7

Lithium-Ion Battery Packs for EVs

John Warner

MAGNA E-CAR SYSTEMS, SALES & BUSINESS DEVELOPMENT, AUBURN HILLS, MI, USA

CHAPTER OUTLINE

1. Introduction ................................................................................................................................... 127

2. Lithium-Ion Battery Design Considerations................................................................................ 129

3. Rechargeable Energy Storage Systems....................................................................................... 132

3.1. Lithium-Ion Battery Cells....................................................................................................... 132

3.2. Mechanical Structure............................................................................................................. 134

3.3. Battery Management System and Electronics .................................................................... 136

3.3.1. High-Voltage Switches, Contactors and Fuses ............................................................... 139

3.3.2. High-Voltage Interlock Loop .......................................................................................... 139

3.3.3. Manual Service Disconnect............................................................................................ 139

3.3.4. Chargers ........................................................................................................................ 140

3.4. Thermal Management System.............................................................................................. 140

4. Testing and Analysis ..................................................................................................................... 143

4.1. Analysis Tools ......................................................................................................................... 144

4.2. Standardization ..................................................................................................................... 145

5. Applications of Electric Vehicle Rechargeable Energy Storage Systems................................. 145

5.1. Nissan Leaf ............................................................................................................................. 145

5.2. Chevrolet Volt ........................................................................................................................ 146

5.3. Ford Focus BEV....................................................................................................................... 147

5.4. Toyota Prius PHEV ................................................................................................................. 148

5.5. Mitsubishi

“I” ......................................................................................................................... 148

6. Conclusions .................................................................................................................................... 149

Nomenclature ..................................................................................................................................... 150

References........................................................................................................................................... 150

1.

Introduction

With the growth of the vehicle electrification market, increasing environmental and
government regulations, growing urbanization trends, rising fuel costs and an emerging
customer market, there is a need to better understand the Li-ion battery that is, quite

Lithium-Ion Batteries: Advances and Applications.

http://dx.doi.org/10.1016/B978-0-444-59513-3.00007-8

127

Ó 2014 Elsevier B.V. All rights reserved.

literally, driving the market. In addition to the growing market drivers, we find that there
are few university programs devoted to battery engineering and design. Both these factors
lead to the greater need to understand the design considerations for large, high-voltage
batteries that are being designed to power the future of vehicle electrification. This
work will attempt to cover some of the main considerations for Li-ion battery design for
the automotive applications.

Battery pack development for electric vehicles (EVs) and plug-in hybrid electric ve-

hicles (PHEVs) includes many of the same considerations involved in the development of
battery packs for hybrid electric vehicles (HEVs). Typical Li-ion battery packs, also called
rechargeable energy storage systems (RESS), generally include four main components: (1)
lithium-ion battery cells, (2) mechanical structure and/or modules, (3) battery manage-
ment system (BMS) and electronics, and (4) thermal management system.

Three different types of Li-ion batteries are discussed in this chapter. HEVs such as

the Toyota Prius use either NiMH or Li-ion power cells. The HEV requires a power
battery in order to generate the acceleration and accept the power created through
the regenerative braking events. However, while high power is required, very low
energy is needed in these batteries; typically, an HEV battery will have a power to
energy ratio of about 20:1 or more and will operate over a relatively small amount of
the battery state of charge (SOC), which enables up to 300,000 cycles or more over the
life of the battery.

PHEVs require both high power and high energy. Yet, as they typically carry much

higher energy than the HEV but not as much as the battery electric vehicle (BEV), the
capacities of their batteries tend to fall in between the two. Also, as the size of the battery
increases so does the power. The PHEV operates at a power-to-energy ratio of about 12:1
or less and operates up to about 80% of its SOC which allows it to achieve up to about
4000 cycles over the life of the battery.

Finally, the BEV requires a much larger energy battery and, as the energy increases, the

power comes along for the ride. In other words, the BEV needs a Li-ion cell, that is, an
energy cell; the power to energy ratio of the BEV is typically in the range of 4:1 or less. The
BEV battery generally operates at around 90% SOC, which enables cycle life of 3000–4000
cycles depending on the cell selected.

This description is important in that it is important to clarify that there is no

single “silver bullet” Li-ion battery that will work for all applications. This must be
understood as the RESS designer begins setting the requirements for their battery
systems.

The quickly growing market for microhybrids is also moving toward the use of Li-ion

batteries. However, as these have much lower voltages (typically less than 48 V) and are
much simpler, they are not covered in this discussion.

This chapter will discuss RESS design philosophy and focus on the main component

systems including the lithium-ion battery cells, the mechanical structure, the BMS and
control electronics, and the thermal management system. The final section will review
several RESS battery applications.

128

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

2.

Lithium-Ion Battery Design Considerations

The overall battery pack design for any application depends greatly on the Li-ion cells
that are used. The Li-ion cell type will determine the mechanical structure, the thermal
management system, the BMS and the overall packaging.

Battery pack system design also varies based on the vehicle architecture and desired

system performance, such as whether it is a ground-up designed EV, or whether it is a
current vehicle architecture that is essentially retrofitted to become an EV. For example,
General Motors Chevrolet Volt is a series HEV, also referred to as extended range electric
vehicle (EREV), with an architecture that is largely new but whose “bones” are based on
an earlier vehicle platform. The Volt includes a small internal combustion engine (ICE)
in addition to an electric drivetrain. In this design, the ICE acts only as a generator,
charging the batteries which provide propulsion to the wheels. This not only enables the
vehicle to package a smaller battery than in a fully EV but also allows a limited electric
drive range before the engine turns on to power the battery. This method of using an
existing architecture required the battery to be packaged into the available area within
the architecture. In this case, the Chevy Volt battery was packaged in the volume pre-
viously used for the transmission (tunnel) and fuel tank. The Nissan Leaf uses a very
different system design in that it is a fully EV with no ICE, running entirely on battery
power at all times. This not only requires a larger battery (24 kWh compared to the Volt’s
16 kWh battery) but also allows for greater electric driving range (100 miles compared to
the Volt’s 40-mile range). Again, the Leaf is based on the “bones” of the existing Versa
platform requiring the battery to get packaged into existing space, in this case, under-
neath the floor and under the seats, thus allowing minimal change to the core vehicle
platform.

As PHEVs and EREVs have different usage and power profiles than BEVs, different

battery pack designs are used to meet the different performance requirements. A PHEV
tends to operate sometimes like an HEV and other times as a BEV. Both PHEVs and EREVs
need to have enough usable energy onboard to enable the vehicle to achieve the required
electric drive range, usually between 10 miles and 40 miles. Once this range is achieved
and the battery pack capacity has been diminished to a predefined level, the PHEV
operates under ICE power with the battery functioning in a hybrid or charge-sustaining
mode, powering the vehicle and accessories during stops. Once the EREV battery reaches
this minimum capacity, the ICE does not directly power the wheels, but instead provides
constant power to the motor(s) to drive the vehicle and power the accessories. This
strategy of integrating a Li-ion battery and electric drive system with an ICE enables
both the PHEVs and EREVs to achieve combined electric and ICE vehicle ranges of
300–400 miles or more, comparable to ICE vehicles. These usage profiles tend to drive a
higher power to energy ratio requirement in PHEV and EREV applications. PHEV batteries
range from about 5 kWh up to 15 kWh in size depending on the desired electric drive
range. EREV battery sizes range from 16 kWh to 20 kWh with most falling at the lower end
of that range.

Chapter 7 • Lithium-Ion Battery Packs for EVs 129

The BEV tends to follow the same design considerations, but as there is no ICE to act as

backup power source, it requires a larger battery, meaning that the entire driving range
must be met with the onboard battery power and with energy coming from brake
regeneration. With the larger battery size, power needs become somewhat less important
because the larger the battery the more power it will contain. This is not because BEVs
need less power, but rather because larger batteries provide more power. Many BEV
batteries have 20–24 kWh of onboard energy and depending on the size of the vehicle can
have as much as 50 kWh for performance vehicles and even up to 100 kWh for some light
commercial vehicles such as all electric delivery vans.

The reason that the power to energy ratio is different for EV and PHEV batteries is

mainly due to the different usage patterns they experience as described above. Energy
density and power density are important factors to consider while evaluating perfor-
mance needs of different battery designs. The more energy a vehicle can carry onboard,
the greater is the driving range that it can achieve. However, one of the main consider-
ations for system designers is the need to minimize the size of the battery as much as
possible. This is desirable for two reasons. First, the larger the battery, the more space it
takes up in the vehicle, which in turn reduces the payload and carrying capacity and
drives more difficult integration exercises. Second, the larger the battery, the greater is its
cost. So system designers are always on the lookout for innovative ways to reduce battery
size and weight while increasing the energy. This leads to higher energy densities (Wh/l)
and specific energies (Wh/kg).

Closely related to energy density is the power density (W/l), which relates to the

battery’s ability to quickly discharge power during acceleration events and accept
charge during regenerative braking events. Power is typically discussed in terms of
“C-rate”. This term is representative of the amount of time it takes the battery to either
charge or discharge. For example, if a 24 kWh battery was fully discharged in 1 h, it
would equate to a 1 C discharge rate, but if that same 24 kWh battery was discharged
over a 2 h period, it would equate to a C/2 or 0.5 C discharge rate. Charging a 24 kWh
battery with a 3.3 kW charger (typical Level 1 charger) would take roughly 7 h, thus
equating to about a 0.14 C charge rate. On the other hand, using a 6.6 kW charger
(typical Level 2 charger) would just about cut the time required to charge the battery in
half, while the C-rate would increase. The higher the C-rate, the faster the power is being
charged or discharged. Initial vehicle acceleration and regenerative braking events can
produce discharge and charge rates of up to 5 C rate for time frames of about 5–10 s.
Cell selection and design must take these repeated high charge and discharge rates into
account. Current Li-ion battery chemistries and technologies vary greatly in the rate at
which they can charge and discharge without causing long-term damage to the batte-
ries. For example, most lithium iron phosphate (LiFePO

4

) chemistries are able to

maintain high discharge rates without suffering any long-term effects, but generally also
have lower energy densities. On the other hand, many of the new nickel manganese
cobalt oxide chemistries being introduced today are beginning to offer both higher
energy densities and higher C-rates.

130

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

Another consideration helping in Li-ion cell selection is the total current flowing in the

RESS. Under typical operations, a Li-ion battery for a BEV can be charged/discharged at
several hundred amperes (

w300 A is not uncommon). This makes the decision on the size

and number of cells in series very important as this will drive pack impedance which in
turn will drive heat generation of the cells. Since the product I

2

R determines the

amount of heating in the pack, it is important to maintain the cell impedance as low as
possible. Impedance values of

w0.1 U or less will help reducing heating and therefore

reduce and simplify the thermal management system.

Design considerations for Li-ion EV batteries also include factors such as the driving

cycle that is to be tested. Examples of government driving cycles include the US06, the
Urban Dynamometer Driving Schedule and the New European Drive Cycle (NEDC). These
cycles, which were originally designed for emissions certification testing, are representa-
tive of specific regional driving scenarios. The US06 test cycle, for instance, is designed to
replicate a combination of aggressive low-speed “city”-type driving and high-speed
“highway”-type driving. The NEDC also includes both city and highway driving cycles,
but includes the “city” portion of the driving cycle first, so it is also considered a good cycle
for testing EVs. It is still to be determined how representative these cycles are for EVs.

The next key design consideration is the electronics management system. Electronics

is responsible for monitoring temperature, voltage, and current of the cells, blocks and
modules within the pack. The other key purpose of the BMS is to ensure that the cells are
balanced within the pack. Balancing the cells refers to monitoring and ensuring that all
cells are at nearly the same capacity. This is important as the battery will only operate at
the capacity of the lowest cell. There are two main concepts around cell balancing, active
and passive balancing. Active balancing refers to a system that actively monitors the
capacity of each cell and moves energy from one cell to the other to keep them balanced.
A passive system also monitors each cell but uses a resistor to burn off excess capacity of
the highest capacity cells in the form of waste heat in order to maintain the balance. The
system electronics also include safety features such as a manual service disconnect
(MSD), used to break the potential of the pack in half (or into smaller “subpacks” with
lower potential), and the high-voltage interlock loop (HVIL) used to activate the MSD fuse
if any housings are opened or connections are broken.

Another factor that must be considered is the region where the vehicle will be sold and

used. This is of key importance as region impacts the ambient temperature requirements
and therefore the thermal management system design. Temperature can have one of the
greatest long-term impacts on lithium-ion battery life, as battery use at constant high and
low temperature reduces the cycle life of virtually all lithium-ion chemistries in the
market today. Constant use under extreme temperature conditions tends to speed
the growth in internal resistance and impedance within the cells, which in turn impacts
the cycle life by reducing the amount of active capacity. Li-ion batteries installed in
vehicles going to colder climates may need less cooling (often air cooling may be
adequate in these applications) but may require additional heating in order to avoid
freezing of the electrolyte at very low temperatures. On the other hand, vehicles with

Chapter 7 • Lithium-Ion Battery Packs for EVs 131

Li-ion batteries going to extremely hot climates may require a more active cooling
methodology but do not need any heating (often a liquid cooling strategy may be used in
these circumstances). And of course, OEMs are designing vehicles to be used across re-
gions and therefore to meet both temperature extremes.

The thermal management system is responsible for maintaining the temperature of

the cells within the pack at consistent levels, usually around 25

C for optimal Li-ion

battery life. Thermal management refers to both heating the battery during cold tem-
perature operation and cooling it during high current discharge and high ambient
temperatures. This may be done by forcing air through the battery (often using air that
has been chilled to about 10

C); alternatively, some systems use liquid heating and

cooling, where aluminum plates are placed between the cells and a mixture of water and
glycol is forced through them to manage the battery temperature.

3.

Rechargeable Energy Storage Systems

The RESS consists of an integration of several key elements, including the lithium-ion
cells, the modules those cells are assembled into, the overall mechanical structure, the
thermal management system, and the electronics consisting of BMS, cell balancing
boards, and the high-voltage connections, switches and disconnects.

Not all RESS systems include all these components as some systems install the bat-

teries directly into the mechanical enclosure, while others use a variety of modules and
monoblocs to subassemble the cells. Some systems break the battery into separate
“packs” that are installed in different locations in the vehicle, while others install all
components into a single “pack”. Some systems integrate cell balancing and monitoring
electronics into the modules, whereas other systems integrate the cell balancing and
monitoring into the core BMS circuitry. Some systems use heated or chilled air thermal
management systems, while others do not actively manage the battery system at all.

The following sections will review each of these main subsystems of the Li-ion battery

in greater detail.

3.1.

Lithium-Ion Battery Cells

Lithium-ion battery cells come in many shapes and sizes, quite literally. The most widely
produced cells for any application today are the 18650 cylindrical (18 mm diame-
ter

65 mm length) types as they are used in portable power applications such as laptops

and similar devices; 18650 cells have capacities in the range 2 Ah to

w3.5 Ah. Therefore, a

higher number of cells are required for EV applications. However, since the EV industry
has emerged, there is yet to be a consensus on any standard cell, concerning size,
chemistry or format.

The benefit of using small cells is that many of them have “built-in” safety features

such as temperature-driven resettable thermal fuses and pressure-driven current inter-
rupt devices and vents. The small cells may also reduce the impact of a single-cell failure

132

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

by reducing the potential for cascading failure propagation. Small cells are also easier to
“bin” during the manufacturing process, thus allowing cells of very close capacities to be
used in a single application and reducing the need for cell balancing. Additionally, as
these cells are today manufactured in very high volumes (billions per year), the cell cost is
lower than for other formats. The main issues and OEM concerns are the reliability
related to the much higher volume of cells (e.g. more than 6800 for the Tesla) needed to
power an EV application and the potential for higher complexity related to this volume.

In addition to the 18650 cylindrical cells, the polymer cell (also known as laminate or

pouch) has become the favorite of many US and European auto manufacturers. Indeed,
many OEMs tend to prefer fewer parts in order to achieve greater reliability and reduce
costs and therefore look for solutions with fewer cells in the pack but with demonstrated
safety. This drives the demand for a “large format” cell, typically falling between 20 Ah
and 100 Ah in capacity. The polymer cell includes an anode, a separator and a cathode
that are assembled into either a “Z-fold” or a stacked configuration. In the Z-fold
configuration, the anode, separator and cathode are folded together in a continuous
run to make a single cell. In the stacked configuration, separately cut anode and cathode
pieces are stacked together with a separator in between to form the final cell. Once
assembled, these cells are referred to as “jelly rolls”, which are then electrically connected
using tabs welded at different locations in the cells to transfer the current to the terminals.
The jelly rolls with assembled tabs are then installed into a soft aluminum laminate-based
“pouch” that is sealed using an adhesive. The drive for high levels of safety demands the
cell to pass nail penetration abuse testing to the European Council for Automotive R & D
Level 3 (no venting, fire or flame; no rupture; no explosion) or less. This safety require-
ment drives the recommendation to use ceramic separator coating technology.

There are several benefits using a pouch design, such as the ability to build cells of

different sizes using the same chemistry and the same basic cell designs, the flexibility to
design the cells into shallow battery packs, the larger cell capacity (15–40 Ah and more
than 100 Ah in some applications) which allows fewer total cells, and the relatively high
energy density of the cell itself. However, as these cell formats are still a relatively new
technology, ramping up the manufacturing to volumes and quality that match those of
the 18650 cell will take time. Other concerns are the lack of integrated safety features, the
need of external devices to exert stack pressure on the cells, the risk of cascading failure
(much more energy is released in a large cell failure), and the risk of damage to the soft
cell packaging. Yet, even with these challenges, laminate cells are the preferred cell format
for many major auto manufacturers today.

Another cell format that is seeing wide use is the “prismatic” one. Cells of this type can

range from very small (

<4 Ah) to very large (>250 Ah) and can greatly vary in cell design

and style as they may use aluminum or plastic enclosures. Prismatic cells generally use
either a rolled of folded cathode/anode/separator assembly installed in either an
aluminum or a plastic housing. Depending on the size of the cell, the anode and cathode
may use a Z-fold, a stack, or a “rolled” design. The rolled design basically replicates the
processes and systems used in the 18650 cylindrical cell manufacturing where the anode

Chapter 7 • Lithium-Ion Battery Packs for EVs 133

and cathode, with a separator in between, are attached to a spindle that rotates and
creates a circular jelly roll. In the prismatic cell design, the components are then pressed
to create a flat, oblong jelly roll. Examples of these cells are reported in

Figure 7.1

.

One of the benefits of the prismatic cell is that it has the higher capacity typical of

laminate cell, but it also may integrate some of the safety features of the small cells such
as vents and, occasionally, thermal fuses. The prismatic cell may also use a mechanical
connection to the cell which reduces the need to weld the bus bars to make the cell to cell
connections in a pack. The issues of this cell format include the risk of cascading failure
(much more energy is released in a large cell failure). In addition, as these cell formats are
still produced in relatively low volumes, ramping up the manufacturing to volumes and
quality that match those of the 18650 format will take time.

All these cell formats are currently being used in either demonstration fleet or

production HEVs, PHEV, and BEVs today.

3.2.

Mechanical Structure

The second key component of an RESS is the overall mechanical structure of the system.
This includes design considerations such as whether to use a single-pack structure or
multiple packs, generally depending on the location in the vehicle where the packs are to
be mounted; the integration with the vehicle frame and body structure; and

FIGURE 7.1 (a) Image of 6.6 V, 8.8 Ah Nanophosphate

Ò

AHR32113 power modules

(Source: A123); (b) 3.7 V, 35 Ah

Boston-power swing key block. (For color version of this

figure, the reader is referred to the online version of this

book.)

(Source: Boston-Power).

134

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

considerations for how the cells will be installed: into a standardized module, a monobloc
design, or directly into the pack structure.

Mechanical integration came to the forefront of pack development in the summer of

2011 after the National Highway Traffic Safety Administration testing on the Chevy Volt
resulted in subsequent fires in the vehicles. These tests drove General Motors to add more
structural components to the Volt to improve protection of the Li-ion cells and increase
its ability to survive side-impact crashes. As many RESS systems are designed into
locations such as trunk space, under front and rear seats and in fuel tank areas, the level of
structural reinforcements that are necessary become essential to RESS survivability and
safety. Basically, when the RESS installation overlaps a crash zone or is integrated into the
frame area, there is a need for greater levels of structural integration in order to protect
the pack.

The Li-ion RESS, whether integrated as a structural element into the vehicle or not,

may have specific requirements, such as material type, mass, impact and crush, elec-
tromagnetic compatibility, shock, vibration, humidity, and liquid intrusion, that will be
unique for each vehicle. Depending on the regional and OEM-specific requirements, the
pack may need to be sealed from the environment to Intrusion Protection (IP69) or
similar criteria. This means that the pack will not allow any dust or liquid intrusion and
will survive submersion. This is most common if the pack is mounted external to the
vehicle. However, if the RESS is mounted inside the vehicle, the IP sealing requirement
may be somewhat lower as there will be less opportunity for the pack to see liquid
intrusion, but it may still need some level of sealing in order to prevent any potential cell
gases to escape the pack during impact, crash scenarios or cell failure.

Another aspect of pack design that is crucial to understand prior to undertaking a

project is whether the pack is to be permanently mounted in the vehicle or is “remov-
able”. There are a number of companies today working on the development of the
replaceable EV battery pack. Perhaps the most widely known is Better Place, this com-
pany has already signed agreements with several auto manufacturers and regional gov-
ernments for the installation of a series of battery changing stations. In this instance, the
EV drives into the changing station in much the same manner as the traditional ICE
vehicle drives into an oil change station. Once in the station and properly aligned, an
automated service detaches the depleted battery and replaces it with a new, fully charged
battery pack. This exchangeability drives a whole new set of requirements into the battery
system. In this instance, the RESS must be mechanically and structurally sound on its
own as it will be picked up and moved from the storage location to the vehicle and back.
In addition to this added structural requirement, the battery design must include auto-
matic interconnections for the high-voltage power supplies, vehicle to battery
communications, and thermal management systems. All these subsystems must be
designed to be able to automatically make their connections in a highly reliable manner,
each and every time.

Evaluating and managing the mass of the RESS will prove to be important as the greater

the mass, the more energy it will take to move the vehicle. Mass is evaluated in much the

Chapter 7 • Lithium-Ion Battery Packs for EVs 135

same manner as it is for the ICE engine, by looking at the material selection for every
component but especially for the largest items such as the enclosure and Li-ion cells.
Reducing the mass of the battery system, while maintaining high-energy levels, will
generate greater driving range. Many manufacturers are working with materials such as
aluminum, magnesium, and structural plastics in order to reduce the weight of the pack.

In parallel with the overall structural integration into the vehicle there, is the need to

develop and use a common cell-mounting strategy. As was noted above, there are a
multitude of cell designs and currently there is no industry standard for module shapes
and sizes. A module is the mechanical structure that interconnects the cells into a single
electromechanical unit. Module sizes vary greatly due to the variety of cells, the class of
vehicles (A segment up to E segment and on to light commercial vehicles), the thermal
management strategy (air vs liquid), etc. Each of these attributes drives great variation in
module design between manufacturers and even within a single manufacturer between
different vehicles. While the size and shape vary greatly, most module designs include
some common attributes such as the manner of connecting the cells together, assembling
them into a mechanical structure, integrating cell or module monitoring and balancing
electronics, and integrating a thermal heating/cooling system into the module.

Another variation on the module concept is being introduced by some manufacturers

of small cells, which is referred to as the “monobloc” or simply as a “block”. This concept
integrates the cell assembly, usually through the use of welded bus bars, into a small
mechanical package with a thermal management system built in (

Figure 7.1

).

3.3.

Battery Management System and Electronics

The BMS is perhaps the most critical component of the RESS as it is responsible for
monitoring the performance of the battery and adjusting the system to match the usage
and environment. The BMS is a complex system in and of itself, including not only the
main system monitoring circuit but also balancing circuits, often mounted at the module
or cell level, communication, safety circuitry and multiple fuses.

The BMS and subsequent algorithmic models used to control the battery are designed

to monitor and control the battery’s behavior in different load conditions, including
monitoring SOC, state of health, state of life, both cell and pack temperatures, and current
demands

[1]

. The design of the BMS should include hardware and software for, at a

minimum, acquisition, storage (or transmission) of performance data; system safety
protections and provisions; measurement and prediction of the current state of the
battery; charge and discharge control functions; balancing of cells; delivering battery
status and authentication to the vehicle system; communication with all battery system
components; and prolonging the life of the battery

[2]

. While there are many character-

istics of the battery that must be managed, those noted above are key to optimizing the
battery system to achieve peak performance and life.

Today, there are several main topologies that are being implemented in BMS devel-

opment, including centralized, distributed and modular BMS

[2]

. In a centralized BMS

136

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

system, the hardware and software are designed into a single physical control unit, with
sensors mounted throughout the battery system to provide feedback on battery perfor-
mance. One potential downfall of the centralized system is the large amount of wiring
that is needed to connect the system. However, it is also potentially the least expensive
system design as less hardware is required. In the distributed BMS system, the main
controller is mounted in one location with cell balancing and monitoring circuitry, often
referred to as a slave board, located separate from the host controller and frequently
mounted on or near the modules and cells. This system benefits from much less complex
wiring as there is only a single series communication set between each unit in the system,
but suffers from higher cost due to the multiple circuit boards needed. In the modular
BMS system, there may be multiple host controllers, each with the ability to manage a
limited number of cells or modules. From a benefit and loss perspective, the modular
system encompasses aspects of both the centralized and distributed system with more
wiring than a distributed system but more control circuits than in a centralized one.

When multiple cells are connected in either series and/or parallel configurations to

achieve the voltage and capacity requirements of the system, they may begin to diverge
from one another during use. And, since all lithium-ion cells vary slightly in voltage and
capacity due to variations in manufacturing and chemical formulations, this divergence
can cause capacity fade over the life of a pack. This happens during both charging and
discharging events. During charge, the BMS will stop charging once the lowest cell in the
pack reaches its full SOC. This means that some cells will not be fully charged—in fact they
will likely all be charged to different levels with none but the weakest getting a full charge, as
shown in

Figure 7.2

. Similarly, during discharge, the weakest cell may complete dis-

charging before the rest of the cells reach their fully discharged state, thus reducing the
overall usability of the battery. While the actual differences in the cells may initially be very
small, they may grow significantly over time. These factors will drive greater and greater
variation in the cells over time and this will drive capacity fade, which in turn will cause
premature aging of the pack, reduced capacity and power, and limited driving range

[2]

.

FIGURE 7.2 Example of unbalanced cells. (For color version of this

figure, the reader is referred to the online version of

this book.)

Chapter 7 • Lithium-Ion Battery Packs for EVs 137

Because of this cell imbalance potential, an effective BMS system must have the ability

to balance the cells within the pack; this is done through one of the two methods: active
and passive cell balancing. Active balancing involves use of a transformer, conductor or
capacitor to equalize the differences between the cells by moving the charge from one cell
to another. In essence, capacity is drained from higher capacity cells and transferred to
lower capacity cells to equalize them. Similarly, passive balancing attempts to equalize
the capacity of the cells in the system use dissipative equalizers such as resistors to
“bleed” off the excess capacity or voltage by transferring the excess energy into heat
energy

[2]

, thereby bringing all cells down to the same level as the lowest cell.

Active balancing is still in the relatively early stages of development and commer-

cialization. Today it is considered a higher cost alternative as each grouping of cells re-
quires a separate set of hardware and with benefits that are harder to quantify. Passive
balancing is the most widely used system in EVs today as it is a well-understood and low-
cost system. However, the passive balancing system will waste energy and generate
significant heat in a battery, so design considerations should be made for thermally
managing this system otherwise they may cause cell heating. As battery management
electronics continue to evolve it is likely that active balancing will become more widely
used and sought after as it has the possibility of extending battery life

[2]

.

Another important variable that the BMS manages is the depth of discharge (DOD) of

the battery system. The DOD is the amount of capacity in the battery that is usable by the
system for propulsion and powering the vehicle. In many EV applications, the DOD is set
somewhere between 80% and 95% on the “top” end of the battery. The reason for this is to
avoid overcharging the battery cells as this can result in safety failures and reduced cycle
life. At the bottom end, the DOD cutoff is generally set at about 10–20% depending on the
usage cycle. The low end of the SOC setting is based on both the need to avoid
overdischarging the batteries and a capacity minimum that enables “limp home mode”
performance of the battery. The BMS also monitors the battery SOC and makes appro-
priate adjustments. For example, most systems will not allow charging through regen-
erative breaking when the battery SOC is above 90% to avoid overcharging the cells and at
around 20% SOC may reduce performance and warn the operator to charge soon—think
of this as the “low fuel” indicator in a traditional ICE car. Additionally, reducing the DOD
enables longer cycle life. For example, some lithium-ion cells can offer over 1000 cycles
when cycled at 100% DOD down to 80% of original capacity. Automotive OEMs generally
consider the lithium-ion battery at the end of its useful life when its usable energy reaches
80% of the original one. But the same cell may achieve additional thousands of cycles by
continuing to reduce the DOD cutoff range of the cells.

Another facet of battery system design is ensuring that the system has low parasitic

power losses. One type of parasitic loss occurs when electronic system components draw
on the energy stored in the battery to maintain their own operation and functioning. A
RESS with high parasitic losses will draw energy from the battery that could otherwise be
used for electric propulsion. While the actual amount of current that is required to
operate the system electronics is generally very small, over time—especially if the vehicle

138

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

is not used—the cumulative impact can be great. In other words, high parasitic losses will
reduce the range of the EV.

3.3.1.

High-Voltage Switches, Contactors and Fuses

Another important part of the high-voltage circuits are the switches, or contactors, that
are closed and opened to energize and de-energize the system. The contactor is an
electromagnetically operated switch that is capable of carrying large amounts of current.
There is typically a set of fixed contacts and a set of moving contacts within the contactor.
When the system is energized, an electromagnet engages a metal rod that makes contact
with the current-carrying parts of the system closing the circuit.

As part of this circuitry, many systems also include a “precharge” circuit that is made

up of a secondary “loop” with a second set of contactors that are opened to “precharge”
the system before the main contactors are closed. This precharge contactor prevents a
large current in-rush into the system.

Fuses are also an important aspect of the high-voltage circuit design: as the battery

stores a high amount of energy, the fuse prevents a direct short across the positive and
negative connections in the system. Without a fusing device, the battery would short and
continue to supply power until the it experiences irreversible damage.

3.3.2.

High-Voltage Interlock Loop

As almost all electrified vehicles operate at high voltages, it is worthwhile to briefly discuss
the components that enable and protect the RESS. One common part of the safety system
is the HVIL. The HVIL is a system of electric switches and circuits that will disconnect the
battery and prevent contact with the high-voltage electrical devices and components
during installation, service and repair. The HVIL has several functions; the first is to
switch the high-voltage circuit loop to interrupt the flow of current through the entire
high-voltage circuit. By opening the circuit a signal is sent to the master, or host, BMS
controller to de-energize the high-voltage system. Another purpose of the HVIL is to act as
a time delay for anyone trying to access the system. By the nature of its physical shape
and overall design, its action allows the system to de-energize the circuits before anyone
can attempt to access. Engaging the HVIL circuit is a prerequisite to servicing the battery
system as it prevents physical contact to the high-voltage connections by de-energizing
the high-voltage circuits, thereby making them safe to repair, service or install.

3.3.3.

Manual Service Disconnect

Another safety feature of the high-voltage component system is the MSD, also referred to
as the mid pack disconnect, manual service disconnect switch, service disconnect switch,
or emergency disconnect switch. Regardless of the name used, they all serve the same
purpose—to disconnect and de-energize the high-voltage RESS system prior to servicing
or accessing the system. The MSD also acts to “break” the potential of the pack in half to
reduce the voltage and current. The MSD also acts as a key part of the HVIL system using a

Chapter 7 • Lithium-Ion Battery Packs for EVs 139

lever-based system to prevent the system from being short circuited and to “break” the
connection somewhere in the middle of the pack, in essence splitting the potential of the
pack in half.

Most “off-the-shelf” MSDs are designed around a built-in fuse designed for the RESS

at the pack level (generally

w450 V and up to 650 A).

3.3.4.

Chargers

EV charging equipment, or EV battery chargers, fall into three main categories ranging
from 110 V systems up to over 400 V systems. Most EVs come with an onboard “Level 1”
charger as a standard feature. The Level 1 charging unit will typically plug into the standard
household outlet using 110 V, and offers about 3.3 kW of power to the vehicle. Next is the
Level 2 charger which typically offers about 6.6 kW of power and requires 220–240 V. The
Level 2 charging unit must be professionally installed but can charge an EV in about half the
time of the Level 1 charger. Finally, Level 3 charging units use very high voltage, typically
about 480 V but can charge an EV battery in about 10 min. Due to the high voltage and
current of the Level 3 chargers, they are not likely to make their way into private homes as a
means of charging EVs, but will more likely be used in public charging stations. In

Table 7.1

,

the main characteristics of Level 1–3 chargers are reported.

Another charging technique that is under development today but is likely to become

more common is based on the use of wireless, or inductive charging, systems. Some
inductive charging systems have been used in very low volume EV demonstration pro-
grams, such as the General Motors EV1 and the Toyota RAV4. The main difference
between the standard charger and the inductive charger is the lack of direct physical
connection in the latter. Indeed, the vehicle charging system only needs to come in to
close enough proximity to the inductive charger and an electromagnetic current is passed
through them charging the battery. In essence, half of the electromagnet is placed in the
vehicle and the other half in the charging station.

With inductive charging, the risk of electric shock is greatly reduced as there are no

open or exposed connections. However, the current technology suffers from high losses
during transmission, with the best inductive chargers being only about 75–85% efficient.

3.4.

Thermal Management System

The thermal management system is another key component of a lithium-ion battery
design as it regulates the temperature of the lithium-ion cells within the battery during

Table 7.1

EV Charging Levels

Voltage (V)

Current (A)

Power (kW)

Type

Level 1

110

16

1.9

AC

Level 2

208/240

32

19

AC

Level 3

480

400

240

DC

140

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

use. Monitoring and regulating the cell temperature extends the useful life of the cells by
keeping them within their optimal operating range. Thermal management systems are
designed based on criteria such as the region where the vehicle will be used and its
performance profile. An example is represented in

Figure 7.3

.

One of the keys to control the pack temperature is to design a thermal management

system that maintains the cell temperatures within a 2–5

C difference throughout the life

of the pack. As mentioned previously, this is the key to long life of the battery pack.

As the lithium-ion cells begin to age, they experience a natural form of capacity fade

based on the increasing internal resistance. So the closer the thermal management
system can maintain the temperature of the cells, the better performance and longer life
will the battery achieve.

While both liquid- and air-based thermal management systems operate under the

same principle, each has different sets of benefits and challenges ranging from perfor-
mance to cost and complexity of the overall RESS. Both systems include similar system
design techniques.

Liquid-based thermal management systems tend to integrate thermally conductive

(usually aluminum) cooling plates in between the cells of the module, which are then
connected with cooling lines. Liquid flows through a heat exchanger (radiator/evapo-
rator) to reduce the temperature of the liquid before it flows into the modules. The liquid
uses the aluminum plates as heat collectors, or heat sinks, to pull the heat away from the
cells. The heated liquid is then routed back to the heat exchanger to be chilled and once
again return to the pack. One addition that can be made is to add a heating element in
order to heat the liquid during cold temperatures to improve cold weather performance.

The liquid-based thermal management system offers the benefit of effective perfor-

mance as liquids facilitate transfer of heat better than air. However, it is likely that the cost
of a liquid-based system will be somewhat higher than an air-based system due to the
amount of aluminum used as heat exchangers and the addition of cooling lines. Another
benefit of the liquid cooled system is that it can be designed as a “closed loop”; in other
words, the system does not draw liquid or air from outside of the system. There are a
variety of cooling/heating mediums that are used in liquid cooled systems. The most
common is the use of a 50/50 liquid mix of water and glycol. It is also common for liquid
refrigerants to be used.

FIGURE 7.3 Example of thermal system schematic. (For color version of this

figure, the reader is referred to the online

version of this book.)

Chapter 7 • Lithium-Ion Battery Packs for EVs 141

For descriptive purposes, systems designed using refrigerants can be included in the

“liquid”-based cooling systems. They work on the same basic principles as the liquid
systems but are able to reduce the temperature of the coolant faster and to a larger extent
than a typical radiation-type heat exchanger. In this system, there is a condenser which
chills the refrigerant that then flows through a heat exchanger attached to the cells, which
may take a variety of forms depending on the cells used.

The air-based thermal management system uses a similar strategy as the liquid sys-

tem, including a heat exchanger (radiator/evaporator) and often integrating a chiller to
reduce the temperature of the incoming air. In air-based systems, the air is channeled
from the heat exchanger into a series of ducts or channels that drive the air through the
cells or modules forcing the heated air out of the pack and back into the heat exchange
system. The air-cooled system may also integrate a heating element into the design for
use in cold ambients; however, this is not very common as heating of lithium-ion cells
entails safety concerns. One type of heating element that has been used with some
success is a thin film, or Kapton tape-based flexible element. This can be used either to
actively heat the batteries (which may give rise to safety concerns) or to more passively
reduce the rate at which the batteries cool.

The main benefit of the air-cooled system is that it is less expensive than the liquid one

because the pack and module designs can integrate the thermal management channels
into the mechanical structure without additional cost to the system. Another innovation
in thermal management is the use of phase change heat exchange mediums. Some are
being used in conjunction with other cooling methodologies, while others are being used
on their own.

Another important factor in designing an effective thermal management system for a

Li-ion battery is to determine the effectiveness of the system’s ability to transfer temper-
ature. One aspect of this is evaluating the pressure drop from the thermal management
inlet to the exit within the pack. The pressure drop is the change in pressure from where the
cooling medium enters the RESS to where it exits. The pressure drop is caused, at least in
part, by the resistance that the cooling medium experiences during its journey through the
pack. This becomes important in cooling Li-ion batteries as the pressure drop determines
the effectiveness of the cooling system. The lower the pressure drop, the more evenly the
Li-ion cells are able to be cooled. And the more evenly they are cooled, the more evenly they
will “age”. This means that the cell’s impedance growth over time, a natural process in Li-
ion cells, will be even thereby preventing premature reduction in capacity. Alternatively, if
the pressure drop is relatively large, the cells will age at different rates, meaning that the
cells furthest from the cooling medium inlet will see a greater growth in internal impedance
and therefore reduced capacity. As the capacity of the weakest cells determines the overall
capacity of the pack, this divergence of cells will cause the overall pack to see reduced
capacity (range, performance, etc.) sooner than it would otherwise be expected.

Another design consideration that affects both the mechanical structure and the

thermal management system is whether to mount the RESS in a single pack or in multiple
packs in the vehicle. This is often based more on the available packaging space than on
any other factor. However, there are still some design considerations that must be taken

142

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

into account when dealing with multiple packs. From a thermal management standpoint,
managing a single pack is much easier than managing multiple packs. With a single pack,
air cooling (and heating) may be adequate to meet the performance needs of the appli-
cation and keep the lithium-ion cells within the desired temperature range. However,
when multiple packs are used, it may be desirable to thermally manage the RESS using
liquid cooling methods. With a liquid-based thermal management system, it may be
possible to maintain the 2–5

C cell temperature gradient even for separate packs much

easier and more reliably than could be done with an air-cooled system.

Active vs passive thermal management is another consideration that must be evalu-

ated based on the RESS performance that is desired. Most systems today are actively
cooled (and heated) meaning that chilled air (or chilled liquid) is actively blown through
the RESS based on the feedback from the various temperature sensors within the pack.
This type of system allows the cooling and heating to be adjusted by the BMS as needed
based on the current temperature readings within the pack. Ambient air that is not cooled
may also be brought into the pack to remove the excess heat generated during operation.
This method, however, is not as efficient especially in hot climates and during extreme
operating conditions.

An alternative to the actively managed system is a passively thermally managed

system. In this type of application, the pack is generally designed with aluminum heat
exchangers throughout the pack to help pull the excess heat away from the cells during
operation. But, in this instance, there is nothing that is done to transfer the heat away
from the cells during extremes of operation. The passive thermal management system
may be optimal for applications with very low C-rates in moderate climates and is
generally the lowest cost thermal system design but may yield shorter cell life especially if
the application regularly operates at high C-rates or higher temperatures.

4.

Testing and Analysis

Testing and validation of automotive Li-ion batteries falls into two major categories. The
first is performance testing intended to determine how the battery will perform under a
variety of conditions and usage models. The second is abuse testing intended to deter-
mine how the battery will react to various abuse situations, such as crash, impact,
penetration, rollover, and high temperatures.

Several of the major industry groups have already designed, in association with

automotive OEMs, battery manufacturers, and several government organizations, several
overlapping sets of abuse test protocols:

• FreedomCAR Electrical Energy Storage System Abuse Test Manual for Electric and

Hybrid Electric Vehicle Applications

• US Advanced Battery Consortium Electrochemical Storage System Abuse Test

Procedure Manual (SAND99-0497)

• SAE J2464, Electric and Hybrid Electric Vehicle Rechargeable Energy Storage System

(RESS) Safety and Battery Abuse Testing

Chapter 7 • Lithium-Ion Battery Packs for EVs 143

In addition to these testing criteria, other industry groups and organizations have begun
offering additional regional safety testing standards and criteria:

• ANSI/UL 1642, ANSI/UL 2054
• IEC 60086-1, 60086-2, 60086-3; IEC 62133; IEC 61851, 61951, 61960, 62196
• IEEE 1625, 1725
• UL2202, UL2231, UL2251
• ANSI C18.1 Part 1, ANSI C18.3 Part 1
• UN Transportation Testing (38.3 T1-T8)
• SAND99-0497
• SAE J2464 EVB
• Nordic Ecolabel (White Swan)

Battery performance testing also has many regional testing requirements, often based on
regional driving standards. Some examples of Society of Automotive Engineers (SAE)
performance testing include:

• Life Cycle Testing of Electric Vehicle Battery Modules (SAE J2288)
• Vibration Testing of Electric Vehicle Batteries (SAE J2380)
• Recommended Practice for Performance Rating of Electric Vehicle Battery Modules

(SAE J1798)

• Determination of the Maximum Available Power from a Rechargeable Energy Storage

System on a Hybrid Electric Vehicle (SAE J2758)

4.1.

Analysis Tools

Computer-aided engineering (CAE) analysis of the structure also provides a very useful
tool in understanding the flow of forces in the pack. CAE tools can be used to assess how
mechanical structure of the RESS will react in both performance and abuse situations.
From a structural perspective, CAE can be used to determine the RESS behavior under
crush scenarios, rollover scenarios and impact scenarios as well as the overall stresses
that the pack will see during various drive cycles.

With an effective thermal model of the cells, modules and overall system, an analysis

of the performance under different situations and load conditions can be evaluated. This
proves to be a very useful tool in the development of the pack as these thermal models can
be input into computational fluid dynamic (CFD) models to determine how the cells will
heat during operation. A good CFD model can be used to determine flow rates, turbu-
lence, and heat transfer within a pack. In addition, it is possible to use a lumped
parameter model to develop a simplified model where the external parameters are
basically ignored and the model is designed using fully adjustable parameters to do
“high-level” evaluations of the thermal effectiveness of a system.

Another useful analysis and development tool is provided by the use of the hardware

in the loop (HIL) technique. HIL testing is based on the use of an “embedded” set of

144

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

hardware tied to a set of test platforms. This allows software and hardware to be devel-
oped at very rapid cycles, reducing the need to build and test hardware in the early
development phases of a project. In the case of the Li-ion battery, cells and the overall
system may be modeled, or portions such as the BMS and related controllers can be
modeled and validated in real time.

4.2.

Standardization

As indicated by some of the testing standards described above, there are many organi-
zations such as the SAE, the Organization for International Standardization, the
International Electrotechnical Commission (IEC) and the American National Standards
Institute (ANSI) that have begun working on developing standards in the areas of battery
testing and battery development. However, standardization of cells, modules and/or
packs will likely take several years to complete, if it is at all possible.

In 2012, SAE released standard J2464 “EV & HEV Rechargeable Energy Storage System

(RESS) Safety and Abuse Testing Procedure”. This is an attempt to standardize the safety
and abuse testing on EV batteries and builds on the work done under the US government-
sponsored “FreedomCAR” program as well as the US Automotive Battery Consortium
(USABC). USABC was created in 1991 with a goal of helping to drive the development of
advanced high-performance batteries for EVs.

5.

Applications of Electric Vehicle Rechargeable
Energy Storage Systems

In this final section, several of the current production electric and plug-in electric vehicles
will be reviewed, focusing on discussing the specifics related to their batteries in relation
to the key areas discussed in this paper.

5.1.

Nissan Leaf

Introduced in late 2010 in Japan and the United States, the Nissan Leaf uses a 24 kWh
lithium-ion battery pack based on polymer cells from Automotive Energy Storage
Corporation. Leaf is classified as a BEV as there is no combustion engine: the vehicle is
propelled purely with the power contained in its Li-ion battery.

The Leaf’s battery system today contains no active thermal management system.

However, the modules which enclose the cells are made of aluminum, enabling them to
act as heat sinks within the battery, so they passively pull heat away from the cells.

The battery pack, shown in

Figure 7.4

, is mounted beneath the vehicle, fitting under

the passenger and driver seats. This location offers a low center of gravity as the pack is
mounted directly in the middle of the vehicle. This also means that the pack must be
sealed to IP69 criteria to ensure that foreign debris, either liquid or dust, is not allowed to
work its way into the pack.

Chapter 7 • Lithium-Ion Battery Packs for EVs 145

From a performance perspective the Environmental Protection Agency (EPA), using

the US drive cycle, estimates the Leaf’s range at about 73 miles with about 34 kWh per
100 mile energy consumption. The EPA also rated the Leaf’s fuel economy at 99 MPGe
(miles per gallon electric equivalent).

5.2.

Chevrolet Volt

Also launched in late 2010, the Chevrolet Volt uses a 16 kWh battery based on LG Chem’s
lithium-ion polymer cells. The Volt is an EREV, which means that the vehicle has both a
1.4-l ICE and a 16 kWh Li-ion battery pack. The ICE acts as a generator operating the
motors once the battery charge has dropped to a minimum level determined by the
system controller. This combination of Li-ion battery and ICE allows the vehicle a total
range comparable to a standard ICE.

The Volt battery is designed to fit into the transmission tunnel and fuel tank areas in a

“T”-shaped pack design (

Figure 7.5

). It mounts outside of the cabin of the vehicle and is

FIGURE 7.4 The 24 kWh Nissan Leaf Li-ion battery pack with AESC Li-polymer cells. (For color version of this

figure, the

reader is referred to the online version of this book.)

FIGURE 7.5 The 16 kWh Chevrolet Volt Li-ion battery pack with LG Chem Li-polymer cells. (For color version of this
figure, the reader is referred to the online version of this book.)

146

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

therefore sealed to the environment to an IP69 level. Based on its location, it also has a
relatively low center of gravity and its position makes it a structural portion of the vehicle
chassis.

General Motors uses a liquid-cooled thermal management system to both heat and

cool the cells. The Li-ion cells are separated with a plastic “frame” which includes an
aluminum heat sink plate in which the cooling liquid passes to transfer the heat away
from the cells. The battery’s liquid cooling system is a separate cooling loop from the
ICE’s cooling system.

While the overall capacity of the pack is 16 kWh, the Volt is designed to use only about

10.3 kWh of that capacity. By reducing the usable energy of the battery, the overall life of
the system is extended. In the 2013 model, Volt’s capacity is increased to 16.5 kWh with
10.8 kWh of usable energy. General Motors claims that the increase is due to a change in
chemistry within the Li-ion cells.

Using the US drive cycle, EPA estimates the Volt’s range at 35 miles of all electric drive

with a total range of 379 miles. EPA also calculates the energy consumption of the battery
at 36 kWh per 100 miles. With the 2013 vehicle, EPA increased the rating of the Volt’s fuel
economy to 99 MPGe and the all-electric range increased to 38 miles.

5.3.

Ford Focus BEV

The Ford Focus BEV was launched in December 2011 for fleet sales, and in May 2012, for
consumers in California, New York, and New Jersey, with the release scheduled for Q3
2012 in 16 other US markets. The Focus is a BEV with a 23 kWh Li-ion battery system
using Li-polymer cells provided by LG Chem and the battery system, including cell,
modules and controls, provided by Compact Power (LG Chem’s US subsidiary).

The Li-ion battery of the Focus EV, shown in

Figure 7.6

, is actively cooled and heated

using a liquid thermal management system, similar to that of the Volt. The thermal

FIGURE 7.6 The 23 kWh Ford Focus Li-ion battery pack with LG Chem Li-polymer cells. (For color version of this

figure,

the reader is referred to the online version of this book.)

Chapter 7 • Lithium-Ion Battery Packs for EVs 147

management loop is designed to “precondition” the battery—precooling the battery on hot
days and preheating on cold days. This strategy is designed to optimize the life and per-
formance of the battery by maintaining it at an optimal temperature throughout its life.

The Ford Focus battery is split into two separate packs, with one mounted under the

rear seat and the second mounted in the trunk immediately behind the rear seat.

The EPA estimates the range of the Focus EV at 76 miles with a combined fuel

economy rating of 105 MPGe. The EPA calculates the energy consumption of the battery
at 32 kWh per 100 miles.

5.4.

Toyota Prius PHEV

The Toyota Prius PHEV was launched in 14 US markets in February 2012. This version is
the first Prius to use a Li-ion battery, as the “traditional” Prius uses NiMH. The 4.4 kWh
battery provides about 14 miles of pure electric driving range and up to 540 miles com-
bined electric and ICE range. The Prius is rated at 95 MPGe equivalent by the EPA.

The 4.4 kWh Li-ion battery is mounted below the rear cargo area floor and weighs

about 80 kg, corresponding to 55 kWh/kg. Using a standard Level 1 (110 V) charger, the
battery can fully charge in about 3 h, and using a Level 2 (240 V) charger, it can fully
charge in about an hour and a half.

The Prius PHEV differs from other PHEVs in that it was designed with only a

10–15 mile electric drive range vs 35–40 miles for, e.g. the Chevy Volt. This puts it on the
lower end of the electric drive range but optimizes the RESS system and cost by being able
to package a much smaller battery in the vehicle compared to the Chevy Volt.

5.5.

Mitsubishi

“I”

The Mitsubishi “I” is a BEV launched into wide-scale production in 2012 after a demon-
stration fleet program that began in 2009 in Japan. The “I”, also referred to as the i-MIEV,
uses a prismatic Li-ion battery designed by Lithium Energy, Japan, a Mitsubishi/GS Yuasa
joint venture. The 16 kWh battery is actually much smaller than other fully EVs most of
which have batteries of

w24 kWh.

The Li-ion battery is installed to the chassis from the bottom of the vehicle and mounts

under both the front and rear seats. The RESS is enclosed in a stainless-steel package that
seals it and protects it from the environment. Similar to other Li-ion RESS batteries, it is
mounted at the lowest possible point in the vehicle in order to ensure a low center of
gravity.

The “I” Li-ion battery uses an active forced air cooling system bringing chilled air in

from the air-conditioner system. The thermal management system is also designed to
actively cool the battery during charging in order to maintain the cell temperatures at
optimal levels and extend the life of the battery.

The EPA has measured the fuel efficiency of the “I” at 112 MPGe, currently the greatest

awarded in the United States, and fuel consumption of 30 kWh per 100 miles. The EPA
also rates the range of the vehicle at 62 mile all-electric drive range.

148

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

6.

Conclusions

As we continue to see greater levels of electrification in the automotive market, growing
regulatory demands and few educational programs dedicated to the engineering and
design of these complex electromechanical systems, there is a growing need to better
understand the interactions of the various component systems within the Li-ion battery.
The new battery engineer and designer needs to have a basic understanding of many
fields including chemistry, and mechanical, electrical, thermal and electronic engineer-
ing, among many other fields.

This chapter has attempted to begin gathering basic design considerations from each

of these fields, describing the design requirements and considerations for the four main
component systems within the large, high-voltage battery used in the HEV/PHEV/BEV:
the Li-ion cell, the mechanical and structural system, the BMS and electronics, and the
thermal system design.

The growing numbers of HEVs, PHEVs, and BEVs that are in the market today have

proved that Li-ion battery technology is a feasible solution to power the future of elec-
trification. However, more Li-ion innovation is still required to reach an energy density of
more than 300 Wh/kg, about twice the actual one, with costs dropping to about one-
quarter of what they are today, thus enabling the market to adopt these technologies in
large quantities.

Mechanical and structural system design requirements of the automotive Li-ion RESS

include considerations for whether the pack is engineered into a single or multiple
configuration and whether the RESS will be “switchable” or permanently mounted in the
vehicle. The design should also include investigations into material types, in terms, e.g. of
materials weight and compatibility. Design must integrate the appropriate types of shock
and vibration considerations for the application. Finally, the mechanical design of how
the cell is built into modules and/or mounted in to the pack is one of the key packaging
considerations as this will drive the overall packaging decisions and drive many of the
other considerations discussed above.

Perhaps the most critical subsystem within the Li-ion RESS is the BMS design. The

BMS is a complex system, including not only the main system monitoring circuit but also
balancing circuits, communication, safety circuitry and multiple fuses. Considerations
must be made as to whether the BMS will actively or passively balance the cells.

Thermal management system design can vary greatly depending on the Li-ion cell, the

application and usage cycle and the overall pack design. The main function of the thermal
management system is to ensure that the Li-ion cells within the battery are maintained
within their optimal operating temperature. This is important as regular operation
outside of the Li-ion cell’s recommended operating temperature will cause premature
degradation of the cell and reduce its useful life.

In summary, each of the four subsystems described above are critical in the auto-

motive Li-ion RESS design as all operate and function in conjunction with the other
subsystems. For instance, a battery design that has an effective thermal management

Chapter 7 • Lithium-Ion Battery Packs for EVs 149

system but does not include enough thermistors to measure the cell temperatures re-
duces the effectiveness of the overall thermal design. The future of automotive electri-
fication includes the Li-ion battery and, while there is still much optimization needed
over the next 10 years or so, Li-ion is the next step along that path.

References

[1]

M. Daowd, N. Omar, J. Van Mierlo, P. Van Den Bossche, Int. Rev. Electrical Eng. 6 (2011) 1264

.

[2]

Y. Xing, E.W.M. Ma, K.L. Tsui, M. Pecht, Energies 4 (2011) 1840

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Nomenclature

AESC

Automotive Energy Storage Corporation

ANSI

American National Standards Institute

BEV

Battery electric vehicle

BMS

Battery management system

C-rate

Ratio between discharge current and battery capacity

Capacity Total amount of Ah the battery can use and store
CFD

Computational fluid dynamic

DOD

Depth of discharge

DOE

Department of energy

EMC

Electromagnetic compatibility

EREV

Extended range electric vehicle

EUCAR

European Council for Automotive R & D

HEV

Hybrid electric vehicle

HPPC

Hybrid pulse power characterization

HVIL

High-voltage interlock loop

ICE

Internal combustion engine

IEC

International Electrotechnical Commission

LFP

Li

y

FePO

4

electrode

LMO

Li

y þ 0.16

Mn

1.84

O

4

electrode

LPM

Lumped parameter model

LTO

Li

4

þ 3x

Ti

5

O

12

electrode

MSD

Manual service disconnect

NEDC

New European drive cycle

NHTSA

National Highway Traffic Safety Administration

PHEV

Plug-in hybrid electric vehicle

RESS

Rechargeable energy storage system

SAE

Society for Automotive Engineers

SOC

State of charge

SOH

State of health

SOL

State of life

UDDS

Urban dynamometer driving schedule

USABC

US Automotive Battery Consortium

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS