

• Also confirmed stable replacement 

NiMH module performance at the 

rated capacity

24 V – 47 V
National Renewable Energy Laboratory 15 Innovation for Our Energy Future

Cycle Start

100A Square 

Wave Cycle:

Aggressive 

upper bound

US06 Bench Cycle:

Anticipated usage

Center Cell

(Max temp location)

Terminal Cells

(Min temp location)

Temperature Performance Summary (25 C ambient)

No heating problems anticipated in application
National Renewable Energy Laboratory 16 Innovation for Our Energy Future

• Controls for Ucap state estimation, safety, etc. 

implemented via rapid control prototyping 

(RCP) with dSpace MicroAutoBox (MABx)

• Pertinent instrumentation, new NiMH battery 

and Ucap system all installed

• Electronic control unit (ECU) calibration 

adjustments and in-vehicle data acquisition 

via ETAS hardware/INCA software

Integration of Ucap System into the Vue Hybrid

* Support from Jim Yurgil (GM) greatly appreciated
National Renewable Energy Laboratory 17 Innovation for Our Energy Future

In-Vehicle Testing: Repeated for both baseline NiMH 

case and Ucap case(s) with adjusted calibrations

• On-road

– Shakedown testing and

calibration setting

• Ambient (24°C) dyno tests

– City (FTP) cycle

– Highway (HFET) cycle

– US06 cycle

• Very cold (-20°C) dyno tests

– City (-20°C FTP) cycle

• Acceleration comparison

– 0-60 mph

– 40-60 mph

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On-road Shakedown Testing and Calibration Setting

Good performance achieved

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Speed 

(kph)

Volts 

(V)

BSE R 

(ohms)

BSE C 

(F)

Volt range: 38 - 47 V

(18 Wh for this 1Ucap config.)

Speed 

(kph)

BSE 

Resistance 

(ohms)

BSE 

Capacitance 

(F)

1Ucap Configuration Over Repeated Test Loop
National Renewable Energy Laboratory 19 Innovation for Our Energy Future

In-Vehicle Ucap Temperature and Cell Voltage 

Performance Consistent with Bench Observations

Volts 

(V)

Temp 

( C)

Primary Ucap Cell 

Voltages (V)

Secondary Ucap Cell Voltages (V)

Secondary Ucap Thermocouple Probes ( C)

Primary Ucap Thermocouple Probes ( C)

1Ucap Configuration Over Same Repeated Test Loop
National Renewable Energy Laboratory 20 Innovation for Our Energy Future

NiMH vs. Ucap In-Vehicle Power Output

Shown for second (hot start) UDDS in FTP-75 test

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0 200 400 600 800 1000 1200 1400

-20

0

20

40

60

80

100

Time (s)

Speed (mph), Power (kW) and SOC

Speed

ESS Power

SOC

0 500 1000 1500

-20

0

20

40

60

80

100

Time (s)

Speed (mph), Power (kW) and SOC

Speed

ESS Power

SOC

NiMH Configuration 1Ucap Configuration

35 Wh 

System

Provided same 

in-vehicle mpg
National Renewable Energy Laboratory 21 Innovation for Our Energy Future

NiMH vs. Ucap Voltage and Cumulative Energy Comparison

Shown for second (hot start) UDDS in FTP-75 test

0 200 400 600 800 1000 1200 1400

-40

-20

0

20

40

60

80

Time (s)

Speed (mph), ESS Volts (V) and Energy (Wh)

Speed

ESS Volts

ESS Cumulative Wh Profile

0 500 1000 1500

-40

-20

0

20

40

60

80

Time (s)

Speed (mph), ESS Volts (V) and Energy (Wh)

Speed

ESS Volts

ESS Cumulative Wh Profile

NiMH Configuration 1Ucap Configuration
National Renewable Energy Laboratory 22 Innovation for Our Energy Future

Voltage Histogram Comparison

Shown for second (hot start) UDDS in FTP-75 test

25 30 35 40 45 50

0

5

10

15

20

25

30

35

40

45

50

Voltage (V)

Fraction of Time (%)

25 30 35 40 45 50

0

5

10

15

20

25

30

35

40

45

50

Voltage (V)

Fraction of Time (%)

NiMH Configuration 1Ucap Configuration

45 V = 2.50 V/cell

47 V = 2.61 V/cell
National Renewable Energy Laboratory 23 Innovation for Our Energy Future

Dyno Testing Comparison for All Three Configurations, 

FTP Drive Cycle (24 C ambient)
National Renewable Energy Laboratory 24 Innovation for Our Energy Future

Dyno Testing Comparison for All Three Configurations 

Highway and US06 Drive Cycles (24 C ambient)
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Very Cold Dyno Testing Comparison

Lowered temperature calibrations enabled a difference in operation

25

1st UDDS

(“cold” start)

2nd UDDS

(“hot” start)

Combined

Caveat: Did not test NiMH with lowered temperature 

calibrations (may obtain same result)
National Renewable Energy Laboratory 26 Innovation for Our Energy Future

Acceleration Performance Comparison:

No difference between NiMH and Ucap configurations
National Renewable Energy Laboratory 27 Innovation for Our Energy Future

Summary

• BAS system provides significant benefit (25% window sticker mpg rise*)

• Designed a low-energy Ucap HEV conversion (no additional DC/DC)

• Performed bench hardware evaluation and verified module performance 

• Implemented Saturn Vue BAS HEV conversion with ability to switch 

between three energy storage configurations

• Found Ucap HEV performance comparable to stock NiMH HEV

– Achieved same fuel economy (generally only using 18-25 Wh)

– Matched driving performance

• Room to optimize design

– Controls tuning and motor sizing

– Take advantage of cold temp capability

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Ucap HEV performed equal or better than the stock 

Saturn Vue BAS battery HEV

* Caveat: Window sticker difference does not necessarily equate to hybridization improvement.
National Renewable Energy Laboratory 28 Innovation for Our Energy Future

Potential Next Steps

• Further experimentation with this test bed

– Evaluate higher power motor 

– Examine air conditioning and/or mountain driving impacts

– Test a smaller/custom Ucap module (decrease number of Ucap cells 

and/or F/cell)

– Further optimize calibration settings

– Artificially force a smaller Wh operating window (by modifying vehicle 

controls) and observe any fuel economy drop off

• Examine a different platform

• Expand platform-specific vehicle modeling to further explore 

the design space
National Renewable Energy Laboratory 29 Innovation for Our Energy Future

Acknowledgements

• GM 

– Jim Yurgil, Damon Frisch 

– Mike Reynolds, Andrew Namou

– Mark Verbrugge, Shawn Hawkins

– Bret Detrick (on-site with dSPACE)

• Maxwell

– Michael Everett, John Miller

– Uday Deshpande

• NREL 

– Mark Mihalic, John Ireland

– Kristin Day, Charlie King

• Department of Energy

– David Howell (funding for initial USABC/ 

DOE simulations laid the groundwork for 

the vehicle conversion project)

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