Heavy Duty Truck Systems by Sean Bennett

Chapter 4 · On-board energy storage systems 241

Using this model, De Levie [De Levie, 1967] has shown that for a straight cylindrical
pore the electrical impedance, in the presence of an electrolyte with constant conductivity,
can be expressed by the following equation:

coth (4.35)

where R0 is the resistance of the electrolyte in Ω/cm, Z0 the impedance at the electrode-
electrolyte interface for a unit of length, and 1 the length of the pore in centimeters.

Given n identical pores in an electrode, the impedance becomes:

coth (4.36)

where ZIF is the interface impedance Ω/cm2, r the pore radius, and p the resistivity of the
electrolyte. In the case of a pure electrostatic supercapacitor, ZIF is equal to the capacitance
C, and we have the ideal equivalent circuit of Figure 4.42. If faradaic reactions occur, as they
do in the case of electrochemical supercapacitors, we need to introduce a faradaic impedance
into the interface impedance ZIF.

A number of models are derived from that of De Levie, which take into account differ­
ent pore shapes (conical, barrel shaped, etc.) and attempt to account for the dynamics of the
complex physical phenomena that take place at the electrodes, especially the ionic diffusion
associated with the concentration gradients of active species, which occur in the presence
of high currents [Levi and Aurbach, 2005; Marie-Françoise et al., 2005; Rafik et al., 2007]

C. Electrothermal Models of Supercapacitors

Thermal modeling of a supercapacitor, analogous to that for batteries (4.2.7.l.C), relies on
an energy balance that incorporates the irreversible and reversible contributions of thermal
power dissipated during a charge/discharge cycle. During discharge especially, ionic mobil-
ity is associated with endothermic processes, whereas electrode charging is dependent on
exothermic phenomena.

An example of 0D electrothermal modeling of the equivalent circuit type coupled to an
energy balance is shown for a commercial battery pack with ten 3,500F supercapacitor ele-
ments mounted in series. The results of simulation, shown in Figure 4.44, based on test bench
measurements at 23 °C over a succession of current pulses (200 A charge for 30 s, followed
by a 10-minute pause, followed by a 200 A discharge for 30 s), show excellent agreement at
high currents and during relaxation phases.

242 Hybrid vehicles

Figure 4.44

Comparison of measured and simulated values in a 3,500F supercapacitor pack
experiencing current pulses while charging and discharging: (a) cell voltage,
(b) skin temperature.
Source: [Prada et al, 2010b]

Depending on the needs of the user, as with batteries, fine multidimensional thermal
models (nD) can be developed where the electrical and thermal models are mathematically
coupled. Prada's work illustrates such an approach in three dimensions on a battery pack
[Prada et al, 2010b], but the user can choose to approximate the physics of the system and
decouple the electrical and thermal aspects (by assuming isothermal operation of the battery,
for example) to reduce processing time.

4.2.8 From the Element to the Battery Pack
There exists a wide selection of commercial storage cells for various applications, but only
some of them are suited for the specific functional requirements of hybrid vehicles in terms
of specific power, energy density, and working life. For each vehicle, the selection of a cell
technology, the determination of pack architecture in terms of safety, energy, and especially
thermal factors, and finally the construction of the pack, are key steps in optimizing the inte-
gration of the storage system. This section will discuss and illustrate these various points.
4.2.8.1 Criteria for Selecting a Battery Cell
Faced with the diversity of hybrid vehicle architectures and battery technologies, different
criteria can be used as a guide to selecting a cell. The principal criteria discussed below are
safety, energy and power performance, rechargeability, durability, and cost.

Chapter 4 · On-board energy storage systems 243

A. Safety

The identification and control of the risks associated with energy storage systems is a key
issue in the development of electrified vehicles [Marlair et al, 2011]. If one or more cells in
a pack are outside the temperature or voltage range specified by the manufacturer, several
undesirable phenomena may occur. Depending on the chemistry of the storage cell, such
phenomena could make the system unusable or even lead to thermal runaway together with
the risk of fire or the release of gas.

For Li-ion storage cells, especially those with a negative carbon electrode, the negative
electrode and composition of the electrolyte play a key role in terms of operation and safety
[Roth et Doughty, 2004]. At 100-120 °C a decomposition reaction takes place in the SEI
passivation layer on the surface of the negative electrode, formed during the cell's initial
charge/discharge cycles {Solid Electrolyte Interphase, 4.2.4.1.A). This initiates the process
of thermal runaway in conjunction with the exothermic reaction that takes place when the
electrolyte is reduced upon coming in contact with the lithiated carbon. In the electrolyte
itself, organic solvents and lithiated salts are responsible for ignition and gas discharge dur-
ing combustion, notably, of hydrofluoric acid (HF), which is toxic, in the presence of fluoride
compounds in the electrolyte and traces of water [Huddleston et al, 2001; Ribiere, 2011].
Finally, extreme charging states can also lead to thermal runaway, especially during over-
charging when the positive electrode is delithiated.

To reduce risk under adverse operating conditions (extreme temperature or charge/dis-
charge levels) or even during an automobile accident, the materials used in a battery cell must
possess chemical and mechanical properties that are stable over time and over a wide temper-
ature range. In this context the choice of chemistry for a storage cell (redox couple and elec-
trolyte) is essential and a number of studies have been conducted to develop and characterize
the materials used. For example, MacNeil [MacNeil et al, 2002] has classified the various
materials used for the positive electrode from the most stable to the least stable (Table 4.10).
This classification is based on calorimetric analysis of the charge state of positive electrode
materials, subject to a thermal ramp, an example of which is shown in Figure 4.45.

Table 4.10. Classification of positive electrode materials for a Li-ion storage cell, from the most stable (1)
to the least stable (7) [MacNeil et ai, 2002]

Relative thermal stability I Material used for positive electrode
1
2
3
4
5
6
7

244 Hybrid vehicles

Figure 4.45
Normalized comparison of rates of thermal runaway of positive electrode mate­
rials in Li-ion systems at the charge state in the presence of an electrolyte.
Source: [Sandia, 2009]

The thermal runaway temperature, indicated by a sudden increase in the rate of tempera­
ture rise at 160 °C for the least stable technology (LiCo02), is specific to each material, as is
the heat given off, which is estimated from the area beneath the curve. The highly exothermic
nature of LiCo02 technology can be explained by the amount of oxygen released by the posi­
tive electrode during degradation, which maintains thermal runaway [Roth and Orendorff,
2009]. Conversely, lithiated iron phosphate has the weakest kinetics and enthalpies of reac­
tion during degradation because it does not release oxygen, unlike the manganese oxides or
cobalt oxides 6.

Given the possible instability of electrode materials, the construction of a battery cell
must ensure safe operation under normal conditions of use, but it must also prevent leaks,
pressure and temperature increases, or even explosion under adverse conditions. Taking a
Ni-MH battery as our example, we see that secondary reactions occur at the electrodes out­
side the voltage range defined for normal use in a HEV, leading to the release of gas. To pre­
vent such mishaps, electrodes are designed to limit these reactions, a system of inter-element
balancing reduces the risks of overcharging and overdischarging, and a vented gas compart­
ment is integrated into each module to protect the battery (Figure 4.46).

6. It is worthwhile indicating the orders of magnitude of the enthalpies of combusion for different Li-
ion battery technologies (Howell, 2006):
ΔΗ = 670 J/g to 1,270 J/g for a positive electrode and its electrolyte,
ΔΗ = 1,552 J/g to 2,151 J/g for a negative electrode.
If we compare these values with the enthalpy of combustion of a hydrocarbon (on the order of
47,000 J/g), which constitutes the other storage system in a hybrid electric vehicle, we find that this is
20 times higher than the most unfavorable Li-ion technology.

Chapter 4 · On-board energy storage systems 245

In Li-ion storage cells, "passive" safety systems are also included in the design of each
element to reduce the risks of thermal runaway. Such methods include the use of separators
capable of irreversibly cutting off circulation of the electrolyte between the electrodes (Roth
and Doughty, 2007) and a vented gas compartment to avoid pressure increases. Specific
electronics can also be included in the cell, but outside the electrochemical volume strictly
speaking (Figure 4.46).

Figure 4.46
Photographs obtained by X-ray microtomography (voxel resolution 40 μηι3) of
a prismatic Li-ion element intended for portable electronic applications.

B. Energy and Power Output

The energy and power output of a cell depends on several criteria: electrode type, cell design,
and balancing the capacity of each electrode.

a. Power and Energy (P/E) Electrodes

When designing an electrode, a variable thickness of active material can be deposited on a
given support surface (4.2.6). If we are designing a power storage cell, that is, one where
the reaction kinetics are fast, we limit the amount of active material on a given exchange
surface, in comparison with an energy electrode. That is why a fixed redox pair can result
in a wide performance range, depending on the power/energy (P/E) relation of the elements
(Figure 4.25). Note that a power element is smaller in size so it can more easily dissipate the
heat generated at power peaks, as in the NCA/C range from SAFT (cells VL 2V, VL12V,
and VL22V, Figure 4.21).

L. Gaines and R. Cuenta (Gaines and Cuenta, 2000) have compared the construction of
Li-ion elements classified as energy (100 Ah capacity) and power (10 Ah capacity) cells.
Part of their analysis, shown in Table 4.11, emphasizes the proportions of active materials
employed. These are significantly higher in the energy cell, while the mass proportions of
current collectors, electrolyte separators, and technological components such as the casing
and safety mechanisms, are appreciably greater in the power cell.

246 Hybrid vehicles

Table 4.11. Comparison of the construction of (mass %) Li-ion elements classified as energy or power
(Gaines and Cuenta, 2000)

Material/C o mp o nent 100 Ah 10 Ah
energy cell power cell
Negative electrode (dry)
Active material (graphite) 16.4 4.3
Current collector (Cu) 4.4 12.8

Positive electrode (dry) 41.0 22.9
Active material (oxide) 1.8 6.0
Current collector (Al) 18.0 13.5
18.4
Electrolyte 40.5

Other components

b. Cell Construction

Cell construction (4.2.1) includes electrochemical and technological components (casing,
separator, electronic and ionic conductors, etc.). The latter have significant mass, which var-
ies depending on whether a power or energy element is involved. The transition from the
theoretical specific capacity to the practical specific capacity depends on the ratio between
the masses of useful active material and the total mass of the cell. For the reversible redox
pairs considered in this chapter, the ratio between the mass of active materials and the total
mass of the cell is limited to 25% for Pb/acid technology (assuming charged active materials)
but can reach approximately 60% for Ni-MH or Li-ion technologies (Caillon, 2008).

c. Balancing the Capacity of Each Electrode

The capacity balance of the positive and negative electrodes is adjustable. In the example of
the LFP/C 2.3 Ah Li-ion storage cell shown in Figure 4.47, we note that the capacity of the
negative electrode is weaker than the capacity of the positive electrode. Consequently, the
negative electrode is said to be limiting during discharge. The residual capacity at the posi-
tive electrode cannot be eliminated because the low voltage limit required by A123 Systems
is 2 V at the terminals of the element. This voltage is achieved when the potential of the nega-
tive electrode increases sharply (the graphite is completely empty). Conversely, a margin is
included in the charged state to prevent the negative electrode from becoming fully charged
(lithiated graphite), which promotes the formation of metallic Li during overcharging. Here,
the positive electrode is said to be limiting during charge.

Chapter 4 · On-board energy storage systems Next Page
247

Figure 4.47

Example of the potential change at the terminals of each electrode and the
resulting potential at the terminals of a 2.3 Ah LFP/C Li-ion storage cell from
A123 Systems.
Source: [Bernard, 2010a]

Balancing affects the theoretical specific capacity of a storage cell, which is calculated
from the theoretical mass capacity of each electrode. For balanced electrodes, the theoretical
capacity of the system is given by the product of the theoretical specific capacities (positive
and negative) divided by their sum. For example, in the case of a LiCo02/graphite Li-ion
system, whose theoretical specific capacities are 155 mAh/g and 330 mAh/g, we calculate
a system capacity of 105 mAh/g, assuming that the two polarities are balanced. If the elec-
trodes were not balanced, this value would be further reduced. Consequently, whenever we
increase the efficiency of a single electrode in a Li-ion storage cell, the overall system gain is
reduced by the simple fact that the other electrode must be balanced.

In closing, it is useful to bear in mind the extent to which battery performance is sensi-
tive to temperature. The customary temperature range in vehicles is relatively broad, from
- 30 °C to +60 °C, if we consider winter and summer conditions at different latitudes. Even
if the customary temperature range is limited to within 0 °C and 40 °C in general, this is suf-
ficient to substantially modify the performance of a battery since the chemical reactions and
diffusion phenomena that take place are thermally activated. In general, this implies the use
of some form of hot and/or cold heat regulation.

Previous Page Hybrid vehicles
248

C. Rechargeability

Beyond the requirement of discharge reversibility imposed on traction batteries, the time
needed to recharge a battery is a distinctive criterion in applications for rechargeable hybrid
vehicles. In such vehicles, the battery is discharged while traveling, which results in the
transfer of part of the hydrocarbon consumption to other primary energy sources in the form
of electricity. This requires that the battery be recharged from the electrical grid and in the
least amount of time possible. However, a complete "rapid" recharge (30 minutes or less)
creates chemical and mechanical stress on electrode materials, which can greatly reduce their
lifespan. That is why only partial recharging is recommended during limited time periods.
Most batteries require several hours (typically, 3 to 8 hours) for a complete recharge if it is to
avoid having a significant affect on lifespan.

D. Durability

An automobile must normally last for 15 years, or 5,000 hours of service. Such durabil-
ity requirements are expected from the battery in the vehicle as well, however, although
Ni-MH technology has been confirmed by experience, we do not have sufficient distance to
adequately evaluate Li-ion technologies. The specificity of the aging mechanisms associated
with each storage cell technology, as well as vehicle use, pose a complex problem. Research
programs in this area together with feedback, will enable us to establish predictive models for
lifespan and improve durability. A storage system must be capable of providing the charge/
discharge cycles required by vehicle operation with reduced loss of capacity, ideally less
than 20% while it is in service. As the depth of discharge is reduced, the number of cycles
increases. Storage when the vehicle is stopped also affects the capacity and impedance of the
battery system, and in this case we refer to calendar aging.

E. Cost

The cost of a storage cell is primarily determined by the cost of its raw materials and com-
ponents, by the processes involved in its fabrication, the cost of integrating the pack into
the vehicle, and the manufacturing volumes involved [Gaines, 2000]. These factors change
rapidly. Additional economic data is provided in Section 7.4.

4.2.8.2 Architecture of a Battery Pack

In highly electrified vehicles, it appears that responsiveness and range are often dependent on
the characteristics of the battery pack. The manufacturer will seek to develop a battery pack
that is best suited to the requirements of its vehicle but, for technical and economic reasons,
will also try to use conventional chemistries that are in widespread use. The construction of
a battery pack made of individual modules that are themselves composed of groups of cells
demands that these constraints be taken into account.

Chapter 4 · On-board energy storage systems 249

A. Definition

Because electric machines operate more effectively at high voltage, the components of a bat-
tery pack most often consist of storage cells mounted in series. Voltages are additive but the
capacity is limited by the weakest storage cell. In assemblies mounted in parallel, however,
voltage is unchanged at the terminals and capacity is additive, but there exists a risk that the
branches will be unbalanced, which can result in the circulation of parasite currents.

Additionally, the integration of a pack is a complex operation given that its construction
makes use of electrical, electronic, and mechanical interfaces, to which must be added the
constraints of thermal management, size, and weight. However, integration of the system by
means of electronic components and enclosures again represents a loss of specific energy
when compared to the characteristics of the unit storage cell, which needs to be minimized.
Therefore, defining the architecture of a battery system is of primary importance in guar-
anteeing the safety and optimizing the packaging, performance (power, energy, durability,
modularity), and thermal management of a battery pack, and in providing the best conditions
for large-scale manufacturing.

As discussed earlier, the definition of battery pack architecture is specific to the intended
vehicle application (Table 4.12). As an example, the comparison of two Li-ion battery pack
architectures, for EV and HEV applications respectively, shows that:

- the storage cell selected for the EV pack uses an energy cell (20 Ah capacitance) while
the storage cell in the HEV pack uses a power cell (5 Ah capacitance);

- to construct a module, cells are generally mounted in series in HEV applications, while
they are coupled in series and in parallel for EV applications;

- to assemble the pack, modules are mounted in series in both applications.

Table 4.12. Examples of Li-ion battery pack architectures based on vehicle use (Hendrix et Buck, 2008)

Characteristic EV mild HEV

Cell (capacity and rated voltage) 20 Ah, 3.7 V 5 Ah, 2.5 V

Module 120 Ah, 30 V 5 Ah, 60 V
(number of cells in series and parallel) (8S6P) (24S1P)
Pack
(number of modules in series and parallel) 120 Ah, 240 V 5 Ah, 120 V
(8 modules in series) (2 modules in series)
28 kWh, 33 kW max 0.6 kWh, 46 kW max

B. Thermal Considerations

The heat from a battery pack is a factor of key importance in defining its architecture. The
integration of cells into modules and modules into packs introduces considerable variability
in the environment of each element, depending on its position in the pack and the method of
heat management used. In addition to the concept of energy management during use, which
will be discussed in Section 4.2.9, it is appropriate to examine the geometry, connectors,
and interfaces of the pack with a view toward reducing thermal gradients in the system and

250 Hybrid vehicles

preventing hot spots. These conditions are required to preserve the uniformity of short- and
long-term performance among the elements, which will ensure the safety of the overall sys-
tem and prevent premature aging.

a. Influence of Connectors

Element design, like the configuration of current collectors [Kim et al, 2008], must be opti-
mized in order to minimize temperature gradients within the system. At the scale of a module,
the connections between elements is also very influential. Pesaran [Pesaran et al, 2005] has
modeled the thermal behavior of Prius battery modules consisting of 6 elementary Ni-MH
cells, one from the 2001 version and the other from the 2004 version. Their work demon-
strates a nonuniform temperature distribution for the 2001 version of the Prius module. The
addition of metal connectors at mid-height between the cells in the 2004 version provided
uniform temperatures within the module (Figure 4.48).

Figure 4.48
Temperature distribution in the Prius modules.
Source: [Pesaran et al, 2005]

b. Impact of Electronics on Thermal Behavior

The question of the location of potential sources of heat in the cells of a battery pack should
not be ignored. The electronics in the system may serve as a heat source leading to the
non-negligible production of heat. Maleki and Shamsuri [Maleki and Shamsuri, 2003] have
studied the impact of heat production from electronic components using a 3D model of a
Li-ion battery pack for a laptop, where the heat flow is made up of reversible and irrevers-
ible contributions. The heat contribution of the electronic components, illustrated in Figure
4.49, emphasizes the appreciable influence those components have in heating neighboring
elements.

Chapter 4 · On-board energy storage systems 251

Figure 4.49
Temperature distribution in cells of a Li-ion battery pack, taking into account
the production of heat by electronic components.
Source: [Maleki and Shamsuri, 2003]
C. Example
The complexity of battery pack architecture is illustrated by the Li-ion battery pack included in the
Mitsubishi i-MieV electric vehicle (Figure 4.50), whose specifications are shown in Table 4.13.

Figure 4.50
Architecture of the Li-ion battery pack beneath the floorboard of the Mitsubishi
i-MiEV.
Source: [Likar, 2008]

2 5 2 Hybrid vehicles

Table 4.13. Specifications of the iMiEV. [Miyashita and Tominaga, 2008]

Vehicle Dimensions 3,400*1,475*1,600 mm
Mass 1,080 kg
Electric machine Seats 4
Battery Maximum speed 130 km/h
Charger Driving range (10-15 mode cycle) 160 km
Energy consumption
Type 100Wh/km
Permanent Magnet Synchronous
Peak power
Peak torque Machine (PMSM)
Type 47 kW
Rated voltage 180 N.m
Rated energy
On-board charger (200V, 15A) Lithium-ion
On-board charger (100V, 15A) 330 V
External quick charger (3-phase 16 kWh
200V-50kW)
7 hours for full charge
14 hours for full charge
30 minutes for 80% charge

The battery, which consists of 88 cells of 50 Ah each in series, developed in collaboration
with GS-YUASA Corporation to meet the car's energy and power needs, operates at high
voltage (330 V). The pack's specifications indicate a specific power of 550 W/kg (60s pulse
at 25 °C and 50% SOC) and a specific energy of 109 Wh/kg. The use of types of modules
improved flexibility and enabled the manufacturer to integrate a high-energy pack (16 kWh,
200 kg) beneath the floorboard (dimensions in millimeters: 1,400 [width] x 700 [height] x
200 [depth]). Note that a DC current of 125 A is used for rapid charging to 80% of full charge
in 30 minutes from an external charger (3-phase, 200 V-50 kW).

4.2.9 Management of Electrochemical Storage Systems

Management of vehicle storage systems is essential in HEV and PHEV applications. Control
of the charge level of battery elements and the heat of the pack are critical for ensuring bat-
tery safety during ordinary use and during periods of severe electrical and heat stress. It is
also needed to provide appropriate power/energy output from the battery pack and to extend
its working life in the vehicle. This section describes the features of battery management
systems (BMS), methods of estimating SOC and SOH, and strategies for controlling heat.

Chapter 4 · On-board energy storage systems 253

4.2.9.1 Battery Management Systems - BMS

A. Management Strategies for Different Types of Vehicle Use

Strategies for managing electrical and thermal behavior in a battery pack are specific to the
type of electric vehicle in question [Nelson, 2000].

- In a nonrechargeable electric vehicle, it is essential to keep the battery at an interme-
diary storage level and operate within a highly limited SOC window during ordinary
use, on the order of 20% for Ni-MH technologies, a figure that was as low as 10% in
its earliest applications [Nagata et al, 2003].

- In a rechargeable hybrid or electric vehicle, the SOC range typically reaches 60 to 80%
during ordinary use, since the vehicle can take advantage of an external charge. How-
ever, the balancing of cells and modules becomes more critical because the SOC range is
wide, even though the cells can easily be balanced at full charge. If we consider the heat
from a pack, we find that HEV and EV uses do not impose the same constraints on the
cooling system, because the heat produced by dynamic stress under power in a hybrid
vehicle is higher than that produced during fully electric use. To this difference should
be added the specific type, power or energy, of cell based on the application, which
appreciably influences battery dimensions and, therefore, the effect of cooling. Finally,
the management strategy used in a pack also depends on the chemistry of the electrodes,
for there are risks associated with each technology, as well as safety requirements.

B. Features of a Battery Management System

A battery management system is a complex electronic component that communicates in
real time with the battery and with the supervisor or operator of the vehicle. In general, the
BMS gathers information about battery characteristics (voltage, temperature, current) and its
components, estimates the SOC and SOH, calculates available power, informs or alerts the
on-board energy controller, and controls various aspects of battery use (charge, discharge,
cell balancing, protection, standby, cooling) based on its initial electrical and thermal set-
tings. Table 4.14 summarizes aspects of the battery environment for three applications [Plett,
2004a] and provides functional specifications for the associated BMS.

Table 4.14. Battery environment and functional specifications for a BMS based on vehicle use (HEV and EV).
Comparison with portable electronics (PE)

Characteristics HEV EV PE
Maximum operating range ±20C ±5C ±3C
Use profile Highly dynamic Very precise Approximate
Predicts available power Yes Yes No
Estimates SOC Very precise Precise Approximate
Balances cells Continuous Continuous or during charge During charge only
Estimates SOH Necessary Necessary Not essential
Working life 10-15 years 10-15 years < 5 years

254 Hybrid vehicles

C. Examples
a. BMS for a Ni-MH Pack

In the environment of the Prius 2 (Figure 4.51), there are several safety systems in place for the
Ni-MH battery, active and passive, for its own protection and that of the user. Passive protec-
tion is provided by a key (manual disconnect) and fuses that are used to disconnect the battery
manually or automatically in the event of high currents or short circuits. Active protection is
provided by relay switches, directly controlled by the vehicle's computer. Power converters
are also controlled by the computer, which makes use of measurements provided by the BMS
and various sensors located on power converters and electrical machines (traction motor and
generator). The exchange between the BMS and the vehicle computer takes place over a multi-
plexed cable known as a CAN (controller area network), which allows a single cable to transmit
a large amount of complex information while minimizing the amount of cable in the vehicle.

Seen from the vehicle computer, the BMS behaves as an intelligent sensor. Using bat-
tery measurements (voltage, current, temperature), it transmits battery fault states as well as
charge state and battery charge and discharge limits. The only control action associated with
the BMS involves the monitoring and control of battery cooling above a certain temperature.

Figure 4.51

View of the BMS in a Prius 2 and Ni-MH pack in the vehicle environment.
Source: [Touzani, 2008]

b. BMS for a Li-Ion Battery Pack

A master/slave BMS architecture, illustrated in Figure 4.52, is generally used to manage
Li-ion packs [Barrie, 2008]. This design has the advantage of being modular: it can be modi-
fied for relatively high-voltage batteries by varying the number of slave units. These see the
balancing currents between the cells but not the battery current.

Chapter 4 · On-board energy storage systems 255

This BMS architecture provides several levels of safety. In addition to the intrinsic safety
devices in each cell (4.2.8.1) and passive external devices (fuses and switches that can be
used to disconnect a cell or battery in the event of overcurrents or short-circuits), the BMS
software tracks key indicators from each cell (including voltage and temperature measure-
ments) and implements the appropriate control actions: balancing, cooling, disconnection.
Finally, the battery will go into standby mode if the low-voltage power source for the BMS
experiences a fault.

Cells Slaves Master

One slave per 50 volt module Monitors current
Monitors the condition (voltage Integrates current over time
and temperature) of each cell Calculates the battery SOC
Provides hardware protection Controls main contactor(s)
Carries out cell balancing Battery protection controlled by
Opto-isolated comms to master software with hardware back-up
System communications
Data logging

Vehicle systems
Energy management, Thermal management, Safety devices, Driver displays

Figure 4.52

Example of the master/slave architecture of a BMS for a Li-ion pack.
Source: [Lawson, 2008]

4.2.9.2 Methods for Estimating State of Charge

A. Current Developments

A precise and reliable estimate of SOC in real time is an essential part of the work of a
BMS. Several factors are involved. For one, it is important to keep the battery within the
predetermined SOC range to avoid the risk of thermal runaway and limit aging. Addition-
ally, conservative battery management is very expensive because it results in overdesigning
the battery and its cooling system. The SOC, which depends on the concentration of active
material within the cells of the battery, is not a quantity that can be measured directly on the
vehicle. A number of methods have been proposed for estimating it, however [Piller et al,
2001, Pop et al, 2005]. They include:

- measurement of residual capacity by means of a discharge test under controlled
conditions;

- coulometer readings (integration over time of the current entering and exiting the bat-
tery, including an estimate of leakage current), a widely used method;

256 Hybrid vehicles

- measurement of no-load voltage, correlated with SOC for some battery technologies,
except when the battery exhibits hysteresis phenomena between charging and discharg-
ing (NiMH) or a constant no-load voltage over the usable SOC range [Pop et al, 2006];

- measurement of internal resistance;
- measurement of dynamic electrical impedance of the battery using electrochemical

impedance spectroscopy (EIS) [Blanke et al, 2005; Huet, 1998; Kuhn et al, 2006;
Thele<^tf/.,2008];
- heuristic interpretation of experimental measurements using analytical techniques
such as linear regression [Verbrugge et Täte, 2004; Verbrugge et al, 2005], artificial
neural networks, or fuzzy logic [Singh et al, 2004];
- the use of Kaiman filtering, an algorithmic technique used to estimate the internal
state of a system from a dynamic mathematical model and measurements of accessible
quantities (voltage at the battery terminals and current) [Barbarisi et al ; 2006, Plett,
2004a; Plett, 2004b; Plett, 2004c; Plett, 2006a; Plett, 2006b].

The principal benefits and drawbacks of these techniques are summarized in Table 4.15.

Table 4.15. Techniques for determining SOC (Piller et ai, 2001)

Technique Application field Advantages Drawbacks
Discharge Easy and accurate;
test Used for capacity independent of SoH Offline, time intensive,
determination at the modifies the battery state, loss
Coulomb beginning of life Accurate if enough of energy
counting recalibration points are
All battery systems, available and with good Sensitive to parasite reactions;
most applications current measurements needs regular recalibration
Online, inexpensive, points
OCV Lead, lithium, Zn/Br OCV prediction
Online, inexpensive, Needs long rest time
EMF Lead, lithium EMF prediction (current = 0)
Online, easy
Linear Lead photovoltaic Needs long rest time
model All systems Provides information (current = 0)
Lead, Ni/Cd about SoH and quality
Impedance Provides information Needs reference data for
spectroscopy about SoH; possibility fitting parameters
of online measurements
DC internal Online Temperature sensitive, cost
resistance intensive
Online
Artificial All battery systems Good accuracy, but only for a
neural All battery systems Online; dynamic short time interval
networks
Needs training data from a
Fuzzy logic similar battery, expensive to
implement
Kaiman All battery systems
filters PV, dynamic Requires memory in real-
application world applications

Difficult to implement a
filtering algorithm that makes
use of all its features, e.g.,
abnormalities and nonlinearities

Chapter 4 · On-board energy storage systems 257

Of these methods, some require the development of a mathematical model of the battery,
most frequently, an equivalent electric circuit or electrochemical model (4.2.7). Techniques
for estimating SOC by measuring no-load voltage, internal resistance, or dynamic electri-
cal impedance make use of models based on electrical analogs, where the elements of an
equivalent electric circuit use SOC and temperature values. The use of Kaiman filters to
estimate SOC requires an electrical or electrochemical model of the battery at concentrated
parameters to reconstruct the internal state of the battery from voltage and current measure-
ments at the terminals.

B. Estimates for Automobile Applications

Current methods for estimating SOC commonly used in hybrid and electric vehicle battery
management systems, known as bookkeeping methods [Piller et al, 2001], are derived from
coulomb counting. They include algorithms that make use of experimental mapping: limited
experimental measurements, typically based on temperature, aging during cycling, autodis-
charge, energy efficiency, and so on, are approached by a series of curves. The best precision
is obtained by directly characterizing the assembled battery pack together with its electronic
protection circuit rather than the individual cells.

Offline, test devices for battery SOC have also been developed that make use of electrical
measurements. The majority of commercial devices apply an alternating voltage or current
disturbance at a given frequency, generally rather high (> 10 Hz). In this way the measured
impedance is close to the cell's ohmic resistance. Electrochemical impedance spectroscopy
has also proven useful in providing a diagnosis of the internal state of Li-ion batteries (SOC,
SOH) using multifrequency or white noise impedance measurements [Sauvant-Moynot et
al, 2010].

C. Outlook: Using Kaiman Filters to Estimate SOC

Research continues in developing a real-time non-intrusive diagnostic method for traction
batteries that would satisfy the requirements for HEV and EV use: a variety of battery and
conductor technologies, extreme diversity of conditions of use in terms of current, tempera-
ture, aging, and so on. One approach that has been widely studied is the use of extended
Kaiman filter algorithms, based on electrothermal models with concentrated parameters (see
inset below).

Specificities of an EKF-based SOC estimator
(Y. Creff, IFP Energies nouvelles)

Estimating the available charge or State Of Charge (SOC) is one of the main tasks of
the Battery Management System (BMS). Traditionally, the state of charge x is considered
to be proportional to the current I, the proportionality factor being the reciprocal of the
battery rated capacity Q. Thus,

(E4.1)

258 Hybrid vehicles

In addition, the voltage V measured across the battery terminals is, to a first approxi­
mation, given by the expression:

(E4.2)

where U0(x) is the open circuit voltage and R0(x) the internal resistance. These quantities
also depend on the temperature.

When the initial value x(t0) of the state of charge is known, it is easy to estimate the
state of charge over time by integrating its evolution equation (a method traditionally
known as Coulomb counting):

(E4.3)

The problem is therefore to find x(t0). In practice, for the reasons explained below,
the expression of voltage given in equation E4.4 is only correct when no current has
been drawn from the battery (I = 0) for a sufficiently long period of time. It can therefore
only be used occasionally, when no current has been drawn for a long time. Obtaining
an approximate value for x at close intervals therefore implies the use of Coulomb count­
ing. The effects of various sources of error (current measurement accuracy, value of Q
where the rated capacity would be less than the maximum capacity, faradaic efficiency
< 1, self-discharge, etc.) can be eliminated through the occasional use of the open circuit
voltage U0 curve.

Due to a number of physical phenomena occurring in the battery, measurement
of the voltage V includes various overvoltages, which progressively disappear when no
more current is being drawn from the battery. These phenomena can be represented in
the description of the battery dynamics via a model by addition of RC circuits. Thus, for
a single RC circuit added, the dynamics are written:

(E4.4)

We see that when I = 0, the overvoltage VRC drops (in absolute value) exponentially
to 0, with the time constant t = RC. By knowing this constant, we can estimate the time
from which VRC becomes very low and x « U^1(V). This knowledge is used to determine
the times when Coulomb counting can be reinitialised.

In practice, it is very difficult to improve upon this estimation technique combining
Coulomb counting and its reinitialisation via the open circuit voltage curve. This may be
necessary, however, so that ageing can be taken into account correctly or to obtain a
better estimation of the available power (based in particular on the real-time estimation of
voltage V). In this case, the observation theory can be applied, in particular the extended
Kaiman filter, often proposed. However, the problems are due less to the tools which
may be used than to the structure and calibration of the battery models. In practice,
the voltage V measured is generally relatively insensitive to the state of charge. In other

Chapter 4 · On-board energy storage systems 259

words, the state of charge is highly sensitive to the voltage: to have a reasonable chance
of obtaining a robust estimation of the state of charge with a non-linear observer such
as the extended Kaiman filter, the voltage predictions of the model used must be highly
faithful and the effective voltage measurements very accurate (Figure E4.1).

Figure E4.1

SOC estimator principle based on an extended Kaiman filter.

Two approaches are mainly proposed, depending on the type of model used. The
most common approach consists in describing the battery by an equivalent electrical
circuit whose parameters can be calibrated for example using electrochemical imped­
ance spectra [Di Dominico et al., 2011]. The main advantage offered by this approach is
calibration by non-intrusive measurements, its main drawback being the difficulty of tak­
ing into account ageing effects. The other approach, currently less developed, consists
in using an electrochemical model with localised parameters (approaches with distrib­
uted parameters are also proposed). Its main disadvantage is the need to calibrate the
numerous physical parameters associated with the model, which may require destructive
measurements. However, this more physical approach seems more suitable to take into
account ageing phenomena deterministically. Figure E4.2 shows experimental results
obtained at IFP Energies nouvelles, concerning an estimation of the state of charge of
an Ni-MH cell, using an extended Kaiman filter applied to an electrochemical model with
localised parameters. Despite incorrect initialisation (unlike the red curve which gives the
result of counting), the state of charge estimation converges towards the value of the

260 Hybrid vehicles
Coulomb counting; the two estimations then give similar results. With an observer of this
type, the voltage measurement can therefore be used continuously.

Figure E4.2
Performance of a SOC estimator based on a Ni-MH 0D model.
Source: [Creff and Di Dominico, 2009]

4.2.9.3 Methods of Thermal Management
When in service, the thermal state of a battery changes with charge and discharge demand;
design and environment also play a role. In these situations, we find the occurrence of ther-
mal gradients between the elements but also within elements.

Thermal gradients between elements depend on the position of each element in the
pack, the geometry of the flow, and the nature of the cooling fluid, as well as the connec-
tions between elements and the electronics inside the pack (4.2.8.2). The thermal gradients
between elements generate functional performance differences (power and energy) inside
the pack. Aside from the safety aspect, it is important to limit gradients among the elements
(< 5 K) to ensure uniform operation and aging within the pack.

Internal thermal gradients result from a core-skin effect between the outside surface of
the battery envelope (the skin), which is generally cooled by a cooling fluid (convection),
and the materials in the battery elements, which are cooled by simple conduction. Here too,
these internal thermal gradients must be limited (< 5-10 K) in order to maintain functional
performance at the element level and ensure safe operation, even though the skin temperature

Chapter 4 · On-board energy storage systems 261

alone can be measured. These internal gradients are even more pronounced for energy ele-
ments, which are more capacitive.

There are several ways to cool a battery pack: natural air convection, circulation of a liq-
uid, or the insertion of elements within a material that has specific thermal properties. These
are generally highly conductive to dissipate heat to the outside or possess high heat capacity
to absorb part of the heat given off by the pack. For example, Khateeb [Khateeb et al, 2006]
has compared four methods of thermal management in a Li-ion battery pack for an electric
scooter: (1) cooling by natural convection, (2) use of an aluminum foam dissipating matrix,
(3) use of a phase-changing material (PCM), where endothermal melting is designed to limit
a rise in temperature in the pack while exothermal crystallization slows pack cooling, (4) a
combination of aluminum foam and a PCM. It turns out that PCMs are the most effective
passive cooling systems for limiting temperature increases in Li-ion cells and maintaining a
uniform distribution of temperature within the module (Figure 4.53).

Kim [Kim et al, 2007] has also worked on the use of phase-changing materials in HEV
and PHEV applications. He has shown that, in certain cases, PCM systems alone are unable
to dissipate sufficient heat. However, coupling a PCM system with air cooling would allow
designers to reduce the size of the traditional air-cooling system.

Figure 4.53

Influence of PCM (phase changing materials) passive cooling systems on the
temperature of a Li-ion battery pack while charging and discharging.
Source: [Khateeb et al, 2006]

262 Hybrid vehicles

4.3 OTHER ON-BOARD STORAGE SYSTEMS

Earlier we described various electric or fuel-based storage systems. This summary pro-
vides a comparative table based on the work of the commission chaired by J. Syrota [Syrota
et al, 2008] (Table 4.16). This table shows in particular:

- Systems that combine a tank filled with an external material, primarily hydrocarbon
or hydrogen, in what can be described as an open storage system. Such systems are
characterized by a fill time that rarely exceeds several minutes.

- Systems in which the recharging operation is symmetrical to the discharge operation.
Such systems are electrochemical or electrostatic, oleopneumatic, or inertial, and take
place in what can be characterized as a closed storage system. Such systems have lim-
ited recharge power, which results in much longer recharging times 7.

- The efficiency range of the converter that sometimes must be added to the different
storage systems in order to obtain energy compatible with the other components of the
transmission system.

Note that major disparities may be observed in the expression of the specific energy and
energy density in some storage systems. This can be explained by the technology used but
also by variations in the limits of the system considered. It could include the casing (with pro-
tection in case of a flywheel), the thermal conditioning or take into account a parallelepiped
containing gas storage bottles, for example.

7. We should point out that battery exchange systems have been proposed for electric vehicles, where
recharging can take place in a few minutes and which resemble an open system. The problem of low
mass energy density, however, is always present and will affect the infrastructure.

Table 4.16. Comparison of various on-board storage systems

Open systems | | Closed systems

Type Gasoline Diesel CNG LPG Hydrogen Batteries Oleo Flywheel
pneumatic

State Liquid Liquid Gas Liquid Gas Liquid Metal
hydrides

Temperature Ambient Ambient Ambient Ambient Ambient - 2 5 3 Ambient Ambient Ambient Ambient
(°C) to 300

Pressure (MPa) Atmospheric Atmospheric 20 to 25 0.5 to 70 0.5 0.2 to 1 Atmospheric 25 to 55 Atmospheric Chapter 4 · On-board energy storage systems
2.5 or vacuum

Specific energy 11,900 11,800 1,000 to 7,080 1,300 to 500 to 400 to 500 25 to 100 2 3 to 6 4 to 12
(Wh/kg) 1,900 1,600 1,000

Energy density 8,900 9,900 1,500 to 4,300 600 to 1,800 2,500 70 to 300 lto2 4 to 10
(Wh/L) 2,300 1,000

Type of converter Spark Compression Spark ignition Fuel cell Hydraulic Electric
ignition ignition engine machine machine
engine engine or CVT

Average energy 20 to 25 25 to 30 22 to 20 to 25 40 to 50 4 80 to 95 5 70 to 85 6 80 to 90 7
efficiency of 27 3
converter (%)

Supplied energy/ Mechanical Electrical Electrical Mechanical Electrical or
exchanged mechanical
energy^

1. Mass and volume of the tank included for open systems.
2. For a complete energy type battery pack.
3. Does not take into account the energy consumed by compression of the NG.
4. Does not take into account the energy consumed by hydrogen compression or liquefaction as well as losses in liquefied state.
5. Average energy efficiency of the battery for a charge-discharge cycle (variable depending on the technologies and conditions of use).
6. Average energy efficiency of the hydraulic machine and the storage for a charge-discharge cycle (variable depending on the technolog ies and conditions of use).
7. Average energy efficiency of the flywheel, electric machine or CVT for a charge-discharge cycle (variable depending on the technoloj>ies and conditions of use).
8. Supplied for irreversible open systems and exchanged for reversible closed systems.

bo

ON

264 Hybrid vehicles

Flywheels
(F. Vangraefschèpe, IFP Energies nouvelles)
Flywheels are being considered as a means of storing mechanical energy. The
kinetic energy of a mass rotating around an axis is given by the expression

where W is the kinetic energy in J
J is the inertia of the mass with respect to the axis of rotation, in kg.m2
Ù is the angular speed in rad/s

Since the vehicle requires mechanical energy to move forward, storage of mechani­
cal energy, for example in a flywheel, should minimise the losses generated by the various
energy conversions required with the other storage means. In practice, this advantage
must be tempered since it is difficult to connect two mechanical systems (the vehicle
wheels and the flywheel) operating at variable speed if the speeds of the two systems do
not vary according to the same parameters: the speed of the vehicle wheels is related to
the vehicle speed and remains controlled by the driver; the speed of the flywheel depends
directly on the energy stored. In this case, a speed adaptation system is required. It
may be mechanical (continuously variable transmission - CVT) or electrical (mechanical
energy in the flywheel converted into electricity by a generator). In this case, an electric
machine must also be planned in the wheels to perform the opposite conversion (as with
the Porsche 911 GT3 R Hybrid).

Figure E4.3
Flywheel associated with a toroidal CVT for Formula 1 applications.

Chapter 4 · On-board energy storage systems 265

Figure E4.4

Implementation of a flywheel in a parallel hybrid architecture.
Source: http://www.flybridsystems.eom/F 1 System.html

Other problems must also be considered with flywheels:
safety: since the flywheel rotates very rapidly, the centrifugal force on the material is
very high. The flywheel must be encapsulated to prevent all the debris generated in case
of breakage from flying out;
minimisation of internal losses due to two causes: mechanical, inherent to a part
rotating on a shaft (traditional bearings could for example be replaced by frictionless
magnetic bearings) and aerodynamic (the most advanced systems use flywheels rotating
in a vacuum).

Flywheels offer high specific powers but store relatively little energy. They are there­
fore installed in vehicles requiring high powers very occasionally, for example buses and
trams when starting to move.

These systems have been considered for use in Formula 1. A 24 kg system with
a volume of 13 litres can provide the 60 kW and 400 kJ (0.111 kWh) authorised by the
regulations.

266 Hybrid vehicles

Figure E4.5
Hybrid system in the Porsche 911 GT3 R Hybrid.
Source: http://www.auto-innovations.com/site/brevetech/whp99.html

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I Hybridization

François Badin \

Hybridization of road vehicles is a complex subject. While it is easy to acquire a broad under-
standing of its principles, there are multiple implementation possibilities.

Consequently, a detailed assessment of the numerous possibilities and limits of the vari-
ous hybridization solutions, necessary to correctly evaluate their respective advantages,
involves an in-depth study of their characteristics, especially by referring to the very large
number of studies published in recent years.

To offer the reader a relevant analysis grid of the possible hybridizations, in this chapter
we propose a three-level approach, based on the following angles:

- the broad principles underlying hybridization and its main implementations,
- the various architectures chosen by car manufacturers, suppliers and laboratories to

integrate the hybridization components in vehicle drivetrains,
- lastly, the features and gains that hybridization, in its various forms, can offer the vehi-

cle driver, passengers or the community.
Readers can refer to the table in Appendix 1 which summarizes the various solutions
according to their architecture and features.

5.1 PRINCIPLES

We must mention here the broad principles underlying hybridization and its main imple-
mentations in order to define the concept of hybridization and describe the factors which
encouraged researchers, companies and public authorities to develop it. We will first outline
the role of a drivetrain (missions and constraints) and the complementarity of thermal and
alternative drivetrains, before introducing the principles of hybridization, its components and
most frequent types of architecture.

5.1.1 Missions and Constraints of a Drivetrain

It is easier to discuss the principles of hybridization when looking at the basic principles
behind installing a drivetrain in a vehicle, i.e. the targeted objectives and the prerequisites.
Globally, the drivetrain system of a road vehicle must perform the following missions:

276 Hybrid vehicles

- constantly adjust the power developed to that demanded by the driver: in this case,
the drivetrain must considered in the broad sense since the aim is both to provide the
power required to accelerate and absorb the power induced by a braking demand,

- supply the energy required for displacement, respecting in particular the constraints
listed below,

- power all auxiliaries allowing the vehicle to move in complete safety while offering its
passengers a level of comfort and services which is constantly increasing.

These various missions must be fulfilled while respecting a large number of constraints,
present right from the vehicle design phase, during its homologation then throughout its use
and even after. The main constraints are as follows:

- the financial interest must be shared between the manufacturer and the end user,
- the local nuisances must comply with applicable regulations: controlled atmospheric

pollutants, noise and electromagnetic emissions,
- the vehicle active safety must be as efficient as possible during braking and acceleration,
- the dynamic performance levels must allow the vehicle to, at least, fit into the traffic

for which it was designed,
- the energy consumption must be as low as possible over a range of uses, to limit the

operating cost as well as greenhouse gas emissions (if fossil fuels are consumed);
recent developments would suggest that, in the future, this constraint on greenhouse
gas emissions could become a regulation, especially for C02,
- the energy must be supplied as rarely as possible (therefore allowing high range), as
quickly as possible (therefore with high power) and under optimum conditions (safety,
feasibility, density of filling stations, costs, energy efficiency), while guaranteeing
maximum compatibility with the existing network and the national or regional energy
policy,
- the reliability and maintenance costs must be compatible with user expectations,
- the use of materials whose availability, under economic conditions compatible with
the automotive industry, may be restricted due to industrial problems or geopolitical
tensions must be minimized,
- the energy consumption and greenhouse gas emissions must be as low as possible
when manufacturing the drivetrain and its components,
- at vehicle end of life, it must be possible to guarantee optimum recycling of the com-
ponents and their precious or rare materials.

As we can see, a drivetrain has a difficult task since the missions to be performed are
complex and accompanied by a very dense array of constraints, some in fact tending to
become more severe over the last few years. In this context therefore, it is not surprising
that alongside the studies conducted on the various types of drivetrain, scientists have tried
to combine them so as to propose optimum solutions best respecting this mission-constraint
duality.

Chapter 5 · Hybridization 277

5.1.2 Complementarity of Thermal and Alternative Drivetrains

Energy storage and conversion are problems common to all types of drivetrain. A road vehi-
cle drivetrain will therefore always have the following characteristics:

- the energy exchanged with the driving wheels is always mechanical, irrespective of the
energy(ies) crossing the drivetrain,

- various components are responsible for storing the energy on board, the energy con-
versions required throughout the transmission and finally the coupling to the vehicle
driving wheels.

Several types of energy storage system (ESS) and energy converters have been described
in the previous chapters (summary in 4.3). We will emphasize here all aspects relating to
their complementarity, being one of the advantages of hybrid drivetrains, with the following
comments:

- energy storage systems:

• the specific energies and energy densities of liquid hydrocarbons (nearly 12,000 Wh/kg
and 9,000 to 10,000 Wh/L for gasoline and diesel) are much higher than those encoun-
tered in alternative solutions such as electrochemical storage (about 100 Wh/kg and
160 Wh/L for an EV battery pack) or hydrogen (about 1,500 Wh/kg or 800 Wh/L for
hydrogen compressed to 70 MPa); storing energy as liquid hydrocarbons will therefore
be far less penalizing in terms of the mass carried and the volume occupied (synthesis
in Table 4.16),

• storage system charging method. As mentioned in Chapter 4, charging may consist
of a reversible operation, symmetrical to discharging, as with closed systems such
as electrochemical or oleopneumatic storage systems. Storage may also be open and
charged by addition of external material, as with a fuel tank or compressed gas bot-
tles, for example. This is an important difference since an electrochemical storage
system can only be recharged, at best, to more than a few kilowatts per kilogram
and with a limitation by any converters which may be necessary or by the grid itself
(a 240 V 16 A domestic power outlet can hardly supply more than 3.5 kW). As a
comparison, the power delivered by a petrol pump nozzle filling up a 60 L fuel tank
in 3 minutes is over 10 MW l.

Remark: For completeness, we may mention the case of cells and semi-open accumula-
tors, such as the metal-air batteries (Li-air, Zn-air or Al-air) which can behave as closed or
open systems. These systems can in fact be charged electrically (accumulator) or mechani-
cally (cell) by replacing the metal, in the form of plates or powder [Broussely, 2007]. In
the early 2000s, company Electric Fuel Ltd manufactured several electric vehicles equipped
with a Zn-air battery which was recharged by replacing the zinc electrode and the electrolyte
[Revel, 2011], [Electric Fuel, 2004]. However, the complexity of the infrastructure to be cre-
ated to collect the products and manufacture zinc plates from the electrolyte has not allowed
this solution to be developed.

1. Note that battery exchange systems are proposed for electric vehicles; in this case, the vehicle can
be charged in a few minutes, but the problem of the stored energy density remains. In addition, the
economic model of this solution has not yet been demonstrated.

278 Hybrid vehicles

- energy converters: between the time the energy is stored and used at the driving
wheels, it must undergo several conversions. For road vehicles, we can identify:
• reversible converters 2; the main converter of this type is the electric machine
which converts electrical energy into mechanical energy when used as a motor and,
inversely, converts mechanical energy into electrical energy when used as a genera-
tor. We may also mention hydraulic machines and compressed air machines which
convert energy related to the pressure of a fluid into mechanical energy (expansion
of the fluid), or inversely (compression). These converters change the type of energy
they transmit, but other converters may performs adaptations only, for example elec-
trical (DC/DC or DC/AC converters) or mechanical (reducers, gearboxes, continu-
ous transmissions) conversions;
• irreversible converters, such as engines (reciprocating internal combustion engines)
which convert the chemical energy of the fuel into mechanical energy by irreversible
combustion;
• converters such as fuel cells, whose principle is reversible but not their use in road
transportation, due to operation or storage optimization considerations.

The complementarity of energy converters can be illustrated by concrete cases. Figure 5.1
shows the operating ranges and the efficiency mappings of three converters that can be used
in vehicles: an engine, an electric machine and a hydraulic machine.

The three diagrams of Figure 5.1 give for each machine the expression of their efficiency
as a function of the speed of rotation and the torque on the shaft.

Note:

- the operating symmetry (reversibility) of the electric and hydraulic machines which,
in negative torque (during braking for example), operate respectively as generator and
pump to recover energy. In the same case of negative torque, the engine only dissi-
pates energy in friction, compressions in the cylinders or during the intake and exhaust
phases (the injection system can maintain zero consumption over a large part of this
range);

- the fact that the reciprocating internal combustion engine does not deliver any torque
below its idle speed and that its torque is very limited at low engine speeds (Chapter 2)
which, in view of road use constraints, means that a clutch and gearbox are required; in
contrast, the other two machines exhibit maximum torque from zero speed of rotation;

- the engine efficiency hardly exceeds 35% in our example and drops very rapidly when
the torque decreases; these limitations are due to the Carnot efficiency which governs
the operation of the engine.

Lastly, there is one characteristic which does not appear on these diagrams: during opera-
tion, the engine emits local nuisances in terms of atmospheric and noise pollution; these nui-
sances are zero, or very limited in terms of noise, for the two alternative solutions described.

2. Reversibility is not to be understood here in the thermodynamic sense, but due to the fact that the
energy can flow in both directions through the converter, obviously with associated losses.

Chapter 5 · Hybridization 279

Figure 5.1

Comparison of various types of converter.

Although brief, this comparison highlights the fact that while the engine proves less
adapted to use in road transport, compared with other alternative solutions, its associated
energy storage by hydrocarbons currently remains the only one capable of allowing multi-
purpose use with no restricting limitation in terms of vehicle range.

5.1.3 Principle of Hybridization

In view of the above, drivetrain hybridization is therefore obtained by associating comple-
mentary components while maximizing the advantages and minimizing the disadvantages of
each one, through the use of a system approach aimed at improving global vehicle perfor-
mance. We will also see that the term "performance" may vary depending on the applications
and include consumption of hydrocarbons, local nuisances, global nuisances (greenhouse
effect) and vehicle dynamics.

280 Hybrid vehicles

Numerous hybridization schematic diagrams have already been proposed in the literature,
with various approaches confirming that this subject is complex and still undergoing consid-
erable changes. For further details, readers can refer to [Fuhs, 2009; Guzzella and Sciarretta,
2007; Beretta et al, 2005; Ehsani et al, 2005].

The diagram of Figure 5.2 must therefore be considered in this context; it was produced
in line with the approach adopted in this document. According to this diagram, a hybrid driv-
etrain therefore includes:

- several energy systems composed of a variable number of Energy Storage Systems
(ESS) and a variable number of energy converters, i.e.:

• an irreversible energy system which must include an open type ESS that can be
charged from the outside,

• at least one reversible energy system that can be charged or can even exchange
energy with the outside if it is connected to a grid,

- at least one reversible mechanical energy link between the vehicle energy system(s)
and the driving wheel axle(s),

- one or more coupling devices handling the exchanges, for energies of the same type,
between the various energy systems.

Charge/energy Reversible energetic system Driving wheels
exchange if connexion

with the power grid

Coupling(s)

Energy charge Irreversible energetic system Driving wheels

Figure 5.2
Schematic diagram of hybridization with two energy systems.

Throughout this chapter, the schematic diagrams of the hybrid drivetrains described will
keep the same syntax as regards the relative positions of the ESSs, the converters and the
mechanical coupling(s) with the vehicle driving wheels, as illustrated on Figure 5.3 3.

It must be possible to apply this definition to a large number of architectures and types of
component potentially usable in a hybrid drivetrain, as described in the next chapter.

3. For a two-wheel drive vehicle, the mechanical couplings to the driving wheels can be in position 1
or 2 and for a four-wheel drive vehicle in positions 1 and 2.

Chapter 5 · Hybridization 281

Storages Mechanical links to
Converters the driving wheels
Charge/energy
exchange if connexion Driving wheels

with the power grid

Coupling(s)

Energy charge Driving wheels

Figure 5.3
Syntax for representing the schematic diagrams of the various hybridizations.

5.1.4 Usable Components

The hybridization concept involves a large number of components implementing different
types of energy for each energy system and coupling, as indicated in Tables 5.1 and 5.2.

Table 5.1. Usable components for the irreversible energy system

ESS types Irreversible converter types Reversible converter types

Liquid or gas fuel tank Engines Electric machines and electric
Metal hydrides Fuel cells 1 converters
Reformers Hydraulic machines
Compressed air machines
Gearboxes, continuous
transmissions

1. Fuel cell operation is reversible. In road transport applications, however, this reversibility is not implemented.

Table 5.2. Usable components for the reversible energy system

ESS types Reversible converter types

Batteries, supercapacitors Electric machines and electric converters
Oleopneumatic Hydraulic machines
Flywheel Compressed air machines
Compressed air Gearboxes, continuous transmissions
Spring

While the main applications commercialized in the field of passenger cars and urban
buses combine a reciprocating internal combustion engine and an electrical system with bat-
tery, numerous other solutions have been studied and are even under development or in
demonstration phase. These solutions use reversible systems based on oleopneumatic storage
systems or flywheels as well as irreversible systems based on fuel cells.

282 Hybrid vehicles

To clarify matters, we will describe below the various types of possible hybridizations,
illustrating them with the most widespread solution, i.e. the thermal-electrical system. Read-
ers are free to imagine how the various configurations described in this chapter could be
considered with other storage systems and other converters.

5.1.5 Series Hybridization
In this type of architecture, the coupling between the two energy systems involves electrical
energy (Figure 5.4). The engine is no longer connected directly to the driving wheels; the
mechanical energy it produces is converted into electrical energy by a first machine, then
back into mechanical energy by a second machine, hence the name "series hybridization"
given to this architecture.

This type of architecture is therefore similar to that of an all-electric drivetrain, the engine
being used as an onboard electrical energy generator.

Figure 5.4
Schematic diagram of series hybridization.

5.1.6 Parallel Hybridization
In this type of architecture, the coupling between the two energy systems involves mechani-
cal energy (Figure 5.5). This type of architecture is similar to that of a conventional driv-
etrain, the electric machine being connected in parallel with the mechanical link of the engine
to the wheels.

5.1.7 Comparison of Series and Parallel Hybridizations
Table 5.3 provides a summary of the main advantages and disadvantages of series and paral-
lel hybridizations.

Chapter 5 · Hybridization 283

Figure 5.5
Schematic diagram of parallel hybridization.

Table 5.3. Comparison of series and parallel hybridizations

Architecture Advantages Disadvantages
Series
• Choice of engine operating point • Low energy efficiency
Parallel (r.p.m., torque, power) • Need to use two electric machines and

• Very high reduction or even cancella- two power electronics systems (cost,
tion of the engine dynamics possible mass, volume)
• Significant modifications compared
• Significant downsizing of the engine with a conventional drivetrain
possible architecture
• Use in all-thermal mode impossible
• Conservation of vehicle dynamic • Larger battery pack
performance in electric mode
• Larger engine operating range
• Battery sizing allowing higher range • Engine dynamic transients not totally
in all-electric mode
eliminated
• Excellent energy recovery during • Fewer possibilities of downsizing the
braking
engine
• Easy installation of components • Lower dynamic performance in all-
(heavy goods and public transport
vehicles in particular) electric mode
• Battery sizing not generally allowing
• Easy transmission management
good range in all-electric mode
• Good energy efficiency • More limited energy recovery during
• Few modifications compared with a
braking
conventional drivetrain architecture • Complex transmission management
• Limited number of components
• Use in all-thermal mode possible (mode changes, dynamics)
• Components difficult to install
• Complex mechanical coupling

284 Hybrid vehicles

5.1.8 Series-Parallel Hybridization

As shown in Table 5.3, series and parallel hybrid architectures exhibit quite complementary
characteristics; researchers therefore found it logical to propose mixed series-parallel archi-
tectures. These associations extend the complementarities between thermal and electrical
drivetrains, by producing a synergy based on the advantages of the two series and parallel
architectures.

As we can see on Figure 5.6, mixed architectures are more complex: they involve three
energy systems and several possible electrical and mechanical couplings between these sys-
tems. Note that the representation proposed respects the order 3/3 parallel hybrid principle
put forward by J. Beretta for the Toyota Prius in 2005 [Beretta et al, 2005]. The differences
between the architectures are based mainly on the mechanical couplings (MCI to MC4 on
Figure 5.6) implemented to connect the various energy systems. In view of the types of com-
ponent used in these mechanical couplings, we can identify two families of series-parallel
association:

- presence of one or more components, such as a clutch, to interrupt the mechanical
couplings and thereby modify the hybridization architecture in series or parallel when
the vehicle is in use (5.2.4),

- use of one or more planetary gears to continuously split the energy in the various
branches of the transmission, which in this case is series and parallel, and can rightly
be qualified as power-split transmission (5.2.5).

A priori, series-parallel architectures exhibit the best gain potentials since they can imple-
ment both types of hybridization. Their success nevertheless depends on maximizing the
advantages of the two series and parallel associations while minimizing their disadvantages,
which requires special effort on the design of components, and hence highly complex man-
agement and control.

This type of architecture is proposed in an increasing number of applications by suppliers
and car manufacturers (5.2.4). In view of the progress made over the last few years in the
field of control command, we can expect that the control of these complex transmissions will
become easier.

As an illustration, Figure 5.6 shows all possible couplings for a series-parallel hybridiza-
tion, each coupling having 3 inputs-outputs. This diagram calls for the following remarks:

- some branches may not exist in the various applications discussed in the following
chapters, thereby simplifying the architecture by deleting some of the couplings,

- depending on the architectures and the number of driving wheels, coupling MC4 can
be fitted in one or more of positions 1, 2 and 3,

- while there is only one type of electrical coupling, we will see that several types of
mechanical coupling are possible (5.2.3).

Chapter 5 · Hybridization 285

Figure 5.6

Summary of the various couplings possible for series-parallel hybridizations
with 2- or 4-wheel drive.

5.2 ARCHITECTURES

5.2.1 Coupling Devices
After describing the various possible associations of drivetrain systems, we will now see that
the type(s) of coupling represents an important feature in a hybrid architecture, identifying
the various types of possible coupling and the corresponding achievements.

5.2.2 Series Hybridization
For these architectures, the coupling mode is extremely simple, by connection on the vehicle
DC network; one or more DC/DC converters may nevertheless be necessary if the voltages
of the energy systems to be connected are different.

286 Hybrid vehicles
5.2.3 Parallel Hybridization
The aim in this case is to achieve mechanical coupling between two energy systems. There
are several possible solutions, which can be divided into two broad types: coupling by addi-
tion of torques and coupling by addition of speeds.
5.2.3.1 Coupling by Addition of Torques
Various technological solutions can be used to achieve coupling by addition of torques:
direct mechanical coupling and through-the-road (TTR) coupling.
A. Direct Mechanical Coupling can be achieved by:

a. Dual Shaft Configuration

where the two shafts from the engine and the electric machine are connected by a pulley-
belt system (belt-alternator system - BAS) or a system of gears (Figures 5.7 and 5.8). The
parameters defining the coupling system (pulley diameter, number of teeth on the gears) can
be used to adjust as accurately as possible the speeds of the two shafts according to the char-
acteristics of the components used (engine, electric machine). In practice, a belt connection
cannot transmit a power of more than about 15 kilowatts; a gear link offers greater potential
but, in this case, the noise aspect must be taken into account from the design phase.

Figure 5.7
Gear coupling, FLEX HYBRID plug-in hybrid lab car produced by IFP Ener-
gies nouvelles.

Chapter 5 · Hybridization 287

Figure 5.8

Pulley-belt coupling, Valeo StARS system.
Source: Valeo

With this type of coupling, the electric machine can be connected at various points of the
transmission, on the primary shaft of the gearbox, either engine side (so-calledpre-transmis-
sion configuration), or on the secondary shaft, differential side (so-called post-transmission
configuration). Note that one financially attractive solution consists in using the power take-
off of a 4-wheel drive system to connect the electric machine to the transmission, thereby
limiting the mechanical modifications.

b. Coaxial (Single Shaft)

Configuration where the electric machine is mounted directly on the drive shaft which, in
this case, goes through it (Figures 5.9 and 5.42). While this configuration represents a com-
pact assembly with direct addition of torques, it induces several constraints:

- the two machines rotate at the same speed, which may pose a problem when starting
the engine due to the lack of gearing, in cold weather or for large engine displace-
ments, especially diesels,

- the powertrain is longer, which introduces integration problems, especially for trans-
verse engines. The electric machine must therefore be designed with a large diameter
and minimum length (Honda claims less than 1 cm per kilowatt-peak). Solutions may
also be found on the engine, by limiting its length, reducing the number of cylinders
(Honda 3-cylinder Insight in 1999) or removing the belt-driven auxiliaries. The gear-
box can also be shortened by adding a shaft or reducing the number of gears which can
be redistributed due to the presence of the electric machine,

- use of "disc-shaped" electric machines which are less efficient due to their diameter/
length ratio, compared with machines of more traditional geometry.

288 Hybrid vehicles

Figure 5.9
Single shaft type coupling, the machine(s) are placed on the engine shaft,
Daimler S400.
Source: [Back, 2008]
B. Through-the-Road (TTR)
Coupling where the thermal and electrical drivetrains are placed on two vehicle different
axles and add their torques to produce the required vehicle motion (Figure 5.10). The speeds
of rotation will depend on the wheel diameters and slip.

Figure 5.10
Through-the-road coupling, engine at the front and electric machine at the rear,
?SA 3008 Hybrid4.
Source: PSA

Chapter 5 · Hybridization 289

With this configuration, vehicle dynamics and trajectory control can be improved
(5.3.2.6). In addition, positioning the electric machine on the rear axle simplifies vehicle
industrialization since fewer modifications are required in the engine compartment.

5.2.3.2 Coupling by Addition of Speeds

Various systems can be used to achieve coupling by addition of speeds, for example use of a
planetary gear or a dual rotor machine.

A. Use of a Planetary Gear
A planetary gear 4 which has with three shafts can be used to connect the engine and the elec-
tric machine to the transmission, as illustrated by the example on Figure 5.11. The gear equa-
tions demonstrate that the output speed is a combination of the speeds of the two machines
(5.2.5). This type of configuration has been proposed in various publications [Chang and
Szumanowski, 2009], [Burke et al, 2009], [Zanasi and Grossi, 2010].

Figure 5.11
Coupling by addition of speeds.
Source: [Zanasi and Grossi, 2010]

Implementation of the planetary gear in hybrid drivetrains is generally more complex and
more optimized than the example shown on Figure 5.11. We may mention:

- use of a combination of clutches and locks of the gear shafts to obtain [Chang and
Szumanowski, 2009]:
• pure electrical mode at low speeds,
• pure thermal mode at high speeds,
• hybrid mode at intermediate speeds.

4. A planetary gear is a mechanical transmission device with two degrees of freedom: it can be used
to combine three shafts with different speeds of rotation, the speeds of two shafts imposing that of the
third. Its main application is use as differential between the two driving wheels of a vehicle.

290 Hybrid vehicles

- the system which is by far the most well-known with the THS transmission developed
by Toyota, which makes ingenious use of a second electric machine to make a power
split (5.2.5 and 5.5.2).

B. Use of a Dual Rotor Machine

Speed coupling can also be obtained through the use of a dual rotor machine (also known
as Dual Mechanical Port electric machine, DMPM or Electromagnetic Split Powertrain,
EMCVT), the speed of the inner rotor, the relative speed of this component with the outer
rotor and the speed of the outer rotor with respect to the stator being added together. These
machines have been described in more detail (3.2.4.4 and Appendix 5). They may include:

- one link to the external electrical circuit, which is equivalent to the configuration
shown on Figure 5.11, the functions of the planetary gear and the electric machine
being replaced by the dual rotor,

- two links to the external electrical circuit, in order to produce a power-split transmis-
sion similar to the solution shown on Figure 5.18 and implemented by Toyota; the role
of the planetary gear and the two machines being performed by the dual rotor machine.

Lastly, as previously, more complex configurations involving clutches can be conceived.
For more details, readers can refer to [Ehsani et al, 2005].

5.2.4 Series-Parallel Hybridization

As already mentioned (5.1.8), this architecture requires the use of a decoupling component,
generally a clutch, used to switch from one mode to another on opening and closing. Simulta-
neous use of the two modes is not advantageous since, all speeds being linked, no additional
degrees of freedom are obtained. Simultaneous implementation may nevertheless be used if
it offers an additional features, such as distributed drive, as on the Nissan Cube (5.2.6) or the
PSA 3008 Hybrid4 (5.3.2.6).

This concept is not new, this type of architecture having been proposed by the supplier
Lucas in the early 1980s on a prototype (Figure 5.12).

Figure 5.12
Implementation of the Lucas series-parallel hybridization.
Source: [Harding et al, 1983]

Chapter 5 · Hybridization 291

In this architecture, the two modes, series or parallel, can be used alternately depending
on the vehicle operating conditions and the battery state of charge. A clutch allows switching
from series mode to parallel mode by connecting the engine and generator assembly to the
electric machine and the wheels.

Figure 5.13

Diagram of the Lucas series-parallel hybridization coupling.

In the coupling diagram shown on Figure 5.13, we can see that couplings MCI and MC2
are not present, MC3 being dual shaft type with pulley-belt (DSC) and MC4 being single
shaft type (SSC). A clutch placed between the engine and the electric machine EMC2 is used
to switch from series mode to parallel mode. The aim of the Lucas project was to develop the
coupling mode management laws and the control electronics; however, no industrialization
solution could be obtained due to the power of the computers and the technology of electrical
drives available at the time. As a result, this highly innovating project was dropped [Harding
et al, 1983], [Badin, 1986]. Note that more recently, in 2000, Nissan commercialized 200
hybrid vehicles based on its Tino model with an architecture using the same principle, but
with a continuous transmission [Jeanneret et al, 2002].

The Canadian electrical component manufacturer TM4 also proposes a series-parallel
architecture in its MÖGEN concept. As can be seen on Figure 5.14, the engine and the two
electric machines are placed coaxially and connected by two clutches. According to the dia-
gram of Figure 5.15, MCI and MC2 correspond to a single shaft coupling, while MC3 and
MC4 are not present. Four different types of operation can be obtained, depending on the
positions of the two clutches (Table 5.4).