National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Plug-In Hybrid Electric Vehicle 

Energy Storage System Design 

Preprint 

T. Markel and A. Simpson 

To be presented at Advanced Automotive Battery Conference 

Baltimore, Maryland 

May 17–19, 2006 

Conference Paper 

NREL/CP-540-39614 

May 2006 
NOTICE

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 1

Plug-In Hybrid Electric Vehicle Energy Storage System Design*

Tony Markel and Andrew Simpson 

National Renewable Energy Laboratory, 1617 Cole Blvd. Golden, Colorado 80401 USA 

ABSTRACT 

Plug-in hybrid electric vehicle technology holds much promise for reducing the demand for petroleum in 

the transportation sector. Its potential impact is highly dependent on the system design and in particular, the 

energy storage system. This paper discusses the design options including power, energy, and operating 

strategy as they relate to the energy storage system. Expansion of the usable state-of-charge window will 

dramatically reduce cost but will likely be limited by battery life requirements. Increasing the power 

capability of the battery provides the ability to run all-electrically more often but increases the incremental 

cost. Increasing the energy capacity from 20-40 miles of electric range capability provides an extra 15% 

reduction in fuel consumption but also nearly doubles the incremental cost. 

Introduction*

The United States is faced with a transportation 

energy dilemma. The transportation sector is 

almost entirely dependent on a single fuel – 

petroleum. The continued role of petroleum as the 

primary transportation fuel should be questioned. 

Today, nearly 60% of U.S. total petroleum 

consumption is imported and results in billions of 

dollars flowing to the economies of foreign 

countries. More than 60% of U.S. petroleum 

consumption is dedicated to transportation.[1] The 

domestic production of petroleum is steadily 

declining while our rate of consumption continues 

to increase; thus imports are expected to continue 

to increase. Meanwhile, petroleum consumption 

rates in the emerging economies of China and 

India are rapidly expanding. Furthermore, experts 

believe world petroleum production may peak 

within the next 5-10 years.[2] The combination of 

these factors will place great strain on the supply 

and demand balance of petroleum in the near 

future. 

Hybrid electric vehicle (HEV) technology is an 

excellent way to reduce our petroleum 

consumption through efficiency improvements. 

HEVs use energy storage technology to improve 

vehicle efficiency through engine downsizing and 

by recapturing energy normally lost during braking 

events. A typical HEV will reduce gasoline 

 

* Employees of the Midwest Research Institute under Contract 

No. DE-AC36-99GO10337 with the U.S. Dept. of Energy have 

authored this work. The United States Government retains and 

the publisher, by accepting the article for publication, 

acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or 

reproduce the published form of this work, or allow others to 

do so, for United States Government purposes.

consumption by about 30% over a comparable 

conventional vehicle.1

 Since introduced, HEV 

sales have grown at an average rate of more than 

80% per year. However, after 5 years of 

availability, they represent only 0.1% of the total 

U.S. vehicle fleet. There are 237,000,000 vehicles 

on the road today and more than 16 million new 

vehicles sold each year.[3] New vehicles will 

likely be in-use for more than 15 years and the 

vehicle miles traveled (VMT) continues to 

grow.[4] It will be challenging to overcome the 

inertia of the vehicle fleet. For instance, if every 

new vehicle sold in 2011 and beyond was a 

petroleum-fueled hybrid, our petroleum 

consumption level 10 years from now would still 

be 6% greater than the current light-duty fleet 

consumption, and it would never drop below 

today’s consumption level. Efficiency 

improvements of HEVs will be insufficient to 

overcome vehicle fleet and VMT growth 

expectations. 

This presents a challenge of how to best displace 

as much petroleum consumption as soon as 

possible while incurring reasonable costs. Many 

industries, including polymer and pharmaceutical, 

have little choice but to use petroleum. There are 

several alternatives to petroleum for a 

transportation fuel source. These include 

hydrogen, ethanol, biodiesel, and electricity. 

Hydrogen and fuel cell technology has advanced 

rapidly but still faces significant cost, 

infrastructure, and technical challenges that could 

limit market penetration within the next 15-20 

years. Both ethanol and biodiesel are used today 

 

1 With additional improvements in aerodynamics and engine 

technology, hybrid vehicles today have demonstrated upwards 

of a 45% reduction in consumption as compared to a 

conventional vehicle.
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 2

and help displace petroleum. However, at current 

production levels and given future expectations on 

cellulosic production potential, biofuels have 

limited ability to end our oil addiction alone but 

may be more successful when combined with 

other displacement technologies. 

Plug-in hybrid electric vehicle (PHEV) technology 

is an option with the potential to displace a 

significant portion of our transportation petroleum 

consumption. A plug-in hybrid vehicle is an HEV 

with the ability to recharge its energy storage 

system with electricity from the electric utility 

grid. With a fully charged energy storage system, 

the vehicle will bias towards using electricity over 

liquid fuels. A key benefit of PHEV technology is 

that the vehicle is no longer dependent on a single 

fuel source. The primary energy carrier is 

electricity generated using a diverse mix of 

domestic resources including coal, natural gas, 

wind, hydroelectric, and solar energy. The 

secondary energy carrier is a liquid fuel (e.g., 

gasoline, diesel, or ethanol). 

PHEV technology is not without its own technical 

challenges. Energy storage system cost, volume, 

and life are major obstacles that must be overcome 

for these vehicles to be viable. The fuel 

displacement potential of a PHEV is directly 

related to the characteristics of the energy storage 

system. More stored energy means more miles that 

can be driven electrically. However, increasing 

energy storage also increases vehicle cost and can 

present significant packaging challenges. Finally, 

the energy storage system duty cycle for a PHEV 

is likely to be more severe from a life standpoint 

than electric vehicles or HEVs. 

The purpose of this paper is to expand on the 

current understanding of the potential benefits, the 

design options, and the challenges related to 

PHEV technology. 

PHEV and HEV Terminology 

Charge-sustaining mode – An operating mode in 

which the state-of-charge of the energy 

storage system over a driving profile may 

increase and decrease but will by the end of 

the cycle return to a state with equivalent 

energy as at the beginning of the period. 

Charge-depleting mode – An operating mode in 

which the state-of-charge of the energy 

storage system over a driving profile will have 

a net decrease in stored energy. 

All-electric range (AER) – The total distance 

driven electrically from the beginning of a 

driving profile to the point at which the engine 

first turns on. 

Electrified miles – Is the sum of all miles driven 

with the engine off including those after the 

engine first turns on. 

PHEVxx – A plug-in hybrid vehicle with sufficient 

energy to drive xx miles electrically on a 

defined driving profile usually assumed to be 

urban driving. The vehicle may or may not 

actually drive the initial xx miles electrically 

depending on the control strategy and driving 

behavior. 

SOC – State-of-charge of the energy storage 

system. The fraction of total energy capacity 

remaining in the battery. 

Degree of hybridization – The fraction of total 

rated power provided by the electric traction 

drive components. 

Utility factor – A measure of the fraction of total 

daily miles that are less than or equal to a 

specified distance based on typical daily 

driving behavior. 

Potential Benefits of PHEVs 

A key reason for exploring PHEV technology is its 

ability to achieve significant petroleum 

consumption reduction benefits. A PHEV has 

essentially two operating modes: a chargesustaining mode and a charge-depleting mode. The 

total consumption benefits of a PHEV are a 

combination of the charge-depleting and chargesustaining mode improvements. 

Figure 1 highlights the relative importance of these 

two modes in achieving fuel displacement. It 

shows the total consumption benefit as a function 

of the improvement in charge-sustaining mode 

consumption for HEVs and PHEVs with several 

electric range capabilities. Several current model 

hybrid vehicles are included. Today’s HEVs do 

not have a charge-depleting mode, so their total 

consumption benefits are derived solely from 

improvements in the charge-sustaining mode. The 

large dots on the plot present three scenarios that 

achieve 50% reduction in total petroleum 

consumption. A PHEV40 that consumed no 

petroleum (all-electric operation) in chargedepleting mode with a fuel economy in chargesustaining mode equivalent to a conventional 

vehicle would consume 50% less petroleum 

because the first 40 miles of driving would be 

done electrically. Likewise, a PHEV20 that 
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 3

Prius (Corolla)

Civic

Accord

Highlander

Escape

Vue

HEV 

+100% cs mpg

PHEV20

+40% cs mpg

PHEV40

+0% cs mpg

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Reduction in Charge-Sustaining Mode Petroleum 

Consumption (%)

Reduction in Total Petroleum 

Consumption (%)

Infeasible region for

HEV technology

HEV

PHEV20

PHEV40

PHEV60

Figure 1: HEV and PHEV Fuel Consumption Benefits by Operating Mode 

consumed 30% less petroleum in chargesustaining mode would also consume 50% less 

total petroleum. An HEV would have to achieve 

50% reduction in consumption in its chargesustaining mode to have an equivalent total 

benefit. It is unlikely that the consumption 

reduction in charge-sustaining mode can be 

reduced beyond 50% cost effectively. Quantifying 

the relative costs of adding electric range 

capability versus improving charge-sustaining 

mode efficiency is important. Moving vertically in 

the figure at a given charge-sustaining mode 

consumption level results in more miles driven 

electrically. Electrification of miles through 

charge-depleting operation in a PHEV is expected 

to be a cost-effective way to continue to reduce 

fuel consumption beyond HEV technology 

capabilities. 

The conclusions drawn from Figure 1 are based on 

national driving statistics shown in Figure 2. 

Figure 2 is a histogram showing the daily driving 

distance distribution and the resulting utility factor 

derived from the 1995 National Personal 

Transportation Survey (NPTS) data. The utility 

factor represents the fraction of total daily VMT 

that are less than or equal to the said distance. The 

utility factor is important for PHEVs because it 

can be used to effectively weight the value of the 

charge-depleting fuel consumption benefits versus 

the charge-sustaining fuel consumption benefits in 

a way that allows the results to be extrapolated and 

applied to the national fleet. 

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Trip Distance (mi)

Percentage of Trips

Utility Factor

Figure 2: 1995 NPTS Data on Daily Driving 

Distance Distribution and Resulting Utility Factor 

PHEVs take advantage of the fact that the typical 

daily driving distance is on the order of 30 miles. 

If most of these miles could be driven electrically, 

a large portion of our petroleum consumption 

would be eliminated. 

Design Options and Implications 

Determination of the energy storage system 

characteristics is a critical step in the PHEV design 

process. The energy storage system design 

variables include the power, energy, and usable 

state-of-charge (SOC) window. These three 

cs=charge sustaining 
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 4

variables will affect cost, mass, volume, life, fuel 

economy, and vehicle operation. 

The usable energy capacity of the energy storage 

system is defined by the desired electric range 

capability. The fuel displacement potential is 

directly related to the electric range capability. 

From Figure 2, a range capability of 20 miles (i.e., 

a PHEV20) would substitute electrical energy for 

petroleum consumption in 30% of total VMT. 

Likewise, a PHEV with 40 miles of range 

capability could displace 50% of total VMT. A 

typical midsize sedan will require ~300 Wh/mi for 

all electric operation. Thus a PHEV20 would 

require ~6 kWh, and a PHEV40 would require ~12 

kWh of usable energy. It is possible to reduce the 

usable energy requirement through aerodynamic 

and lightweight vehicle designs but not 

substantially. 

For design purposes, the usable SOC window 

relates the total energy capacity to the required 

usable energy capacity. A PHEV is likely to incur 

at least one deep discharge cycle per day and as a 

result will need to provide 4000+ deep discharge 

cycles in its 10-15 year lifetime. Figure 3 is a 

curve-fit to data presented by Rosenkranz showing 

the expected cycle life performance of lithium-ion 

(Li-ion) and nickel-metal hydride (NiMH) 

technology as a function of the discharge depth.[5] 

It shows that when a battery is discharged more 

deeply, the cycle life decreases. The horizontal, 

shaded box is the typical depth of discharge 

cycling that HEV batteries today incur while the 

vertical, shaded box is the range of cycles that a 

PHEV battery will need to endure for a 10-15 year 

vehicle life. 

0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000

Number of Cycles

Depth of Discharge (%)

NiMH

Li-ion

Typical PHEV Cycles 

During Life of Vehicle

Typical HEV

Operating Window

Figure 3: Cycle Life Characteristics of Varta 

Energy Storage Technologies [5] 

The data indicate that NiMH can achieve 4000 

cycles when discharged to 70% depth of discharge 

repeatedly. To achieve the same number of cycles, 

Li-ion technology could only be discharged to 

50% depth of discharge on a daily basis. Assuming 

a 70% usable SOC window for a PHEV20 then 

requires 8.6 kWh of total energy capacity. This 

battery would have 5-10 times more energy 

capacity relative to that found in current hybrid 

vehicles. The PHEV40 will need 17.2 kWh. To 

minimize total energy storage capacity (and thus 

cost and volume), it will be important to maximize 

the usable SOC window for PHEVs while 

satisfying cycle life requirements. 

The energy storage system cost, mass, and volume 

are strong functions of the energy storage system 

power to energy ratio. Representative specific 

power and power densities are provided for both 

Li-ion (based on Saft products [6]) and NiMH 

(based on Cobasys products [7]) technologies in 

Figures 4 and 5. Current and projected specific 

cost relationships are provided in Figure 6. The 

cost projections are those suggested by Electric 

Power Research Institute.[8] For fixed energy 

storage capacity, as power to energy ratio 

decreases so do cost, volume, and mass. The 

question is how does reducing the power 

capability affect the fuel consumption reduction 

potential? 

0

200

400

600

800

1000

1200

1400

0 5 10 15 20

Power to Total Energy Ratio (kW/kWh)

Specific Power (W/kg)

NiMH

Li-ion

Figure 4: Typical Specific Power of Energy 

Storage Technologies 

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20

Power to Total Energy Ratio (kW/kWh)

Power Density (W/L)

NiMH

Li-ion

Figure 5: Typical Power Density of Energy 

Storage Technologies 
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 5

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20

Power to Total Energy Ratio (kW/kWh)

Specific Cost ($/kWh)

Projected

Current Status

Figure 6: Typical Specific Cost of Energy Storage 

Technologies 

To achieve a desired all-electric range (AER) 

capability, the energy storage system and motor 

will need to provide sufficient power to propel the 

vehicle without assistance from the engine. On an 

urban driving profile, the peak power is ~40 kW 

and the power is typically less than 15 kW as 

shown in Figure 7 for a typical light-duty vehicle. 

0 2 4 6 8

-20

0

20

40

60

Distance (mi)

Power (kW)

Peak Power

Power

Energy

0 2 4 6 8

0

0.5

1

1.5

2

Energy (kWh)

Figure 7: Power and Energy Requirements for AllElectric Range Capability on Urban Driving 

In Figures 8 and 9, the power and energy required 

for all-electric range capability on several driving 

profiles is provided. The Urban Dynamometer 

Driving Schedule (UDDS) and Highway Fuel 

Economy Test (HWFET) cycles are used by the 

U.S. Environmental Protection Agency to 

represent urban and highway driving behaviors in 

reporting the fuel economies of today’s vehicles. 

The Unified Driving Cycle, also called LA92, and 

the US06 (part of the Supplemental Federal Test 

Procedure) are more aggressive urban and 

highway driving cycles respectively that are likely 

to be more representative of current driving 

behaviors. The energy to provide 20 miles of allelectric range on the UDDS and HWFET cycles 

only provides 10 and 15 miles of range capability 

on the US06 and LA92 cycles, respectively as 

shown by the dashed line in Figure 9. A PHEV on 

the UDDS would need 40 kW of battery power 

while it would require more than 60 kW on the 

LA92 cycle. Adding battery power beyond the 

peak power requirement is unlikely to provide 

additional value. Acceleration requirements will 

place additional constraints on the energy storage 

system power requirements depending on engine 

sizing. 

0 10 20 30 40 50 60

0

50

100

150

UDDS

US06

HWFET

LA92

Distance (mi)

Peak Power (kW)

Figure 8: Peak Power Requirements for AllElectric Range Capability for Typical Mid-size 

Car on Several Duty Cycles 

0 10 20 30 40 50 60

0

5

10

15

20

Energy (kWh)

Distance (mi)

UDDS

US06

HWFET

LA92

Figure 9: Usable Energy Requirements for AllElectric Range Capability on Typical Driving 

Profiles for a Mid-size Car 

To maximize charge-sustaining fuel economy, it is 

desirable to minimize the power rating (downsize) 

the engine as much as possible. The engine in a 

PHEV will likely be sized to provide continuous 

performance capability. If it is assumed that the 

vehicle must achieve a continuous top speed of 

110 mph and continuous gradeability at 55 mph on 

a 7.2% grade using 2/3 of peak engine power then 

the minimum engine would need to be 80-85 kW 

for a typical sedan. Now to achieve a 0-60 mph 

acceleration time of 8.0s or less, the energy storage 

system would need at least 45-50 kW of power 

capability at the low SOC operating point. As a 
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 6

result, with maximum engine downsizing, the 

power to energy ratio would be ~5 for a PHEV20 

and 2.8 for a PHEV40. 

The sizes described so far are only necessary to 

achieve large all-electric range capabilities. 

Significant fuel can be displaced without large allelectric range capability. As shown in Figure 7, the 

urban drive cycle power requirements are typically 

less than 15 kW. Since cost, mass, and volume of 

the energy storage system can be reduced by 

reducing the power to energy ratio, it is 

worthwhile to explore the fuel displacement 

potential of low power energy storage systems for 

PHEVs. 

The lower bound on the energy storage power will 

be a function of the lowest power to energy ratio 

modules available. The lowest power to energy 

ratio for typical Li-ion or NiMH technology today 

is ~1. Therefore, the minimum power will be on 

the order of 10-15 kW. 

Employing the low-power option limits the allelectric range capability of the vehicle. However, 

if, when the engine is on, it only provides 

supplemental power beyond the capabilities of the 

energy storage system; substantial fuel 

displacement can still be achieved via a strategy 

where energy storage and engine operate in a 

blended manner. The blended approach was 

proposed in an early paper [9] and will be referred 

to as a blended strategy in the remainder of the 

paper. 

Figures 10 and 11 provide a comparison between 

operating characteristics for PHEVs with allelectric-range-focused versus blended operating 

strategies. In Figure 10, the battery and motor 

(dashed line) have sufficient power to propel the 

vehicle until about 22 miles at which time the 

engine (solid line) is turned on. In Figure 11, the 

engine turns on within the first mile but when on, 

it only provides supplemental power, and the 

battery still provides most of the power. Thus, the 

battery discharges over approximately the same 

distance and displaces nearly as much fuel. 

Figure 10: Urban Cycle Operating Characteristics 

of an All-Electric Range Focused PHEV20 

Figure 11: Urban Cycle Operating Characteristics 

of a PHEV20 with a Blended Strategy 

The charts that follow summarize the tradeoffs of 

power, energy, SOC window, and operating 

strategy on the cost, efficiency, and fuel savings 

potential of a PHEV20 and a PHEV40. All 

components in each vehicle scenario were sized 

first for an all-electric range scenario and second 

for a blended scenario. And for each of these four 

scenarios, a 50% SOC window and a 70% usable 

SOC window were considered. To define the 

blended scenario, a power to energy ratio was 

chosen that was half that of the all-electric range 

scenario. 

Figure 12 summarizes the energy storage system 

power and energy characteristics of the eight 

vehicles considered. The SOC window only 

slightly impacts the power requirement while the 

AER case needs twice as much power as the 

blended case as designed. Battery energy is 

slightly more than a factor of two due to mass 

compounding. 

-30

-20

-10

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35 40

Distance (mi)

Power (kW)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

SOC (%)

engine

motor

SOC

-30

-20

-10

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35 40

Distance (mi)

Power (kW)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

SOC (%)

engine

motor

SOC
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 7

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Energy (kWh)

Power (kW)

PHEV20 - Blended

PHEV20 - AER

PHEV40 - Blended

PHEV40 - AER

70% SOC Window 50% SOC Window

Figure 12: Power and Total Energy Characteristics 

of the Energy Storage System 

Incremental cost of the vehicle is likely to be a 

significant barrier to PHEV technology 

acceptance. The main reason for trying to use a 

lower power to energy ratio energy storage system 

would be to reduce cost while providing the same 

amount of energy. Figure 13 shows that reducing 

power to energy ratio and moving from an AER to 

blended strategy reduced incremental cost. 

However, increasing the usable SOC window 

seemed to more strongly impact the incremental 

costs. 

$-

$1,000

$2,000

$3,000

$4,000

$5,000

$6,000

$7,000

0123456

Power to Total Energy Ratio (kW/kWh)

Incremental Cost ($)

PHEV20 - Blended

PHEV20 - AER

PHEV40 - Blended

PHEV40 - AER

50% SOC Window

70% SOC Window

AER

Blended

Figure 13: Relationship between Incremental Cost 

and Power to Energy Ratio 

For a given range (e.g., PHEV20) and SOC 

window (e.g., 50%), moving to a lower power to 

energy ratio not only reduced the incremental cost 

but also reduced the fuel consumption reduction 

potential as expected. The fuel consumption 

benefits of the blended strategy are about 6% less 

than the AER strategy for both PHEV20 and 

PHEV40 cases as shown in Figure 14. 

Interestingly, expanding the usable SOC window 

has minimal impact on fuel consumption reduction 

potential but substantially reduces incremental cost 

and thus should be emphasized. 

$-

$1,000

$2,000

$3,000

$4,000

$5,000

$6,000

$7,000

20% 30% 40% 50% 60% 70%

Percent Reduction in Petroleum Consumption (%)

Incremental Cost ($)

PHEV20 - Blended

PHEV20 - AER

PHEV40 - Blended

PHEV40 - AER

50% SOC Window

70% SOC Window

Figure 14: Cost-to-Benefit Relationship for 

PHEVs 

As shown in Figure 15, there are efficiency 

tradeoffs between a blended and an all-electric 

range focused strategy. In the AER approach, the 

engine is as small as possible and when it is on, it 

will operate at higher load fractions which 

typically correlate to higher efficiencies. The 

energy storage system in the AER scenario is a 

higher power to energy ratio with lower internal 

resistance and thus less loss. On the other hand, 

the motor in the blended approach is smaller, and 

thus running at higher load fractions with higher 

efficiencies. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Engine Motor Battery

Urban Cycle Avg. Efficiency (%)

PHEV20 - Blended

PHEV20 - AER

Figure 15: Component Efficiencies for AER and 

Blended PHEV Strategies 

The purpose of these case scenarios is to 

demonstrate that there are many options in the 

design of a PHEV. Each of these options has 

associated tradeoffs. Ideally, the design should 

find a balance between petroleum consumption 

reduction potential and incremental costs if it is to 

be successful. Our results demonstrate that a 

blended approach combined with an expanded 

SOC window effectively reduced cost while 

displacing nearly as much fuel in comparison to an 

all-electric range focused PHEV. These cost 

reductions are critical for market viability. 
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 8

Analysis Refinements 

This analysis provides only a simplified view of 

the PHEV design space and challenges. There are 

many uncertainties associated with these 

conclusions. In particular, there is uncertainty 

associated with the life cycle data, the battery cost 

data, and the vehicle usage patterns. It will be 

important to identify how each of these 

uncertainties will affect the PHEV design and 

operation. 

The life cycle data available at this time have been 

collected as constant discharge cycles to a 

specified depth of discharge repeatedly. The 

batteries in HEVs today can be expected to 

encounter tens of thousands of small depth of 

discharge cycles at a moderate to high SOC. 

PHEVs will on the other hand encounter at least 

one deep discharge cycle on a daily basis. In 

addition, a fully utilized energy storage system 

will also encounter shallow depth of discharge 

cycles both at high and low SOC levels. It has 

been assumed that the daily deep discharge cycle 

will be the overriding factor that will determine 

life cycle performance. It is unclear how the 

shallow cycling behavior may contribute to the 

degradation of the energy storage system. 

Today, the cost of hybrid battery technology is 

high, and tax incentives are used to make hybrids 

cost competitive with comparable vehicle options. 

The energy storage system costs contributing to 

the incremental cost analysis presented earlier 

assume future high-volume production. Current 

costs are estimated to be 4-5 times higher than the 

long-term assumptions. Since this is a pivotal 

assumption, it is possible to turn the analysis 

around and look at what battery costs might need 

to be to provide a cost effective vehicle.

Figure 16, shows the specific costs that the energy 

storage technology would need to achieve for the 

fuel cost savings over 5 years to offset the initial 

incremental cost. The chart includes both a present 

fuel cost ($2.15/gallon gasoline and 9 ¢/kWh 

electricity case) and a future fuel cost scenario 

($4.30/gallon and 9 ¢/kWh). At today’s fuel costs, 

to be cost neutral, PHEV20 batteries would need 

to be at the projected long-term cost goals (labeled 

as Projected Battery Costs in Figure 16). 

However, in the future fuel price scenario, both 

PHEV20 and PHEV40 energy storage systems 

only need to reach the $750 to $500/kWh range to 

be cost neutral respectively. 

$-

$500

$1,000

$1,500

$2,000

$2,500

$3,000

0 5 10 15 20 25

Power-to-energy ratio (kW/kWh)

Battery specific cost ($/kWh total)

NiMH

Li-Ion

PHEV3.5

PHEV5

PHEV10

EXPANDED 70% SOC 

WINDOW FOR ALL PHEVS

Projected battery costs

Current battery costs?

HEV0

HEV0 PHEV3.5

$4.30 / gal.

$2.15 / gal. PHEV20

(19,000 mi/year, 52 mi/day avg.)

Assumed electricity cost = 9c/kWh

PHEV40

$-

$500

$1,000

$1,500

$2,000

$2,500

$3,000

0 5 10 15 20 25

Power-to-energy ratio (kW/kWh)

Battery specific cost ($/kWh total)

NiMH

Li-Ion

PHEV3.5

PHEV5

PHEV10

EXPANDED 70% SOC 

WINDOW FOR ALL PHEVS

Projected battery costs

Current battery costs?

HEV0

HEV0 PHEV3.5

$4.30 / gal.

$2.15 / gal. PHEV20

(19,000 mi/year, 52 mi/day avg.)

Assumed electricity cost = 9c/kWh

$-

$500

$1,000

$1,500

$2,000

$2,500

$3,000

0 5 10 15 20 25

Power-to-energy ratio (kW/kWh)

Battery specific cost ($/kWh total)

NiMH

Li-Ion

PHEV3.5

PHEV5

PHEV10

EXPANDED 70% SOC 

WINDOW FOR ALL PHEVS

Projected battery costs

Current battery costs?

HEV0

HEV0 PHEV3.5

$4.30 / gal.

$2.15 / gal. PHEV20

(19,000 mi/year, 52 mi/day avg.)

Assumed electricity cost = 9c/kWh

PHEV40

Figure 16: Battery Cost Requirements for Fuel Savings to Offset Incremental Cost within 5 Years 
 Presented at Advanced Automotive Battery Conference 

 Baltimore, Maryland - May 17-19, 2006 

 9

Additional research completed at the National 

Renewable Energy Laboratory clearly shows that 

there is a significant connection between the 

vehicle usage pattern and the consumption 

reduction benefits of a PHEV both over a 

conventional vehicle and an HEV. The analysis 

presented assumes utility factor weighted fuel 

economies based on the UDDS and HWFET 

driving profiles. It is fair to assume that neither of 

these driving profiles (developed in the 1970s) 

accurately represents typical driving habits of 

today. In addition, the utility factor is used to 

weight the relative value of the electric range 

capability of a PHEV. However, the utility factor 

is based on data from national personal travel 

surveys conducted in 1995. More recent data are 

available and need to be analyzed. It’s likely that 

travel behavior is evolving. In addition, the 

existing survey data typically only represent a 

single day of the year and do not account for 

variation daily or seasonally. PHEV benefits are 

likely to be significantly influenced by these 

variations in driving habits.

Conclusions 

PHEVs have the potential to dramatically reduce 

future U.S. transportation petroleum consumption. 

To overcome the implementation challenges of 

PHEV technology, a systems perspective should 

be employed. This study sheds light on the 

systems design tradeoffs as they relate to energy 

storage system technology for PHEVs. 

Specifically, it evaluates the impacts of reducing 

power to energy ratio and expanding the usable 

SOC window on incremental cost and fuel 

consumption reduction benefits. 

Based on the analyses, we conclude that: 

• Plug-in hybrids provide potential for reducing 

petroleum consumption beyond that of HEV 

technology. 

• There is a spectrum of PHEV design options 

that satisfy performance constraints but with 

tradeoffs in incremental costs and fuel 

consumption reduction potential. 

• Expansion of the usable SOC window while 

maintaining energy storage system life will be 

critical for reducing incremental costs of 

PHEVs. 

• The fuel consumption reduction benefits are 

only slightly reduced while the battery size 

and cost are significantly reduced when a 

blended strategy is chosen relative to an allelectric range focused strategy. 

References 

1) Energy Information Administration 

www.eia.doe.gov. 

2) Hirsch, R., Bezdek, R., and Wendling, R. 

“Peaking of World Oil Production: Impacts, 

Risks, and Mitigation.” February, 2005. 

3) “Highway Statistics 2004.” U.S. Department 

of Transportation. Federal Highway 

Administration. 

http://www.fhwa.dot.gov/policy/ohim/hs04/in

dex.htm. 

4) Davis, S., Diegel, S. “Transportation Energy 

Databook: Edition 24.” December, 2004. 

5) Rosenkranz, C. “Deep Cycle Batteries for 

Plug-in Hybrid Application” Presented at 

EVS-20 Plug-in Hybrid Workshop. Nov. 15, 

2003. Monaco. 

6) Saft website. www.saftbatteries.com. 

7) Cobasys website. www.cobasys.com. 

8) Duval, M. “Advanced Batteries for Electric 

Drive Vehicles.” EPRI Technical Report 

#1009299. May, 2004. 

9) Markel, T. and Simpson, A. “Energy Storage 

Considerations for Grid-Charged Hybrid 

Electric Vehicles.” IEEE Vehicular 

Technologies Conference. September 7-9, 

2005. Chicago, IL. 

Acknowledgements 

The authors would like to acknowledge the 

programmatic support of the U.S. Department of 

Energy Office of Energy Efficiency and 

Renewable Energy FreedomCAR and Vehicle 

Technologies Program. 
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

May 2006 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Plug-In Hybrid Electric Vehicle Energy Storage System Design: 

Preprint 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-540-39614 

5e. TASK NUMBER

FC06.2000 

6. AUTHOR(S)

T. Markel and A. Simpson 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-540-39614 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

This paper discusses the design options for a plug-in hybrid electric vehicle, including power, energy, and operating 

strategy as they relate to the energy storage system. 

15. SUBJECT TERMS

Plug-in hybrid electric vehicle; hybrids; PHEV; energy storage 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18