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.

Jeff Gonder and Ahmad 

Pesaran

jeff.gonder@nrel.gov

National Renewable Energy Laboratory 

David Howell

U.S. Department of Energy (DOE)

Harshad Tataria

USABC Program Manager,

General Motors (GM)

NREL/PR-540-47682

Lower-Energy Requirements for Power-Assist HEV 

Energy Storage Systems—Analysis and Rationale

Presented at the 27th International Battery Seminar and Exhibit

Fort Lauderdale, FL

March 18, 2010
National Renewable Energy Laboratory 2 Innovation for Our Energy Future

Executive Summary

• Performed simulations and analyzed test data in conjunction with an 

EES TT Workgroup in response to a USABC request

• Results suggested power-assist (PA) HEVs can still achieve high fuel 

savings with lower energy requirements and potentially lower cost

• Clarified energy definition

– “Available” energy where power requirements simultaneously met, vs.

– Energy window for vehicle use

• USABC/EES TT established new set of ESS targets and issued a 

Request for Proposal Information (RFPI)

– The goal is to reduce the cost of fuel-saving HEV systems

– Open to any ESS technology (very high power batteries, electrochemical 

double layer capacitors, or asymmetric supercapacitors)

EES TT = The FreedomCAR Electrochemical Energy Storage Technical Team

USABC = United States Advanced Battery Consortium

HEV, ESS = hybrid electric vehicle, energy storage system
National Renewable Energy Laboratory Innovation for Our Energy Future

Background

• Historic PNGV, FreedomCAR, and/or USABC targets

– For the energy storage system (ESS) in a power-assist (PA) hybrid 

electric vehicle (HEV)

– Established in the late 1990s and early 2000s

– Call for 0.3-0.5 kWh of “available” energy

• Limitations of NiMH batteries result in large design margins 

– To achieve at least 8-10 year life

– Lead to high-cost battery packs with total energy of 1.2 to 1.5 kWh

• Pursuing more cost-effective energy storage

– EES TT and USABC assembled a workgroup to re-evaluate the energy 

required for PA-HEVs and milder hybrids

– NREL was asked to provide analysis support

3

PNGV = Partnership for a New Generation of Vehicles

USABC = United States Advanced Battery Consortium

NiMH = Nickel metal hydride

EES TT = The FreedomCAR Electrochemical Energy Storage Technical Team
National Renewable Energy Laboratory Innovation for Our Energy Future

Motivation for this work:

Could required energy be reduced to save $ and expand technologies?

4

* From USABC ESS Goals: http://www.uscar.org/guest/article_view.php?articles_id=85

Total available energy (over DOD range where power goals are met) = 0.3-0.5 kWh
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Approach Outline

Supporting FreedomCAR/USABC Workgroup’s re-evaluation of 

HEV ESS requirements/goals per DOE’s funding

• HEV simulations

– ESS energy vs. fuel use trends

– Various degrees of hybridization and drive cycles

• Analysis of vehicle ESS test data

– Production HEV dyno runs

– Sanity check on ESS use over standard cycles

• Reprocessing of simulations for power analysis

– Characterize power pulses

• (More closely examine energy definition)

– “Available” energy over which pulse power goals met, vs.

– Energy window for vehicle use

5

Dis. 69 Wh Goals Both Met = 300 Wh Chg. 56 Wh

Energy Window for Vehicle Use = 425 Wh

Dis. reqmt. (25 kW * 10 s) met: 300 + 56 = 356 Wh

Chg. reqmt. (20 kW * 10 s) met: 300 + 69 = 369 Wh

Workgroup

H. Tataria

R. Elder

C. Ashtiani

J. Belt

V. Bataglia

R. Spotnitz

C. Bae

K. Snyder

J. Deppe
National Renewable Energy Laboratory Innovation for Our Energy Future

Base modeling process/assumptions

Used Powertrain System Analysis Toolkit (PSAT) simulation software

• Midsize parallel HEV

• Multiple degree of hybridization (DOH) cases

– Based on consistent 0-60 mph acceleration

• Multiple drive cycles (UDDS, US06 & Colorado foothills)

6

FA = 2.27 m2 Test mass = 1675 kg

CD = 0.30 Accessory power:

Crr1 = 0.008 Electrical = 500 W

Crr2 = 0.00012 Mechanical = 230 W

DOH = 10% DOH = 19% DOH = 38%

15 kW motor

135 kW eng

25 kW motor

110 kW eng

45 kW motor

75 kW eng

Platform assumptions -- held constant

(Technology-neutral assessment of ESS capability)

0 500 1000 1500 -20

0

20

40

60

80

time (s)

Vehicle speed (mph)

Cycle grade (%)

0 200 400 600 0

20

40

60

80

100

time

Vehicle speed (mph)

time (s) 0 500 1000 1500 0

20

40

60

time (s)

Vehicle Speed (mph)

DOH = Motor Power/

(Engine + Motor)Power
National Renewable Energy Laboratory Innovation for Our Energy Future

Simulate and analyze ESS in all base cases

Identify actual energy window used to support HEV functions

• Investigate range of ESS energy content

– ESS power limited by lesser of motor power or driving demand

• Constant SOC-based controls  changing Wh control window

• Capture in-use energy window for each simulation (charge sustaining)

7

Smallest 

ESS case

Largest 

ESS case

Smallest 

ESS case

Largest 

ESS 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)

Total 

energy

“Available”

energy

Energy

window
National Renewable Energy Laboratory Innovation for Our Energy Future

8

 

 

0 100 200 300 400 500 600

-100

-80

-60

-40

-20

0

20

40

60

80

100

time

Vehicle speed (mph)

Energy - large ESS case (Wh)

Energy - small ESS case (Wh)

Example in-use ESS energy comparison

Launch Assist

Comparison of large and small ESS cases over US06 cycle

Energy window = 125 and 27 Wh, respectively

Vehicle speed (mph)

Energy – large ESS case (Wh)

Energy – small ESS case (Wh)

time (s)

Speed (mph) or Cumulative Energy (Wh)

Regen Capture

125 Wh 

window

“Idle-off” 

accessory 

power

8
National Renewable Energy Laboratory Innovation for Our Energy Future

Simulation Results: Trends from fuel consumption results:

* Fuel savings increase with higher DOH (engine downsizing)

* Fuel savings increase, but taper with larger energy window 

6.50

7.50

8.50

9.50

10.50

11.50

12.50

0 100 200 300 400 500

ESS Energy Window (Wh)

Fuel Consumption (L/100 km)

18.8

24.8

27.7

31.4

36.2

22.4

20.5

(mpg reference)

9

DOH = 10%

DOH = 19%

DOH = 38%

14% decrease

29% decrease

13% decrease

UDDS Cycle Results

10% decrease

Significant benefit with energy window 

out to ≈50 Wh.

Most additional benefit with energy 

window out to ≈150 Wh or so.
National Renewable Energy Laboratory Innovation for Our Energy Future

10

DOH = 10%

DOH = 19%

DOH = 38%

3% decrease

3% decrease 12% decrease

6% decrease

US06 Cycle Results

6.50

7.50

8.50

9.50

10.50

11.50

12.50

0 100 200 300 400 500

ESS Energy Window (Wh)

Fuel Consumption (L/100 km)

18.8

24.8

27.7

31.4

36.2

22.4

20.5

(mpg reference)

Simulation Results: Similar trends for other drive cycles

10
National Renewable Energy Laboratory Innovation for Our Energy Future

Simulation Results: Some additional, limited benefit for 

higher energy window use in foothills-type driving

6.50

7.50

8.50

9.50

10.50

11.50

12.50

0 100 200 300 400 500

ESS Energy Window (Wh)

Fuel Consumption (L/100 km)

UDDS Cycle

US06 Cycle

Foothills Driving

19% DOH case

18.8

24.8

27.7

31.4

36.2

22.4

20.5

(mpg reference)

11
National Renewable Energy Laboratory Innovation for Our Energy Future

Actual driving data from existing commercial HEVs

Used to complement and provide a sanity check on the simulation results

12

Chassis 

dynamometer 

(dyno) test cell
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Consistent findings from analysis of production HEV dyno data*

* Thanks to ANL for providing access to some of the raw dynamometer test data

Results adjusted for round-trip efficiency (to provide actual ESS energy state)

* In-use energy window for charge-sustaining 

tests: same range as simulation results

* Total “nominal” battery energy much larger, 

some of it used occasionally
National Renewable Energy Laboratory 14 Innovation for Our Energy Future

• Even small energy windows can provide HEV fuel savings

• Significant fuel savings can be achieved with a ≈150-200 Wh energy 

window (less than the previous 425 Wh minimum goal)

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

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

– Achieving longer cycle life from reduced SOC swings

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

temperatures)

– Note that power dominates cost in HEV batteries (high P/E ratio)

Summary of in-use ESS energy window analysis

Next question: What goals for ESS power capability tests should be 

combined with a reduced in-use energy goal?
National Renewable Energy Laboratory Innovation for Our Energy Future

Power pulse analysis considerations/approach

• Standard lab test vs. in-vehicle ESS capability

– Actual use depends on DOH, drive cycle, engine vs. motor control decisions, etc.

• Analyze pulses from past simulation cycles, observe trends

15

10 s

25 kW

Pulse 

power

Pulse time

Process for translating 

complex pulse profiles 

into “standard pulse 

test” equivalents
National Renewable Energy Laboratory Innovation for Our Energy Future

Example complex in-vehicle power profile*

16

* From aforementioned ANL dyno testing

Zoom in on next slide
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Quantifying pulse characteristics - 1
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Quantifying pulse characteristics - 2
National Renewable Energy Laboratory 19 Innovation for Our Energy Future

Quantifying pulse characteristics - 3
National Renewable Energy Laboratory Innovation for Our Energy Future

Example results from power pulse analysis

• Bound observations for in-use 

power pulses

– Max discharge

– Max charge/regen

20

US06, highest 

DOH and energy 

window case
National Renewable Energy Laboratory Innovation for Our Energy Future

Summarized pulse power bounds for multiple cases 

• EES TT Workgroup: Use 2 sec and 10 sec values from highest ESS 

power and energy case on US06 as basis for power goals

21

UDDS

US06

Each DOH configuration

DOH = 10%

DOH = 19%

DOH = 38%

Two in-use energy window cases

Smallest energy:

Largest energy:
National Renewable Energy Laboratory Innovation for Our Energy Future

Energy Window = 165 Wh

Proposed method to obtain value for battery testing 

from an “energy window for vehicle use”

1) Begin with the stated “energy window for 

vehicle use” (e.g., 165 Wh)

2) Calculate energy for pulse requirements

a) Discharge (e.g., 10 sec x 20 kW → 56 Wh)

b) Charge (e.g., 10 sec x 30 kW → 83 Wh)

3) Subtract pulse energy from ends of vehicle use 

energy (e.g., 165 Wh – 83 Wh – 56 Wh = 26 Wh)

4) This gives “available energy over which pulse 

power requirements must be met” (e.g., 

perform size factor analysis with ≥ 26 Wh 

bounded by 10 s power requirements)

5) Repeat if needed for other pulse power levels 

(e.g., if energy from 2 sec power requirements 

happens to be greater than that from the 10 sec 

power requirements)

6) Note: restrict “energy over which pulse power 

requirements must be met” to ≥ 0

22

Energy Window

Dis.

Chg.

Dis. Chg.

Dis. reqmt met

165 – 56 = 109 Wh

Chg reqmt 

met: 165 –

83 = 82 Wh

Reqmts both met

26 Wh

(“Available Energy”)
National Renewable Energy Laboratory Innovation for Our Energy Future

Comparing New Proposed Lower Energy ESS requirements 

with other existing requirements 

23

Previous USABC requirements 

Proposed requirements 
National Renewable Energy Laboratory Innovation for Our Energy Future

From Analysis to New Requirements 

• New set of targets/goals established for PA-HEVs

– Selected by EES TT and USABC

– Based on the analysis and discussions of the Workgroup

– Referred to as lower-energy energy storage system (LEESS) goals

• USABC announces a Request for Proposal Information (RFPI)

– Asking for proposals for technologies meeting the LEESS requirements

– Systems satisfying the requirements could be based on batteries, 

symmetric or asymmetric capacitors, or some other device

– http://evworld.com/news.cfm?newsid=22459

– http://www.uscar.org/guest/article_view.php?articles_id=87
National Renewable Energy Laboratory 25 Innovation for Our Energy Future

Summary

• Performed simulations and analyzed test data in conjunction with an 

EES TT Workgroup in response to a USABC request

• Results suggested power-assist HEVs can still achieve high fuel savings 

with lower energy requirements and potentially lower cost

• Clarified energy definition

– “Available” energy where power requirements simultaneously met, vs.

– Energy window for vehicle use

• USABC/EES TT established new set of ESS targets and an RFPI 

– With goal of reducing cost for fuel-saving HEV systems

– Open to any ESS technology (batteries, ultracapacitors, hybrid batteryultracapacitors)
National Renewable Energy Laboratory Innovation for Our Energy Future

Acknowledgments

• Funded by DOE Energy Storage Program Manager

– David Howell, Office of Energy Efficiency and Renewable Energy, 

Vehicle Technologies Program

• Direction provided by USABC Managing Committee

• Technical discussions with Workgroup of FreedomCAR 

Electrochemical Energy Storage Technical Team

– H. Tataria; R. Elder; C. Ashtiani; J. Belt; V. Bataglia; 

– C. Bae; R. Spotnitz; K. Snyder; J. Deppe

26