Conference Paper

NREL/CP-540-45779

May 2009

Integration Issues of Cells into 

Battery Packs for Plug-In and 

Hybrid Electric Vehicles

Preprint

A.A. Pesaran, G.-H. Kim, and M. Keyser

Presented at EVS-24 International Battery, Hybrid and Fuel Cell 

Electric Vehicle Symposium

Stavanger, Norway

May 13-16, 2009
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1 

EVS24

Stavanger, Norway, May 13-16, 2009

Integration Issues of Cells into Battery Packs for Plug-in 

and Hybrid Electric Vehicles

Ahmad A. Pesaran1, Gi-Heon Kim, and Matt Keyser

National Renewable Energy Laboratory

1617 Cole Blvd, Golden, CO 80401 USA 1

Tel: +1 303 275-4441; Fax: +1 303 275-4415; Email: ahmad_pesaran@nrel.gov

Abstract

The main barriers to increased market share of hybrid electric vehicles (HEVs) and commercialization of 

plug-in HEVs are the cost, safety, and life of lithium ion batteries. Significant effort is being directed to 

address these issues for lithium ion cells. However, even the best cells may not perform as well when 

integrated into packs for vehicles because of the environment in which vehicles operate. In this paper, we 

discuss mechanical, electrical, and thermal integration issues and vehicle interface issues that could impact 

the cost, life, and safety of the system. We compare the advantages and disadvantages of using many small 

cells versus a few large cells and using prismatic cells versus cylindrical cells.

Keywords: Lithium battery, battery management, cooling, electric drive, modelling, thermal management

1 Introduction

Hybrid electric vehicles (HEVs) and plug-in 

hybrid electric vehicles (PHEVs) have the 

potential to reduce a significant amount of 

petroleum used for transportation in the next 10-

20 years. The major obstacle to increasing the

market share of HEVs and to the mass

production of commercial PHEVs is the battery.

Batteries for these vehicles must be highperforming, inexpensive, long-lasting, and safe. 

Lithium ion batteries have the potential to meet 

those challenges. A significant amount of R&D 

is being performed around the world to improve 

the attributes of the cells. However, poorly 

integrating even good cells in modules and packs 

may lead to lower performance, life, and safety 

and increased cost. Integration must address 

mechanical, packaging, electrical, thermal, 

safety, monitoring and control and interface with 

the rest of the vehicle. In the case of PHEVs, 

packs need to be interfaced with an on-board 

charger obtaining electricity from an alternating 

current grid. In the terminology used here, which

is widely used by battery developers (Figure 1), a

“cell” consists of the electrochemical unit with the 

lowest voltage of the associated chemistry. A

module consists of several cells to produce midrange voltage, up to 50 V, and a pack consists of 

many modules (or cells). The pack is housed in a 

container with electronic and thermal control that 

creates the total system that interfaces with the rest 

of the vehicle components. Each module or pack 

has its appropriate packaging, mechanical, 

electrical, and thermal controls. In the initial 

design of a battery pack, one must consider the 

following: safety (abuse tolerance), cost of 

packaging for the whole system, impact packaging 

and control on life and durability, packaging for 

recyclability and reuse, manufacturability for 

lower cost, maintenance/repair considerations, 

thermal management (since temperature impacts 

life and performance), electronics for monitoring
2 

 

and control, and gauges that show the capacity, 

power, and degradation rate. Battery pack 

integration must be achieved while meeting 

multiple requirements and balancing multiple 

inputs and outputs [1]. In this paper, we will 

explore the use of small versus large cells and 

prismatic/laminate versus cylindrical cells. 

Battery modelling design tools will be discussed.

Figure 1: Cell, module, and pack

2 Battery Integration

Figure 2 shows how the battery pack or the 

energy storage system (ESS) integration could 

impact or influence other parameters or 

components. 

Figure 2: Components in battery pack integration and 

interface with vehicle, adapted from [1]

Mechanical elements and packaging should 

consider how cell-to-cell interconnects are 

designed; the particular cell, module, and 

electronic assembly; the structural protection 

points for shock and vibration; how the pack is 

crush-protected; the desired attachment points to 

the vehicle; and the mechanical interfaces with 

other components [2]. Electrical management 

includes the interfaces with the vehicle so that

the DC power can go back and forth from the 

battery to the inverter or motor controller; how 

voltage, current (pressure), and temperature are 

measured and monitored; and how the state of

charge (SOC), capacity, available energy, power 

capability, state of health, impedance, and rate of 

degradation are measured. Electrical safety also 

has to be considered by the appropriate use of 

fuses and automatic interconnects. Thermal 

management is a must, because, as the battery is 

used and power goes in and out, the inefficiency of 

the battery, even if it is less than 5%-10%, could 

lead to heating of the cells (sometimes not 

uniformly), and high temperatures could lead to 

lower life and reduced vehicle fuel economy. 

2.1 Battery Safety

Safety is another big concern for lithium ion 

battery packs. During abuse tests, many have 

observed that overvoltage, overdischarge, 

overcharge, heating, arcing, crush, nail penetration, 

external short, and internal short due to defects 

could lead to thermal runaway and electrolyte 

leaks, smoke, venting, fire, and even explosion [4]. 

Although many of the electrical and mechanical 

controls could almost eliminate or significantly 

reduce the probability of these events, such as 

overvoltage and crush, thermal runaway due to 

internal short because of native contamination is 

still a concern. It is estimated that today’s 18650 

Li ion cells (produced in high quantities for 

computers and other electronics) have an internal 

short failure rate of 1 in 5-10 million. We estimate 

that each battery pack for future (P)HEVs may 

have between 100 and 200 larger cells, and of 

course much larger ampere-hour (Ah) capacity for 

PHEVs. Although the battery packs in HEVs and 

PHEVs use much larger cells, and their production 

quality is not yet established, one can assume the 

same failure rate of 18650 cells for larger cells. 

This means that roughly 1 cell in roughly 50,000 

packs may have an internal short leading to a 

safety event. This is too high. The back could be 

designed in such a way that, as a cell goes to 

thermal runaway, design features could prevent 

propagation from cell to cell, eventually reducing 

the pack safety events to 1 in 1 million or more. 

2.2 A Few Large Cells vs. Many Small 

Cells 

Another integration issue that has to be considered 

is using many small cells versus using fewer larger 

cells for PHEV and EV applications. For example, 

EnergyCS uses more than 2000 small 18650 cells 

(2 Ah) for their PHEV pack conversion. Chrysler 

uses 200 much larger cells (41 Ah) for its PHEV

 
3 

 

conversion pack. Using many small cells has 

advantages, including lower cell cost 

(commodity market), improved safety (faster 

heat rejection), the small magnitude of safety 

events because of the smaller cell, and higher 

quality production. However, disadvantages 

include many interconnects, much higher 

integration and assembly costs, lower weight and 

volume efficiency, lower reliability (many 

components, but some redundancy), and costly 

electrical management. Using fewer large cells 

has advantages that include lower assembly cost, 

higher weight and volume efficiency, and higher 

reliability (a lower number of components). 

However, the disadvantages include higher cell 

cost, lower quality, more difficult thermal 

management, and the large magnitude of safety 

events. The final decision about which system to 

use must depend on a trade-off analysis for 

specific application. A comparison of thermal 

performance is given in Section 4.

2.3 Cylindrical vs. Prismatic Cells 

Another item to be considered for integration is 

the use of cylindrical cells versus prismatic (or 

laminate) cells. Cylindrical cells can be 

produced in high volumes and with high quality. 

But as the need for various shape-factors

changes, the cost advantages may be less. 

Cylindrical design is robust and structurally 

strong, particularly for handling, shock, and 

vibration, and it has the ability to maintain 

pressure and venting to prevent safety events. As 

their size gets larger, the ratio of their external 

surface area to volume decreases; thus, heat 

transfer ability goes down and the internal 

temperature gradient can increase. The prismatic 

or laminate design could have a higher heat 

transfer surface area to volume ratio and could be 

thermally managed easier. Prismatic cells could 

be packaged with better volume efficiency than 

cylindrical cells. However, if soft pouch 

packaging is used, attention must be paid to 

preventing local stresses. Handling, shock, and 

vibration must be considered in the design. These 

could add volume and weight and could reduce 

the volumetric advantage of pouch prismatic 

cells. A comparison of thermal performance is

given in Section 4.

3 Battery Management System

3.1 Battery Electrical Control System 

The battery management system for a battery pack 

is used to monitor and control the voltage 

differences between cells and the temperature of 

individual cells within the pack. The battery 

management system ensures that the cells are not 

allowed to exceed the manufacturer’s specified 

voltage and temperature for the battery system. In 

order for the cells to remain balanced during and 

after cycling, a balancing circuit is required. Most 

balancing circuits consist of a buck-boost circuit or 

a buck/resistive balancing circuit. The balancing 

resistors for large capacity cells may need to be 

larger in order to handle the voltage differences 

between cells during HEV and PHEV cycling –

more energy may be wasted to bring the cells into 

balance. Furthermore, a cooling circuit may be 

required for the balancing board. Also, when 

smaller cells are placed in parallel to increase the 

capacity of the pack, the voltage of the parallel 

string is monitored as compared to the voltage of 

each individual cell. By monitoring the parallel 

string of cells, the failure of a single cell may be 

missed and may lead to a damaging situation. 

Figure 3 shows an example of electrical 

components in a battery management system.

Figure 3: Electrical components of a battery pack, from [3]

3.2 Battery Thermal Control

Thermal management of batteries is critical in 

achieving the desired performance in a lowtemperature environment and the desired life in the 

high-temperature environment. High temperatures 

degrade the life of the Li ion batteries, while cold 

temperatures reduce power and energy capabilities,

thus limiting their driving range or performance 

capabilities. A thermal management system is 
4 

PRISMATIC Base (AP) Small (BP) Thin (CP) 

Capacity 1 x 20Ah 3 x 6.7Ah 1 x 20Ah 

Dimension(mm) 100x140x15 100x140x5 140x200x7.5 

CYLINDRICAL Base (AC) Small (BC) Thin (CC) 

Capacity 1 x 20Ah 3 x 6.7Ah 1 x 20Ah 

Dimension(mm) R:42.2 H:150 R:28.5 H:110 R:36.6 H:200 

thus needed to either heat the batteries in cold 

temperatures or cool them in high temperatures. 

Either of these cooling and heating systems adds 

cost, weight, and volume to the battery pack. 

Electrochemistries that are more tolerant to low 

and high temperatures are being pursued, but it is 

a challenging R&D effort. To cool the batteries, 

air, liquid, refrigerant, or fin cooling is 

considered. For all of these systems, heat has to 

be rejected outside the vehicle. Some air cooling 

techniques, such as those in the Toyota Prius,

pass cooled cabin air (by the vehicle’s air 

conditioner) through the batteries. In liquid or 

fin cooling systems, a secondary refrigeration 

loop to reject the heat may be needed. In most 

cases, either air or liquid cooling, with the aid of 

a refrigeration system, is sufficient for keeping 

the battery temperature below damaging limits. 

However, depending on the location of the 

battery pack in the vehicle and the magnitude of 

cooling availability, neither air nor liquid cooling 

is sufficient, and a phase-change material should 

be used. 

One important factor in thermal management is 

not only to maintain the maximum battery 

temperature below the high limits, but also to 

maintain uniformity of temperature between 

cells. It is desirable for this variation to be in a 

tight range (for example, less than 5°C) for 

improved balancing of the pack.

4 Cell Thermal Analysis 

Figure 4 shows schematics of procedures for a 

power profile evaluation of a 20 Ah cell with the 

US06 driving scenario for a mid-size PHEV10 

application. During the initial drive (about the

first 10 minutes), the vehicle consumes electric 

energy stored in the on-board battery system; this 

is the charge-depleting (CD) drive. After that, the 

system maintains the state of charge of the 

battery and drives the vehicle in normal hybrid 

mode; this is the charge-sustaining (CS) drive. 

So, the battery is used more intensely during the

initial drive. 

Impacts of the size (capacity) of unit cell and 

form factor on the thermal response of the cell 

are investigated, focusing on CD drive conditions

through simplified thermal analysis; 

electrical/electrochemical impacts of designs 

were not considered. (Capacity and dimensions 

of the compared prismatic and cylindrical cells 

are shown in Table 1 and Table 2.) 

Figure 4: Schematics for 20 Ah cell power profile 

evaluation with PHEV10 application (120 Wh/kg cell, 

78 BSF)

Table 1: Comparison of the capacity and dimension of 

prismatic cells

Table 2: Comparison of the capacity and dimensions of 

cylindrical cells

The heat generation rate per 20 Ah cell was 

calculated and is presented in Figure 5. For 

modelling purposes, time-averaged values were

evaluated for given period of time, as shown in the 

graph, and applied as inputs to the following 

thermal simulation analysis. All external 

boundaries of each cell are assumed to be used for

surface cooling where the convective heat transfer 

coefficient is fixed at 20 W/m2

K for 30oC ambient 

temperature. Orthotropic thermal conductivities 

(27 W/mK in the electrode layer parallel direction 

and 0.8 W/mK in the layer normal direction) are 

applied to composite jellyroll volume. 
5 

0 5 10 15 20

0

10

20

30

40

50

60

Time(min)

Heat Generation (W)

instantaneous

averaged

Figure 5: Instantaneous and averaged heat generation 

rate per cell (20 Ah)

Figure 6(a) compares average temperature rise 

and internal temperature variation of Cell AP and 

Cell BP. They are prismatic cells having the 

same base area (100 mm by 140 mm) but 

different thicknesses and capacities. As shown in 

the graph, constructing a battery system with 

multiple small cell parallel banks (Cell BP) 

rather than with a single large cell string in series 

(Cell AP) could allow for better thermal 

management. The large cell (AP) shows about a 

15o

C temperature rise with a 2oC internal 

temperature difference, while the small cell (BP) 

shows less than a 10o

C temperature rise with a 

1o

C or less spatial temperature imbalance due to

the increase in available cooling surface area and

the decrease in thermal diffusion distance.

Using small cells quickly increases the 

complexity and the cost of the coolant flow 

pathway design and system assembly, however. 

Cell AP and Cell CP have the same capacity but 

a different form factor. Cell CP is thinner and has 

a wider base area, to have a larger cooling 

surface and shorter thermal diffusion length in

the layer normal direction. 

Making a cell thinner would help in thermal 

management, as seen in Figure 6(b), but a larger 

base area would cause severe current 

convergence and nonuniform material use issues

where tabbing design becomes critical for cell 

performance and degradation. Figure 6(c) shows 

the importance of a material property change. 

When thermal conductivity in the layer normal 

direction decreases fourfold, the internal 

temperature difference increases from 2o

C to 5oC 

for a given case.

Figure 6: (a) Comparison of average temperatures (solid

line) and maximum/minimum temperatures (dotted) time 

variation for Cell AP (blue) and Cell BP (red) with midsize PHEV10 US06 driving scenario; (b) same for Cell AP

(blue) and Cell CP (red); (c) same for Cell AP with 

different layer normal conductivity, 0.8 W/mK (blue) and 

0.2 W/mK (red) 

Thermal management is more difficult for large 

cylindrical format cells than it is for prismatic 

cells. The thermal responses of a large cylindrical 

cell (AC) and a small cylindrical cell (BC) are 

compared in Figure 7(a), and impacts of form 

factor are compared in Figure 7(b). In large 

cylindrical cell (AC), the temperature still 

increases during the CS drive, which implies heat 

rejection from the cell is slower than heat 

generation, even in CS mode. The temperature 

difference between the cell’s center and its surface 

reaches 5o

C for AC and 3oC for BC. A tall, thin 

cylinder would be better in thermal aspects, as 

seen in Figure 7(b). However, a thin cylinder 

could cause a longer electron current path, leading 

to degradations in performance and life. 

(a)

 (b)

(c)
6 

Battery Thermal Responses

3D

Component

Analysis

System

Analysis

Cell Characteristics

Module Cooling 

Strategy

Operating

Conditions

Design Process

0 5 10 15 20

30

35

40

45

50

55

Time(sec)

Temperautre (oC)

0 5 10 15 20

30

35

40

45

50

55

Time(sec)

Temperautre (oC)

0 5 10 15 20

30

35

40

45

50

55

Time(sec)

Temperautre (oC)

Figure 7: (a) Comparison of average temperatures 

(solid) and maximum/minimum temperatures (dotted) 

time variation for Cell AC (blue) and Cell BC (red) 

with mid-size PHEV10 US06 driving scenario; (b) 

same for Cell AC (blue) and Cell CC (red)

Figure 8 simply compares the thermal responses 

of nominal 20 Ah prismatic (AP) and cylindrical 

(AC) cells.

Figure 8: Comparison of average temperatures (solid) 

and maximum/minimum temperatures (dotted) time 

variation for Cell AP (blue) and Cell AC (red) with 

mid-size PHEV10 US06 driving scenario

5 Computer-Aided Engineering 

Tools

Because of the complexity of the integration 

issues associated with thermal, mechanical, 

electrical, safety and packaging of cells into 

battery packs, we believe that seamless design 

tools that combine all of these aspects into one

integrated software would increase the speed at 

which the battery packs could be designed. This in 

turn would reduce the product cycle time as well 

as the pack cost. For thermal design and 

management of cells and packs, NREL has 

developed a set of analysis tools that are used to 

aid engineering in order to understand the thermal 

performance of cells and batteries under various 

loads and environmental conditions as well as 

cooling/heating approaches. This design 

process/tool, depicted in Figure 9, uses various 

commercial software (codes) but it is not 

seamlessly integrated. Results from one tool need

to be linked to another tool by an expert, and 

iterations could be lengthy. 

Figure 9: Example of a battery thermal design and 

analysis process [2]

This or similar processes need to be integrated into 

one code/software so it would be easier to use and

to train engineers for the eventual goal of 

seamlessness. In addition to thermal design, the 

electrical design/analysis and control must also be 

added to this tool. However, including all 

temperature distributions of a cell into an electrical 

model may be too tasking computationally, so 

simplification may be needed. For example, at 

NREL we have developed a thermal and electrical 

integration network model of a multi-cell battery

that assumes that each cell can be represented by 

five thermal nodes. The nodes provide 

temperature-dependent output resistance to the 

electrical model of a cell represented by an 

equivalent circuit modeling approach (Figure 10). 

Further work to develop these integrated 

(a)

(b)
7 

Unit Cell Model

Multi-Cell Thermal & Electrical 

Network Model

5-Node

Thermal 

Model

Electrical 

Performan

ce Model

Thermal 

Network 

Model

Electrical 

Network 

Model

electrical/thermal interaction electrical/thermal interaction

computer-aided engineering tools is needed to 

advance the state of the art of battery pack 

integration and to make battery packs less 

expensive.

Figure 10: Example of an integrated thermal and 

electrical network model of a multi-cell battery

6 Concluding Remarks

Integration of cells into modules and then into 

battery packs is critical in achieving optimum

battery cost, performance, and life. Many 

electrical, thermal, and mechanical issues must 

be considered. The smaller capacity cells allow 

for easier packaging of the battery system but 

result in a higher number of interconnects and 

failure points. The smaller capacity cells are also 

easier to keep isothermal due to their smaller 

size; however, providing cooling air of a 

consistent velocity and temperature to the cells 

becomes a more difficult problem. Incorporating 

larger capacity cells in an HEV or a PHEV limits 

the number of interconnects but also limits how 

the cells can be packaged within a vehicle. They 

also make it more difficult to keep the center of 

the cell isothermal because of the thermal path 

length. A computer-aided-engineering tool must 

be developed to address integration issues.

Acknowledgments

This work was sponsored by the United States 

Department of Energy, Office of Renewable 

Energy and Energy Efficiency, Vehicle 

Technologies Program. We would like to thank 

David Howell of Energy Storage R&D for his 

support.

References

[1] Kevin Konecky, “Battery Integration Hybrid 

Vehicles,” Proceedings of the Advanced 

Automotive Battery Conference, Tampa Bay, 

Florida, May 2008.

[2] Ahmad Pesaran, “Battery Pack Integration for 

Plug-In Hybrid Vehicles,” Proceeding of the PlugIn 2008 Conference and Exhibit, San Jose, CA, 

July 22-24, 2008

[3] Delphi, “Electrical Control of HEV Batteries,” 

Proceedings of the Advanced Automotive Battery 

Conference, Tampa Bay, Florida, May 2008.

[4] G.H. Kim, A. Pesaran, “Analysis of Heat 

Dissipation in Li-Ion Cells and Modules for 

Modelling of Thermal Runaway,” Proceeding of

3rd International Symposium on Large Lithium 

Ion Battery Technology and Application, Long 

Beach, California, May 15-18, 2007.

[5] A. A. Pesaran, G. H. Kim, K. Smith, and E. Darcy, 

“Designing Safe Lithium Ion Battery Packs Using 

Thermal Abuse Models,” Proceedings of Lithium 

Mobil Power Conference, December 7-8, 2008, 

Las Vegas, NV. 

Authors

Ahmad A. Pesaran is a Principal 

Engineer at NREL and leads the 

energy storage team. Ahmad manages 

several projects for Department of 

Energy and industrial partners, which 

include thermal characterization and 

analysis of batteries as well as

modelling and simulation for hybrid 

and plug-in hybrids. He holds a Ph.D. 

in thermal engineering from UCLA.

Matt Keyser is a Senior Engineer at 

NREL and manages the Energy 

Storage Lab for performing thermal 

and electrical characterization of 

batteries and ultracapacitors. Matt has 

tested many Li ion batteries and 

recently has designed and fabricated 

the largest battery calorimeter in the 

world that can measure heat from large 

modules and battery packs.

Gi-Heon Kim is a Senior Engineer at 

NREL. His recent research interests in 

advanced vehicle energy storage 

system tasks include development of a 

three-dimensional lithium ion battery 

thermal abuse model, a 3D 

electrochemical modelling of Li ion

cells, and modelling hybrid electric 

vehicle/electric vehicle battery thermal 

management systems. He holds a 

Ph.D. from Colorado State University.
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14. ABSTRACT (Maximum 200 Words)

The main barriers to increased market share of hybrid electric vehicles (HEVs) and commercialization of plug-in 

HEVs are the cost, safety, and life of lithium ion batteries. Significant effort is being directed to address these issues 

for lithium ion cells. However, even the best cells may not perform as well when integrated into packs for vehicles 

because of the environment in which vehicles operate. This paper discusses mechanical, electrical, and thermal 

integration issues and vehicle interface issues that could impact the cost, life, and safety of the system. It also

compares the advantages and disadvantages of using many small cells versus a few large cells and using prismatic 

cells versus cylindrical cells.

15. SUBJECT TERMS

lithium battery; battery management; battery cooling; electric drive; battery modelling; vehicle thermal management; 

hybrid electric vehicles

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