Heavy Duty Truck Systems by Sean Bennett

142 Hybrid vehicles

system (id, i ) et (vd, v ) which have no physical reality but which are deduced mathemati­
cally from the true currents and voltages of each of the 3 phases (ia, ib, i ) and (v , vb v ).

The machine equations can then be written in the coordinate system (d, q):

- electrical equations: (3.4)

- magnetic equations: (3.5)

- mechanical equations: (3.6)

To determine the reluctance torque due to the machine saliency (Ld-L ), the electromag­
netic torque given in equation (3.6) can be expressed as follows:

(3.7)

where: T electromagnetic torque
Tferimct friction torque
> v d stator voltages in coordinate system (d, q) Tload torque imposed by the load
id' iq stator currents in coordinate system (d, q) J rotor inertia
Rs resistance of a stator coil
cor mechanical speed of rotation
d, q flux along axes d, q
Φ magnetic flux P number of pole pairs

L dm' aLgq cyclic inductances 10 along axes d, q

In equation (3.7), the first term expresses the torque produced by the interaction of the
stator and rotor fluxes, while the second term represents the reluctance torque. The reluctance
torque increases with the machine saliency, which is expressed by the difference between the
inductances measured along the direct axis d and the quadrature axis q (Ld - L ).

For variable reluctance machines, as indicated in paragraph 3.2.2.4, the first term is zero;
the rotor is therefore designed so as to maximize the second term related to the saliency,
which means that the ratio between inductances Ld and L must be as large as possible.

These equations, especially the steady-state voltage equations, are often represented on a
Fresnel diagram, to allow vectorial construction of the various quantities (Figure 3.19).

10. The cyclic inductance expresses the influence of the other stator (or rotor) windings on the flux
through each winding; it is equal to the inductance of one phase in the presence of the mutual induct­
ances of the other phases.

Chapter 3 · Electric drivetrain 143

Figure 3.19
Fresnel diagram corresponding to the steady-state voltage equations.

The machine operating state is limited in current and voltage as follows:

- current: I = A /i2 + i2 < I . The value T v can be defined in the machine according to

s ^ d q max max =>

its heating resistance (Joule effect) and/or by the design of the inverter,

- voltage: V = J v 2 + v 2 < V . This value results from the voltage limitation of the

0 y d q max ö

current source (generally the battery) after conversion by the inverter.

These two operating limits, represented in the plane id i as quantities reduced with respect
to the maximum current, are illustrated on Figure 3.20.

The currents id and i must therefore be controlled both to generate the torque by maxi-
mizing the efficiency and to keep the machine voltage within the range defined by the operat-
ing limits mentioned above.

144 Hybrid vehicles

Figure 3.20
Operating limits in the plane (d, q).
Source: [Abdelli and Le Berr, 2011]

3.3.2 Thermal Aspects
An electric machine intended for application on a hybrid vehicle (and to a lesser extent on
an electric vehicle) cannot be designed without taking into account the thermal aspects; in
this application, the power demands of the electric machine are short (a few tens of seconds),
unlike the case in an industrial context. On vehicles, therefore, it is quite reasonable to con-
sider smaller machines which will be used above their nominal performance (therefore in
magnetic saturation and beyond what the machine can withstand for an extended period of
time), provided however that thermal monitoring is possible on these machines.

As regards operating safety, certain hot spots or certain points sensitive to magnetic satu-
ration must be monitored in the machines (stator windings, rotor magnets in some configura-
tions, etc.). The thermal models required to calculate the temperature of these spots are more
or less complex, but share the same logic: the losses inside the machine (Joule effect losses,
core losses, etc.) form heat sources. This heat must be evacuated to the exterior by a cooling
circuit to prevent damage to the machine.

Chapter 3 · Electric drivetrain 145

Figure 3.21

Examples of thermal modeling.
Sources: (a) [Raynal, 1998] and (b) [Legranger, 2009]

The main difficulty with the above model is finding the internal parameters of the elec-
tric machine (Ld, L (j)mag, etc.). They can be calculating using a finite element code, pro-
vided however that the internal geometry of the machine and the characteristics, especially
magnetic, of the materials forming it are known very precisely. This is therefore the logical
procedure when designing the machine. If the necessary data is not available, an analytical
approach based partly on experience can be adopted to estimate the machine internal param-
eters. This approach also offers the advantage of being able to combine the analytical models
with an optimization system, in order to seek the best technical definition to meet precise
specifications. When considering an existing machine, one alternative is to identify these
parameters by specific tests on an electric machine characterization bench.

Modeling also provides a means of assessing the thermal operating conditions of the
machine depending on how it is used in the vehicle. This approach proves to be important
since, as already mentioned, the operating conditions of a machine for a road vehicle, and
especially in a hybrid architecture, are highly specific, with very high motor and generator
power peaks over very short periods of time. Heating phenomena can be analyzed by includ-
ing a thermal model of the machine in a complete model of the vehicle, in order to design the
machine to match a given set of specifications as closely as possible and check its behavior
for various types of use.

The modeling may be more or less complex, depending on the data available to produce
and validate the model. The most basic models allow an initial simplified approach, consider-
ing the machine as a homogeneous assembly; more complex models may take into account
the rotor and stator separately, discretize the machine along its length and allow coupling
with the cooling system of the machine and the power electronics, even the vehicle.

Figure 3.22 shows as an example a graph of the average temperature of the electric
machine in a hybrid vehicle according to a use cycle, for two different sized machines. We

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146 Hybrid vehicles

observe that the power demands generate very sharp temperature peaks, the peaks being more
pronounced on the machine with the lower peak power. By knowing the temperature peaks
and their frequency of occurrence, the machine design can be fine tuned to its intended use.

Figure 3.22
Graphs of the temperature of two electric machines of different powers for the
same vehicle load.

3.4 POWER ELECTRONICS

While various types of machine can be used to drive a vehicle, they all require an energy
supply controlled by power electronics.

In road transport, the energy for the electrified transmissions is stored by electrochemi-
cal components which supply a direct current that varies depending on the conditions of use
(refer to Chapter 4 for further details). In order to operate, the electric machine requires one,
or even two, power supplies whose voltage, current and frequency (for alternating signals)
must be constantly adjusted, to comply with the torque and speed references. The role of the
power electronics is therefore to convert and adapt the electrical energy between the storage
and the various machines or consumers.

The power electronics are therefore required whenever an electricity supply must be
adapted to the specific operating conditions of a component, or for drivetrain applications.
Power electronics perform three broad types of function:

- conversion of a DC signal into a different DC signal (DC-DC converter or chopper):
this type of converter is required to adapt the voltage supplied to a DC machine, to
auxiliaries operating on 12 V or to an onboard network operating at a different voltage
(3.4.3.2);

- conversion of a DC signal into an AC signal (DC-AC converter or inverter): this type
of converter is required to power an alternating current machine (3.4.3.1);

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Chapter 3 · Electric drivetrain 147

- conversion of an AC signal into an DC signal (AC-DC converter or rectifier): this type
of converter is required to charge a battery from an AC network (3.4.3.3).

This chapter provides a brief description of the various components of the power elec-
tronics, their characteristics, their application and the bases of their modeling. For further
details, readers can refer to specialized books such as [Séguier et al, 2011; Lasne, 2011;
Lavabre, 1998].

3.4.1 Power Components

The key parts of the power electronics converters are switches made from semiconductor
components; in the current state of the art, diodes and two types of transistor are used: MOS-
FET (Metal Oxide Semiconductor Field Effect Transistor) and IGBT (Insulated Gate Bipolar
Transistor). Bipolar junction transistors (BJT), used for many years in these structures, often
in Darlington type assemblies n , have been totally overtaken by IGBTs, now the main com-
ponent in the three types of converter used in cars (AC-DC, DC-DC, DC-AC).

The "switches" used in power electronics can be characterized by the maximum voltage
they can withstand, the maximum permissible current and their maximum state change fre-
quency 12. Figure 3.23 classifies the various components used in electric drivetrains accord-
ing to these three criteria.

Figure 3.23

Classification of power semiconductor components according to power and
frequency criteria.
Source: [Theolier, 2008]

11. The Darlington pair is a combination of two transistors used when the power switched is high.
12. A state change is an opening or closing of the power component, also called switching.

148 Hybrid vehicles

One essential aspect in the implementation of power electronics converters is the creation
of gate drive systems for semiconductor components. These interfaces are used to convert
low-level information, from the main control, into electrical signals capable of switching
the component. These switching operations are used to manage the power flows, which may
reach several tens of kilowatts, under the best possible conditions. These drives must be
perfectly adapted to the specific features of the components considered, whose performance
levels largely depend on those of their gate drives.

3.4.1.1 Diode

The diode is a unidirectional electric dipole with two electrodes: anode and cathode (Fig-
ure 3.24); current can only flow through the diode if the anode potential is greater than the
cathode potential. In direct polarization (conducting diode), the diode behaves like a closed
switch. In reverse polarization, the diode behaves like an open switch.

Cathode

Figure 3.24

Symbol of the diode.

Due to these characteristics, the diode can be used to rectify alternating current (diode
rectifiers) or to perform certain operating phases of the power electronics converters by con-
nection in antiparallel with the active components (MOSFET, IGBT).

3.4.1.2 MOSFET

MOSFETs are more suited to low or medium power applications. The conduction resist-
ance of this field effect component increases significantly with the direct breakdown voltage.
Currents of a few hundred amps are permissible for breakdown voltages of 50 V to 200 V;
the maximum breakdown voltage cannot exceed 1,000 V (with a few amps of switchable
current).

This component is often used in low voltage applications (alternator, starter-alternator,
adaptation of the 14 V or 42 V vehicle network), with chopping frequencies possibly exceed-
ing 50 kHz.

This component is known for being fast and easy to control. Since it does not involve
bipolar mechanisms (doping with only positive or negative unipolar charges), no charges are
stored. Due to its insulated gate, it is often referred to as a component with zero control cur-
rent. However, the high values of its capacitance CGS between gate and source, but also its

Chapter 3 · Electric drivetrain 149

gate-drain capacitance at low voltage (for states close to conduction), may lead to the appear-
ance of high gate capacitive currents during switching. Figure 3.25 symbolizes a MOSFET
and its control.

Figure 3.25

Symbol of the MOSFET with its control.

3.4.1.3 IGBT

The IGBT (Insulated Gate Bipolar Transistor) is a kind of cross between the MOSFET and
bipolar transistors, combining the characteristics of fast low-energy switching of the first
with the very low on-state voltage drop of the second (especially for high voltages). The sub-
stantial development work conducted on its technology since the start of the 1990s led to the
emergence of highly varied ranges with breakdown voltages of several thousand volts and
switching currents of several thousand amps. Its switching performance is quite respectable
and it can be used at chopping frequencies from a few kilohertz to several tens of kilohertz.

In view of its characteristics, it has become the essential component in power converters
in a power range which now extends from a few kilowatts to a few megawatts. Its voltage
control and encapsulation, as modules (including legs, even three-phase structures), particu-
larly suited to power wiring, have probably played a major role in its success.

Although in its internal structure it resembles the bipolar transistor, the IGBT is charac-
terized by an insulated gate control similar to that of the MOSFET (Figure 3.26). Concerning
its gate drive, the same comments as those made for the MOSFET apply, noting however
that, being bipolar, it is naturally slower, especially on opening. Consequently, its control
does not have to take full advantage of the speed of the MOSFET input stage and may be
less efficient.

150 Hybrid vehicles

Figure 3.26
Symbol of the IGBT with its control.

3.4.2 Commutation
As already mentioned in the previous paragraph, the basic components of power electronics
operate as two-state switches: closed (on) or open (off). Commutation is the transition from
one state to the other: when the component closes, we speak of turn-on, when it opens we
speak of turn-off. Depending on the conditions under which these commutations take place,
we can identify two types of commutation: natural or forced.
3.4.2.1 Natural Commutation
Commutation results from changes to the current and voltage conditions outside the compo-
nent; it therefore takes place independently of any control signal. This commutation mode
governs in particular the operation of rectifier circuits.
3.4.2.2 Forced Commutation
When the component state change, especially opening, is caused by a control signal, we
speak of forced commutation which governs the operation of standalone inverters and chop-
pers, for example.
3.4.2.3 Dead Time During Commutation
Controlled semiconductors (IGBT, MOSFET, etc.) do not switch instantaneously; they
require a finite time to change state, as illustrated on Figure 3.27 for a MOSFET.

Precautions must therefore be taken when determining the control of switches, especially
to prevent the possibility of a short-circuit when two components are placed in series in the
same branch of a structure.

Chapter 3 · Electric drivetrain 151

Figure 3.27
Experimental readings, switching times: turn-on and turn-off for a MOSFET
transistor (AP6030BVR, 600 V, 21 A).

A safety delay, or turn-on delay, is therefore introduced in the control of each semicon-
ductor so that the additional switch can recover all its turn-off capacities; this safety delay is
called the dead time.

3.4.3 Electrical Conversion Structures

This chapter describes the main power electronics structures used in electric vehicles: invert-
ers (DC-AC structures), choppers (DC-DC structures) and rectifiers (AC-DC structures), and
an example of concrete implementation in a vehicle.

3.4.3.1 DC-AC Conversion Structures

A voltage source inverter is a static converter of electrical energy which converts direct cur-
rent into alternating current. Current source inverters also exist, but in this book, we will only
discuss the voltage inverter, the most widely used to control electric machines for automotive
applications.

For automotive applications, a voltage source inverter consists of three elementary cells
(Figure 3.28.a), also called inverter legs (one leg for each phase of the electric machine).

If we observe the operation of a two level inverter leg [0, E] (E is the voltage at the
inverter leg input), we see that the low-frequency components of the output voltage (exclud-
ing harmonic components due to modulation) must necessarily lie in the range [0, E], maxi-
mum values that the average value can reach over the chopping period of this output voltage.

152 Hybrid vehicles

Figure 3.28
Diagram of a two level voltage source inverter leg and output signals.
In the highly traditional configuration of sine wave generation, the wave form which
seems to be the most obvious (corresponding to purely sinusoidal modulation) is that shown
on Figure 3.29.b. The maximum amplitude of the sinusoidal signal generated (excluding
the DC component) is about E/2. Compound voltages 13 of maximum amplitude V3.E/2 can
therefore be obtained.
When controlling electric machines with pulse width modulation inverters (3.5.2), the
alternating voltage output from the inverter consists of a series of voltage steps whose width
varies depending on the reference and the modulation strategy. The fundamental of the cur-
rent to be imposed for the machine is smoothed by its inductance. Figure 3.29.b illustrates the
change in compound voltage U A B and phase current at the output of the three-phase inverter
shown on Figure 3.29.a.

Figure 3.29
Graph of current/voltage signals in a two-level inverter structure.
13. The compound voltage between two phases is the difference between the simple voltages of these
two phases.

Chapter 3 · Electric drivetrain 153

Two-level voltage source inverters are by far the most widely used power structures in
vehicle electric traction. These power structures now exist in modules available to the general
public, but nevertheless have major disadvantages:

- the semiconductor components (switches) must be dimensioned with respect to the
maximum voltage of the DC bus,

- switching losses are high,
- the quality ofthe alternating signals is rather poor and depends on the control technique.

These disadvantages are all the more penalizing since the trend in the automotive indus-
try is currently moving towards increasingly high DC bus voltages to reduce the size of the
machines, as we will see below.

Depending on the control technique used, these inverters deliver voltage steps which vary
within the interval [-E +E] (Figure 3.29b). These voltage steps supply the A, B and C phases
(Figure 3.29.a) of the machine, with the inductive effect in the windings creating a roughly
sinusoidal current of the required amplitude and frequency (Figure 3.29.b).

The efficiencies of the inverters currently used in electric and hybrid vehicles generally
range between 85% and 97%. This efficiency is usually estimated on steady-state operating
points. Note, however, that variations in DC bus voltage and machine current may reduce this
stated efficiency by some ten per cent.

Research studies in power electronics have led to other inverter topologies which,
although now increasingly understood, are not yet applied in the automotive industry. They
include in particular multilevel inverters, whose main advantage lies in the possibility of
increasing the voltage supported by the inverter while keeping the same components. In addi-
tion, these solutions reduce the harmonics generated [Song Manguelle, 2004].

3.4.3.2 DC-DC Conversion Structures

A. Principle

A DC-DC converter transfers energy between a direct voltage source and a direct current
source, as shown on Figure 3.30.

In general, the DC-DC converters used in cars consist of a single leg with two switches
(IGBT, Figure 3.30) interfacing a voltage source in input and a current receiver at output.

Concerning the circuit, the type of converter can be considered as a "transformer" of
direct electrical quantities. Depending on the input and output quantities, it performs two
functions.

If U{ > U0 =^> step-down DC-DC converter (buck).
If U0 > U{ => step-up DC-DC converter (boost) (e.g. DC-DC in the THS).
For this type of converter, we speak of current bidirectionality when the input or output
current is capable of changing direction while the converter is operating.

154 Hybrid vehicles

Figure 3.30
Diagram of a DC-DC converter and schematic diagram of its automotive
application.

B. Application as Voltage Booster
A DC-DC converter can be used as voltage booster. In this type of application; it decorre-
lates the battery voltage of an electric or hybrid vehicle from that of the DC bus connected to
the inverter powering the electric machine(s) (boost). Toyota used this configuration on the
Prius 2 in 2004 in order to reduce the battery voltage from 275 V to 200 V and increase the
DC voltage seen by the 275 V inverters to a maximum of 500 V. Toyota has since extended
the use of this component to its entire range, while increasing the maximum network voltage
to 650 V (Figure 3.31).

Figure 3.31
Role of the DC-DC converter in the THS; Prius 2 and RX400L
Source: [Killmann, 2006]
The advantages of using the DC-DC converter in this configuration can be summarized
as follows:
- possibility of using a low-voltage battery,
- use of inverters with very high DC bus voltages, to reduce the size of the machine
size and optimize its operation at high speed, in particular by reducing the need for
defluxing,

Chapter 3 · Electric drivetrain 155

- broader speed and torque variation range on the machine (Figure 3.32),
- better total efficiency of the electric conversion chain for some operating points.

The disadvantages include higher cost, weight and volume, as well as the presence of
additional losses in the electric conversion chain.

Figure 3.32

Tm_v and P„_v curves of the 2003 Prius 2 electric machine.
Sornudrxce: [Okrnadmx ura et al, 2003]

A voltage booster adds an extra degree of freedom by constantly adjusting the network
DC voltage. When the vehicle is used, the onboard management system tries to determine the
network voltage required to reach the operating point of the electric machines and minimize
the machine and inverter losses, especially by reducing the current required for defluxing. A
compromise must nevertheless be reached since the voltage booster creates additional losses
in the drive, which rise with the voltage increase ratio, as illustrated on Figure 3.33. This
curve was plotted by modeling the current-reversible boost DC-DC converter used for the
Lexus LS600h (power 37 kW, input voltage 288 V).

156 Hybrid vehicles

Figure 3.33
Graph of efficiency against voltage conversion ratio.
C. Coupling of Battery-Supercapacitor Sources

DC-DC converters can also be used to couple onboard energy storage units: for example, to
couple a battery (energy source) and a supercapacitor (power source), as illustrated on Fig-
ure 3.34. In this configuration, the battery imposes the voltage at converter output. The con-
verter current is controlled directly so that the supercapacitor supplies or absorbs the power
peaks when acceleration and braking, respectively. A current-reversible boost converter is
used, since the voltage from the supercapacitor can drop by 50% (in order to use a range of
75% of the supercapacitor energy).

Figure 3.34
Example of using a DC-DC converter for battery/supercapacitor coupling.

Chapter 3 · Electric drivetrain 157

D. Supplying a Low-Voltage Network

DC-DC converters can also be used to interface the vehicle power battery (several hun-
dred volts) with the 12 V network of the onboard auxiliaries (buck). For safety reasons, the
onboard network (12 V) must be separated (isolated) from the power network, for example
by using a DC-DC topology providing galvanic isolation. This topology has an intermedi-
ate stage (transformer inserted in the DC-DC converter) which provides galvanic isolation
and offers the possibility of operating with very high switching frequencies. In this case, the
output and input static potentials are independent.

3.4.3.3 AC-DC Conversion Structures

AC-DC converters (rectifiers) are used to convert an alternating voltage system (e.g. the AC
grid) into a direct voltage to supply DC loads. For automotive applications, this type of con-
version structure is used in particular in the battery chargers for electric vehicles and plug-in
hybrid vehicles. We will consider the case of a single-phase 3 kVA charger dedicated to
charges at home or at work, which represents the majority of applications for private vehicles.

The difficulty with a single-phase grid is due to the fact that the power supplied fluctuates
more than with a three-phase grid, bearing in mind that standards on harmonic disturbances
(IEC-EN-61000-3-2) require sinusoidal absorption of the current and correction of the power
factor. While respecting these requirements, the charger must supply the battery with a spe-
cific current and voltage profile, in order to protect the cells and optimize their lifetime and
the charging time. In addition, to protect persons and equipment, the charger must provide
galvanic isolation between the grid and the vehicle. Not forgetting one of the main automo-
tive constraints: small size so that it can be carried without penalizing the vehicle.

Isolation transformers operating on 50 Hz are relatively large and heavy. For example,
a Legrand single-phase transformer delivering 1.6 kVA weighs 25 kg; the weight of the
20 kVA model increases to 140 kg.

Chargers therefore operate at higher transformation frequencies (medium frequency
transformers), bearing in mind that the size of the magnetic circuit decreases as the frequency
increases. Increasing the frequency may reduce the AC-DC converter efficiency, however.
An extra DC-DC conversion stage must therefore be added. The efficiency of a 50 Hz single-
phase transformer is typically in the region of 98% in this power range, whereas the effi-
ciency of a transformer followed by a bridge rectifier is typically about 93%.

Figure 3.35 shows the possible configuration of galvanic isolation in a battery charger.

Figure 3.35
Example of galvanic isolation in a battery charger.

158 Hybrid vehicles

Controlling the charging power is more complex in a single-phase grid than in a three-
phase grid. In three-phase, the input power is virtually constant and the power injected in the
battery is easily controlled; in single-phase, the input power fluctuates.

Two battery charging strategies are mainly used:
- charging at constant current/constant voltage (CCCV),
- charging by a series of short charge/relaxation/discharge sequences.
In the first case, unidirectional current rectification structures are used. In the second
case, a current-reversible charger must be used; two situations may then occur:
- the converter structure comprises a passive storage circuit to store the negative current
pulses,
- the current-reversible converter directly reinjects current in the grid.
Numerous studies are currently underway on reversible charger structures offering energy
exchange possibilities between vehicle storage and house (Vehicle To House - V2H concept),
even an exchange between the vehicle and the grid (Vehicle To Grid - V2G concept). These
points are detailed in paragraph 5.3.2.5.

3.4.3.4 Example of Implementation in a Vehicle
The various manufacturers use different types of electric drive architecture. The most com-
plete architectures are proposed, in particular, by Toyota and Lexus.

For example, Toyota's architecture for the Prius 2010 (figure 3.36) consists of a DC-DC
converter used as voltage booster, to create a 650 V DC network and two DC-AC convert-
ers supplying the electric machines of the power-split transmission. Note that some models
may include a third inverter to power an electric machine on the rear axle in a 4WD version,
especially on the Lexus RX450h (Chapter 5).

Figure 3.36

Electric architecture of the Toyota Prius 2010.
Source: [Olszewski, 2011]

Chapter 3 · Electric drivetrain 159

The figure highlights the importance of the passive components (chokes, capacitors) fit-
ted between the chopper and the inverters, required to adapt the sources (current/voltage) and
filter the power signals. With these components, the undulation rate obtained on the voltage
of the network supplying the inverters is negligible.

In the latest models of plug-in hybrids, an AC-DC converter is also fitted to charge the
battery from the grid, further improving the power electronics. Solutions are nevertheless
being investigated to use the electric machine and the inverter together for charging, to limit
the onboard power electronics.

3.4.4 Losses in the Power Converters

Miniaturization of the power electronics devices involves reducing the size of the heat sinks.
This can only be achieved if the losses dissipated in these components are reduced. Excessive
heating is in fact likely to impair the performance of the component, reduce its lifetime and
possibly even cause a failure.

A better understanding and optimized management of the losses are therefore necessary
in order to choose the best conversion structure, the components used, the control devices and
modes, as well as the method used to cool the power electronics device.

The main losses in the power converters are:

- losses in the active components (the switches controlled):
• by conduction,
• by commutation,
• due to the control (losses in the gate),

- losses in the passive components (chokes, capacitors, etc.):
• by Joule effect,
• by hysteresis,

- losses in the converter wiring and related to the length and cross-section of the wires.

A model representative of the converter in all its operating phases must be built in order to
calculate the losses in the power converters. These phases are characterized by the converter
topology and the control law used. The variables required (current, voltage, temperature,
etc.), can be obtained by analytical or numerical resolution of this model, in order to calculate
the losses in the power structure considered.

3.4.5 Modeling the Power Converters

Devices based on power electronics components are generally modeled to:
- develop prototypes, dimension the elements, adjust and optimize the parameters,
- specify the electrical constraints on the components of a system,
- validate the prototypes in normal operation and in disturbed modes (short-circuits,
voltage dips or harmonic disturbances),
- understand and analyze the electromagnetic phenomena involved,

160 Hybrid vehicles

- study interactions between systems,
- develop control laws adapted to the topology concerned.
Most simulation software packages sold (Saber, EMTP, Matlab/Simulink, Simplorer,
ARENE, etc.) propose ready-to-use component libraries including various power component
models. For applications with more specific requirements, customized models sometimes
have to be built in order to optimize the computation time, improve the representation of the
components and obtain better system observability, all variables taken into account by the
model then being accessible.
Several approaches can be adopted to model converters based on power electronics com-
ponents. Depending on the modeling objectives (control, calculation of losses, etc.), several
types of model can be developed: topological models and mean models.

3.4.5.1 Topological Model
A topological model describes the phenomena related to the component commutation; with
this objective, each power switch is accurately represented, including the elements modeling
its imperfections, such as the resistance in on-state, the junction capacities, etc. In this type
of modeling, the passive components and the connections can also be represented by fine
models, such as magnetic saturation in wound inductive elements and the impedance of the
connections. Figure 3.37 gives a representation example for an IGBT type switch.

Figure 3.37

Detailed switch model: example of an IGBT.

These very fine representations are mainly used to study the constraints on the compo-
nents and the main electromagnetic disturbances in order to assess the system electromag-
netic compatibility (EMC). Due to the very long computation times, these studies generally
focus on simulating very short operating durations of the component.

The following procedure must be implemented to create a power converter topological
model:

- choose state variables (usually currents in the chokes and voltages in the capacitors or
a linear combination of these variables),

Chapter 3 · Electric drivetrain 161

- write the differential equations governing the system as a function of the switch states
(generally, by using Kirchhoff s laws for the derivatives of the variables, respectively
voltage loops and current sums at nodes),

- format a system of differential equations with discontinuous inputs (showing the
switching functions - chopping due to modulation - which indicate the switch states).

One way of simplifying the topological models is to model the power switches by perfect
components, switching instantaneously and controlled by switching functions determining their
state: on (state 1) or off (state 0). This simplification reduces the computation time while keep-
ing the ability to study the harmonics due to the chopping frequencies. This modeling level is
therefore dimensioning for the power stage, the dynamics, the antiharmonic filters and the con-
verter protections. Simplifying the topological model in this way reduces the computation time.

3.4.5.2 Mean Model

A mean model is created using the sliding mean equation (3.8), applied to the switch control
signals and to the state variables of the topological model during a chopping period Td.

(3.8)

By applying the Laplace transform to equation (3.8), we obtain a representation in the
form of a transfer function (3.9):

(3.9)

This sliding mean method has the effect of filtering the signals and results in a loss of
information. This is expressed by a banof the sliding mean function. Attenuation of this func-
tion depends on the input signal frequency "f ' and is expressed using equation (3.10):

Attenuation (3.10)

Note that, with this type of modeling, the harmonics due to commutation are not rep-
resented. The simulation time of these models is much shorter than that obtained with the
topological models, since the signals simulated have few discontinuities and contain fewer
harmonics at high frequency. This method offers the advantage of allowing complete mod-
eling of the control loops, in order to validate the operation, adjust the parameters of the cor-
rectors used and analyze the converter dynamics.

Another type of mean modeling consists in using the Park rotating coordinate system
(3.3.1). With an inverter controlling a three-phase electric machine, if we consider that the
Park coordinate system rotates at the machine electrical frequency, all alternating quantities
projected on this coordinate system become direct in steady state. This type of modeling is
useful if we are only interested in the signal fundamental during one electrical period, typi-
cally in "system" type modeling where the aim is to study the interactions between several
devices based on power electronics.

162 Hybrid vehicles

3.5 CONTROLLING ELECTRIC MACHINES

In the previous chapters, we have described various types of electric machine and the asso-
ciated power electronics. We will now discuss the most important point, i.e. control of the
machine using its power electronics.

Simply speaking, the control system of an electric machine for an automotive application
consists of two main parts (Figure 3.38):

- a "power" part (inverter in the case of AC machines),
- a "control" part which includes:

• a control unit based on chips (DSP, microcontroller, FPGA, etc.) containing the
control algorithms and the security functions, mainly protection and management
of downgraded modes depending on the thermies of the power electronics and the
electric machine,

• a modulation unit which, depending on the references from the control, generates the
low-level control signals of the power stage switches,

• a measurement unit, which acquires and processes the electrical (currents and volt-
ages) and mechanical (position) signals required for control; temperature measure-
ments are also taken for protection purposes.

The various functional units of the control part shown on Figure 3.38 are detailed in the
following chapters.

Figure 3.38

Schematic diagram of an electronic control unit (ECU).

Chapter 3 · Electric drivetrain 163

3.5.1 Control

The control unit generate the control quantities from the references and instantaneous state of
the system in order to reach the operating point under optimum conditions (efficiency, etc.).

For permanent magnet synchronous machines, which are by far the most widely used
type in hybrid vehicles, field oriented control (FOC) is the most common (Figure 3.39). The
machine is controlled by measuring the currents in two of the stator phases and measuring the
angular position of the rotor with a synchro-resolver.

In field oriented control, an AC machine is controlled like a separately excited DC
machine, which offers the advantage of being able to control the machine torque and field
independently from each other. "Field oriented control" owes its name to the fact that, with
this type of control, the field is oriented along the "d" axis in the Park coordinate system, so
that the torque can be controlled by the current i along the "q" axis. Control algorithms are
determined using equations (3.4) to (3.7) described in paragraph 3.3.1.

Direct Torque Control (DTC) can also be used. It consists in controlling the closing and
opening of the inverter switches directly from calculated values of the torque and field.

This choice, generally based on the use of hysteresis regulators, avoids the need for the
modulation unit and the position sensor. This control strategy has various major disadvan­
tages, however: most are due to the fact that the switching frequency is highly variable,
which may lead to EMC, harmonic distortion and noise problems.

Figure 3.39
Block diagram of field oriented control (FOC).

With Ia, Ib Currents in machine phases a, b

Tref Reference torque V , Vb, Vc Voltages of machine phases a, b
ΨΓ6ί Reference field
Test Estimated torque andc
Ψ6§ί Estimated field
9r Machine rotor position Sa, Sb, Sc Control signals of the three-phase
id, i Machine currents along axes d, q
voltage inverter

164 Hybrid vehicles

3.5.2 Pulse Width Modulation (PWM)

3.5.2.1 Principle

Pulse width modulation is required to convert directly the control quantities generated by the
control into signals to turn the power switches on and off.

These control signals are generated at a given frequency (chopping frequency) and are
used to close and open each switch during a chopping period (notion of duty cycle 14). They
are used to define the amplitude, frequency and phase of a voltage or current to be imposed
on the machine.

Several methods are used to generate the control signals; we will give a brief description
of the most frequently used methods:

- the so-called intersective modulation method (also known as triangular wave or sine
wave PWM), which consists in generating the switch control signals by comparing
two signals:
• a high-frequency triangular wave, called the carrier,
• and a wave representing the signal value to be applied to the machine, called the
modulator (Figure 3.40.a);

- The Space Vector Modulation (SVM) method, which is based on the vector repre­
sentation of the three-phase voltages to be applied to the machine by a single space
vector defined in a two-phase plane (α, β). During each modulation interval, the best
possible approximation for this vector is obtained by combining the two adjacent vec­
tors defined by its current sector. Figure 3.40.b represents the six sectors bound by
vectors each representing an active state of the inverter. Vector modulation is widely
used in industrial applications, even through it requires fast digital electronics (micro­
controller, DSP or FPGA).

- For this and the previous method, the chopping frequency and the duty cycle are per­
manently managed by the control system;

- the hysteresis method, in which the power switches are controlled so that the reference
signal to be applied to the machine is permanently maintained within a hysteresis band
bounding the reference signal; the ease of implementation and its operating principle
are its main advantages, but the fact that the switchings change freely within the hyster­
esis band with an uncontrolled switching frequency represents a major disadvantage.

Far from being of secondary importance in the control of electric machines, modulation
plays a key role with consequences on all system characteristics: the drive, performance,
losses in the inverter or in the machine, acoustic noise, electromagnetic noise, even destruc­
tion of the system, due for example to overvoltage, overtemperature or overcurrent.

14. The duty cycle ratio is the ratio of the switch closure time over a chopping period.

Chapter 3 · Electric drivetrain 165

Figure 3.40
Intersective modulation and space vector modulation.

3.5.2.2 Practical Case of Modulation (Toyota THS II)
We will now give a brief description of the special case of Toyota. For more details, readers
can refer to [Séguier et al, 2011].

The modulation strategy applied in the Toyota THS II is based on the machine speed
range and the voltage level of the inverter DC bus. At low speed, the voltage of the inverter
DC bus is controlled by the DC-DC converter at 300 V. During this operating phase (low fre-
quency), PWM is used to improve the quality of the electrical signals (currents/voltages). In
the second operating area, the DC bus voltages changes to 500 V, increasing the base speed.
Within this relatively high frequency range, the THS II uses overmodulation15 to avoid very
fast voltage pulses which, since they contain a large number of harmonics, will increase the
switching losses in the inverter. In the third operating range, the DC bus voltage is controlled
at 500 V (even higher base speed). Within this area, full wave control (only one voltage step
per alternation) optimizes the inverter efficiency and maximum voltage at high speed.

With this strategy, the THS II DC-DC converter translates the electric machine torque/
speed characteristics horizontally and vertically, thereby optimizing the inverter efficiency
by modulation strategies adapted to each operating area.

15. We speak of overmodulation when the reference amplitude is greater than that of the bus voltage.
This operating mode is used at high speed to increase the voltage fundamental.

166 Hybrid vehicles

Figure 3.41
Modulation techniques used in the Toyota THS II according to the electric
machine speed/torque.
Source'. [Yaguchi and Sasaki, 2005]

3.5.3 Angular Position Measurement

Amongst all the measurements used to control a synchronous machine, the position measure-
ment is the most important and also the most difficult. A position sensor is therefore required.

Controls without position sensor nevertheless exist; they use calculation algorithms to
estimate the angular position by measuring other accessible parameters (currents, voltages).
At low speed, however, these estimation methods are not perfectly accurate.

This chapter describes two position sensors frequently employed in industrial applica-
tions: the optical encoder and the resolver.

3.5.3.1 Optical Encoder
The principle of position optical encoders is based on detection of an optical beam; the light
emitted by a LED (light emitting diode) passes through a rotating disc fastened to the motor
shaft and containing a series of transparent and opaque fixed slots, before reaching an array
of photodiodes. When the encoder shaft rotates, the various slots move in front of the photo-
diodes and the signal is delivered as a series of pulses. The angular position of the machine
is determined by counting the pulses.

Two types of encoder are available, depending on whether they measure the absolute
position (absolute encoder) or the relative position (incremental encoder):

- the incremental encoder supplies relative position information via three signals: two
quadrature signals and a third signal which only appears once per rotation to set the
index pulse. Incremental encoders are highly sensitive to mains failures (loss of power
supply), since the encoder must be reinitialized in view of the loss of true position,

- the absolute encoder provides absolute position information specific to the actual posi-
tion of the disc; it supplies a binary code of n bits corresponding to the image of the true
position of the machine to be controlled. The absolute encoder offers two advantages

Chapter 3 · Electric drivetrain 167

compared with the incremental encoder: its insensitivity to mains failures, since the
measured position is independent of any position reference before and after the mains
failure, and its insensitivity to line disturbance, since there is no counting system.

3.5.3.2 Resolver

A resolver is an inductive position sensor which, subject to a specific excitation signal, sends
two analog signals; it consists of one coil on the rotor and two coils on the stator.

The primary excitation signal is a high-frequency (a few kilohertz) sinusoidal voltage
which supplies the winding attached to the rotor. The voltages induced in the two orthogonal
stator windings have a sinusoidal shape modulated by the reference signal, and therefore by
the rotor position.

To obtain the digital value of the angular position, two approaches can be used:
- the first consists in using a Resolver-to-Digital Converter (RDC); this conversion tech-

nique provides a digital output with good noise immunity; the delay introduced by the
filter inside the RDC must nevertheless be taken into account;
- the second approach, based on software, uses a microcontroller or a digital signal
processor (DSP); in this case, the sine and cosine voltages of the resolver secondary
windings are applied to two analog/digital converters; the sampling times must be
synchronized with the positive and negative peaks of the sinusoidal excitation wave,
in order to carry out demodulation.

Resolvers offer clear advantages over encoders. They can provide the position and speed
like an encoder. In addition, they are highly robust under strong vibrations, temperature
vibrations and shocks due to the load.

Resolvers nevertheless exhibit certain weaknesses: complex acquisition (need for spe-
cific acquisition electronics), low precision and low resolution, especially under conditions
of high dynamics.

3.6 ELECTRIC MACHINE AND POWER ELECTRONICS INTEGRATION

CONSTRAINTS

The main improvements in recent years concern reductions in the cost, mass and volume of
power converters and electric machines. The challenge is to ensure excellent reliability under
severe utilization environments. The power electronics and electric machine assembly must
also be integrated in a confined environment with high thermal constraints and significant
vibratory stresses. For example, in Figure 3.42, the IISB describes the trend in electrome-
chanical integration and the associated variations in cost as well as thermal and vibratory
constraints since 1997, year when the Prius was launched [Maerz, 2007]. The new genera-
tion of Valeo "i-StARS" starter-alternators is a concrete example of this electromechanical
integration (see Valeo insert on page 316).

The data in Table 3.7 summarize the trend in the integration of power electronics and
electric machines for the Toyota Hybrid System, between 2003 and 2009, on three hybrid
models: LS 600h, Camry and Prius.

168 Hybrid vehicles

Technology maturity, production volume

Figure 3.42

Trend in electromechanical integration.
Source: [Maerz, 2007]

Table 3.7. Comparison of mass/volume/power data of the power electronics and electric drive for various
Toyota and Lexus models [Olszewski, 2009; Olszewski, 2011]

Vehicle Prius LS 600h Camry Prius
(2009) (2008)4 (2007)4 (2003)
Peak power in kW1
Power electronics2 60 110 70 50

Mass in kg 3.6 7.4 7.5 8.8

Volume in L 5.4 6.4 6 8.7

Power-to-weight ratio in kW/kg 16.6 14.9 9.3 5.7

Power-to-volume ratio in kW/L 11.1 17.2 11.7 5.7

Traction electric machine

Nominal speed in r.p.m. 13,500 10,230 14,000 6,000
Mass in kg3 36.7 44.7 41.7 45
Volume in L3 12.5 16.7 14.8 15.4

Power-to-weight ratio in kW/kg 1.6 2.5 1.7 1.1

Power-to-volume ratio in kW/L 4.8 6.6 4.7 3.2

Mass of magnets in kg (NdFeB) 0.768 1.349 0.928 1.232

Mass of stator copper in kg 4.93 3.59 5.6 6.8

1. Measurement over 18 s
2. Without the DC/DC converter
3. With casing and cooling system
4. Note that the Oak Ridge National Laboratory which conducted the tests did not find the maximum powers
announced by the manufacturer (respectively 165 and 105 kW). One of the reasons put forward is the lack of
standard to determine the power performance of electric machines (with respect to thermal problems in particular).

Chapter 3 · Electric drivetrain 169

These data were recorded by the Oak Ridge National Laboratory, which performed in-
depth studies on the drives of the first vehicles sold (for further information, readers can refer
to their site http://www.ornl.gov/). These studies have the advantage of using the same meth-
odology, which is important to compare different systems using mass and volume quantities
in particular. We may observe the progress made in terms of power-to-weight and power-to-
volume ratio on the components of each new model, as well as the effect of maximum speed
on the motor characteristics.

Lastly, integration of electric machines and power electronics is an extremely vast sub-
ject. To discuss the most relevant aspects (from our point of view), we will consider the
integration constraints for electric machines and power electronics separately.

3.6.1 Integration of the Electric Machine

Using an electric machine in transport application raises two types of problem: one thermal,
the other concerning the location in the vehicle.

Thermal problems are mainly due to the highly variable power stresses, inherent to
transport applications (as opposed to most uses in industry where the powers required are
much more stable). Although electric machines are more efficient than internal combus-
tion engines, significant power is dissipated as Joule effect losses, in a very small volume.
Two types of cooling system are generally found: natural or forced air ventilation for small
electric machines (alternators, actuators, etc.) and cooling by heat exchange fluids flowing in
the stator. Hydrogen cooling (used for example in high-power alternators of several hundred
megawatts) is dismissed by the automotive industry for power and safety reasons.

The second constraint with the electric machine is related to its location in the vehicle
and its mechanical strength. The location of the electric machine in the drivetrain will in fact
depend on the architecture and the degree of integration, but will govern in particular the
machine speed range, compactness, mechanical strength with respect to vibratory stresses
and thermal properties. Note that the location of the machines in the vehicle may also be an
important factor with respect to the dynamics, especially when they are fitted in the wheels
(unsprung mass) and/or may drive 2 or 4 wheels individually (active dynamics [5.4.2]).

Note also that, without modifying the location of the electric machine, its speed of rota-
tion can be increased through the use of a reducing gear, as is the case with the latest genera-
tion Toyota Prius. Using a reducing gear of ratio 2.6 increases the maximum speed of the
traction machine from 6,400 to 13,500 r.p.m. while increasing its power from 50 to 60 kW
and reducing its length by 4% and its mass by 20% [Yaguchi et al, 2009]. The speed increase
has been achieved by raising the DC bus voltage from 500 V to 650 V.

Other manufacturers, such as Honda, have managed to reduce the length (more than 20%)
or mass (15%) by optimizing the winding or design of their machines [Ogawa et al, 2003].

170 Hybrid vehicles

3.6.2 Integration of the Power Electronics
Integration of the power electronics concerns three functions: control and sensors, power
components (semiconductors and passive components) and cooling.

In the Lexus LS 600h electric conversion chain, for example, we observe that integrating
all the power electronics in a single unit results in a significant integration gain, by sharing
the source cooling and decoupling functions. It also simplifies the electromagnetic compat-
ibility aspects. This degree of integration involved extensive technological studies on the
components, with respect to temperature, and on the semiconductor manufacturing technolo-
gies (cooling and integration of the functions).

Manufacturers and suppliers make further optimizations whenever a new hybrid model
is launched. Toyota, for example, announced a gain of over 35% in mass and volume on
the power electronics unit of its new Prius [Yaguchi et al, 2009], while Honda had already
achieved a gain of nearly 40% on the volume of the electronics on its Civic [Ogawa et al,
2003]. Valeo achieved outstanding integration on the power electronics of its starter-alterna-
tor system (see Valeo insert). Another example of an innovating architecture is the platform
developed by Fraunhofer: a motorized axle contains two motors and the control and power
unit on the shaft (Figure 3.43).

Figure 3.43
Example of a control and power unit on a motorize axle.
Source: [Maerz et al, 2010]

3.6.2.1 Control and Sensors
Miniaturization of the power electronics has benefited considerably from the advances made
in microelectronics over the last few years. Problems concerning integration of onboard pro-
cessors, acquisition electronics, programmable logic and sensors are tackled in other fields,
such as telecommunication, robotics, etc. Regarding vibratory stresses, the automotive sector

Chapter 3 · Electric drivetrain 111

has built up know-how acquired in integration of conventional powertrain ECUs and all
onboard electronic functions in the current internal combustion engines. For the automotive
sector, however, the integration challenge is now how to use this electronics near the electri-
cal power, at minimum cost, while guaranteeing functional robustness (EMC).

Figure 3.44 shows the control electronics of the Lexus LS 600h (2008) inverters and
DC-DC converter; we can see the digital signal processors (DSPs) and the signal processing
chips of the resolvers (Tamagawa).

Figure 3.44

Control electronics of the inverters and DC-DC converter on the Lexus LS 600h
(2008).
Source: [Olszewski, 2009]

3.6.2.2 Power Components

The first efforts to integrate power semiconductors were made with the power modules, in
which several identical or complementary chips were associated to create an inverter leg for
example. All types of integration can be divided into two families: hybrid integration and
monolithic integration:

- hybrid integration is the association, on a substrate, of power, control and protection
elements using appropriate surface assembly; it is suitable for high powers (voltage
and current greater than 600 V and 30 A respectively),

- monolithic integration is the association, in a semiconductor crystal, of power, control
and protection elements using a technology derived from microelectronics; it is suit-
able for lower powers (voltage and current less than 600 V and 30 A respectively).

172 Hybrid vehicles

Recent hybrid and electric vehicles have adopted hybrid integration of power semi-
conductor chips. The first generations of these vehicles were equipped with power mod-
ules formed from semiconductor chips (MOSFET, IGBT and diodes), based on metallized
ceramic substrates.

The substrate is bonded to a base acting as mechanical support for the assembly and
providing thermal transfer to the cooling plate. The electrical connections inside the power
module are made by wire bonding. Figure 3.45 shows the IGBT chips and diodes with their
wire bonding connections in a 200 A, 600 V open module.

Figure 3.45

Open module of an IGBT integrated converter (200 A, 600 V).
Source: [Vagnon, 2010]

Electrical interconnections in power modules are currently made using wire bonding,
highly sensitive to thermal fatigue and frequently causing failures in the power modules. To
overcome these disadvantages and improve the power density, manufacturers are increasingly
considering the possibility of integrating power components (IGBT modules and diodes) in
three-dimensional (3D) structures. Several technologies are available to implement this inte-
gration, including the bump technology which consists in using copper beads or cylinders
(bumps) instead of connection wires. Figure 3.46 gives an example of an elementary switch
(IGBT, diode) where the bumps make the connection between the upper and lower sub-
strates. This type of integration is used in the Lexus LS 600h, according to an original prin-
ciple of semiconductor chips where the emitter and collector are soldered on plates directly
attached to the power busbar and the cooling plates.

Chapter 3 · Electric drivetrain 173

Figure 3.46

3D structure of an elementary switch using the bump connection technology
(developed jointly by Pearl-Alstom).
Source: [Batista, 2009]

Another way of improving the integration of power components consists in working on
new materials to replace silicon. Silicon carbide (SiC), for example, offers highly promising
integration perspectives. Its main advantages compared with silicon are:

- electric field about four times higher,
- space charge zone ten times thinner,
- resistance up to 700 times lower,
- operation at much higher temperatures.

The main objectives as regards integration of passive components (capacitors and coils)
is to reduce their mass and volume. These components, essential to all types of power con-
verter, are in fact large, heavy, expensive and generate losses.

The studies focus mainly on the operating frequency range versus the quality of the mate-
rials used. For example, the choice of switching frequencies always stems from a compro-
mise between reducing the volume of passive components and maintaining an acceptable
level of losses. In microelectronics, integration of passive components on silicon or within
the crystalline structure is relatively well understood. Porting these integration techniques to
power electronics, however, is less obvious and is lagging behind.

174 Hybrid vehicles

3.6.2.3 Thermal Behavior
Temperature behavior is key factor in the robustness, reliability and performance of power
converters. Temperature therefore plays an important role in the behavior of power electron-
ics components (active and passive).

While the trend is to miniaturize these components, which also offer higher and higher
powers, heating impacts their lifetime and reliability. Cooling the components is therefore a
constant source of concern.

Components can be cooled, for example, by contact with a solid, cooled support (sub-
strate). The quality and architecture of the contact together with the choice of materials are
then determining factors for efficient cooling. The critical parts are mainly the switch junc-
tions (IGBT, MOSFET, etc.) as well as the capacitors and filter chokes, which are also highly
sensitive to heating.

The active and passive components currently used in the automotive industry are designed
for continuous operation in temperature ranges of between - 55 °C and +125 °C. For exam-
ple, the maximum operating temperature of active components, such as IGBTs and MOS-
FETs, varies depending on the technology used but is generally in the region of 125 °C.
Generally therefore, the maximum permissible current in steady state is not limited by elec-
trical and physical considerations intrinsic to the components, but instead by the environment
and in particular their cooling system. An efficiently cooled IGBT can easily dissipate heat
fluxes of several hundred watts per square centimeter.

In confined environments such as the automobile, expecting to evacuate such high heat
flux densities with, in addition, total losses often reaching several hundred watts with simple
air dissipaters, is unrealistic. Liquid cooling (e.g. glycol water) therefore stands out as the
best solution for integration of power electronics.

Figure 3.47
Double-sided cooling structure of power modules in the Toyota LS 600.
Source: [Maerz etal, 2010]

Chapter 3 · Electric drivetrain 175

One example of efficient cooling is the system adopted in the Toyota LS 600, whose
architecture is represented on Figure 3.47. It consists of double-sided liquid cooling, where
each power arm is sandwiched between two blades of the cooling circuit. The first advantage
of this solution is that, for a given volume, the heat exchange surface is double that of a tra-
ditional one-sided cooling module. The second advantage is that the distance and interfaces
between the chips and the coolant are reduced. With this type of solution, the power modules
and the cooling system must be designed simultaneously.

3.7 PERSPECTIVES

Owing to their lower polluting emissions, hybrid/electric vehicles are becoming an unavoid-
able alternative to reach European and world objectives regarding sustainable development.
One major challenge to optimize these vehicles consists in combining as efficiently as pos-
sible the energy storage, power electronics, electric machine and control aspects with clearly
defined perspectives.

3.7.1 Power Electronics

Research studies in power electronics dedicated to hybrid/electric vehicles are being con-
ducted in several fields:

- design of new innovating topologies: the future topologies of power electronics dedi-
cated to the automobile will be highly efficient and integrate several functions (DC/
AC, DC/DC, etc.);

- use of new materials, such as silicon carbide (SiC), to manufacture power converters;
these new materials will allow faster switching and reduce the power dissipated by 30
to 50% compared with traditional silicon-based (Si) components. In addition, these
components can intrinsically operate at higher temperature;

- progress in component integration: 3D integration offers promising perspectives since
it allows double-sided exchange favoring fast evacuation of the power dissipated;

- reduction in volume and mass, any volume and mass gain remaining an important
objective in the automotive industry; to achieve this, studies are focusing for exam-
ple on reducing the size of the passive components (condensers, chokes) in static
converters;

- development of innovating charging techniques such as induction charging (static or
dynamic if the road is equipped).

176 Hybrid vehicles

3.7.2 Electric Machines

There are considerable perspectives for improving the electric machines used in hybrid/elec-
tric machines, especially a regards their miniaturization and better efficiency across the entire
operating range. The research directions in this field include:

- Machines running at higher speed, in order to reduce the mass and the volume. With
this approach, however, the entire system must be taken into account since problems
may be induced on the reduction gear and the power electronics;

- For permanent magnet machines, the search for super-powerful magnets with innovat-
ing geometries which can improve efficiency and size while offering better perfor-
mance (linearity, resistance to demagnetization, etc.);

- Research on machines that can operate without magnets (wound rotor, variable reluc-
tance, etc.) in order to limit the purchase cost and the use of rare earths;

- Development of new materials (plates, insulators) which will allow higher coil tem-
peratures and/or converter frequencies;

The solutions implemented will depend on the targeted application, they will differ
depending on whether the aim is to obtain a highly efficient machine for high-power hybrid
architectures or a more standardized machine for low-cost applications such as the micro or
micro-mild hybrids. The solutions for machines used on all-electric vehicles may also be
different.

3.7.3 Control

More knowledge is required to efficiently manage the various associations of energy sources,
power electronics and electric machine. The areas which could offer interesting perspectives
include:

- the notion of Hardware In the Loop (HIL), which will become essential to rapidly
assess components or even a complete system,

- the development of new optimum control strategies, required to optimize performance
and overall efficiency.

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I On-Board Energy
Storage Systems

Valérie Sauvant-Moynot |

4.1 STORAGE REQUIREMENTS

On-board energy storage is a key issue in the development of hybrid vehicles. As part of the
drive system, a storage system must provide both the reserve energy required to achieve the
desired vehicle range and enable that energy to be used at power levels capable of supplying
the dynamic performance levels established for the vehicle. In a conventional drive system,
storage is provided by a tank containing a hydrocarbon in liquid or gas form. In alternative
solutions, storage may appear, also in the form of a liquid or gas contained in a tank, as
hydrogen connected to a fuel cell, as a biofuel connected to an internal combustion engine,
or as electrochemical storage connected to an electric drive system.

As will be shown in greater detail in Chapter 5, hybrid engines, which make use of sev-
eral on-board storage systems, allow us to isolate and enrich those storage functions. They
include:

- Range, which is provided by high mass and high volume energy density storage that
results in very fast refueling, such as a hydrocarbon reservoir;

- Optimization of the operating conditions of the ICE and regenerative braking system,
which make use of a form of reversible storage that can supply and absorb power
with the best possible energy output. The corresponding features are described in Sec-
tion 5.3, but it is important to point out that the power and energy levels involved will
vary with these functions. This allows us to categorize reversible storage systems, as
shown in Table 4.1.

The value ranges correspond to the different vehicle classes, from lightweight vehicles
(low values) to vans (high values). For purposes of comparison, we have given the character-
istics of a battery pack for an electric vehicle, which alone provides range and dynamic per-
formance, with a power/energy (P/E) ratio that is much lower than it is for hybrid solutions.

180 Hybrid vehicles

Table 4.1. Impact of features on reversible storage characteristics of an individual vehicle in Europe

Description Feature1 Power Total P/E ratio
(see Chap. 5) inkW energy inh1
2.5 to 4 in kWh >20
Micro hybrid Stop-Start 4 to 6 <0.5 >20
Micro-mild hybrid 15 to 20 <0.5 >20
Mild hybrid + Low regenerative braking 0.5 to 1
20 to 60 15-20
Full hybrid + Average regenerative braking 1 to 2 or 3
and assistance to ICE 20 to 60 3-10
Plug-in hybrid 5 to 15 ~2
Electric vehicle + Good regenerative braking, 30 to 60
assistance to ICE and limited 15 to 30
electric mode ("supervisor")

+ Range in "driver" electric mode
and possible external recharge of
reversible storage

Range and dynamic performance

1. These ones are dealt with in detail in Chapter 5, a synthesis is presented in appendix 1

This chapter presents the operating principles, characteristics, and challenges associated
with the principal reversible storage systems used for highway transportation. Given the
importance of developments associated with vehicle electrification, battery and supercapaci-
tor solutions will be described in detail, but other solutions, such as flywheels or oleopneu-
matics, will also be covered. We also describe some nonreversible on-board storage solutions.

4.2 ELECTRICAL STORAGE

Among the devices used to store electrical energy and put it to later use in vehicles, storage
cells can convert chemical energy to electrical energy reversibly by means of electrochemical
reactions - electrochemistry being the science that describes chemical phenomena coupled
to the exchange of electrons among atoms or molecules. Supercapacitors provide a means of
direct electrical storage in electrostatic form, by accumulating static electrical charges.

4.2.1 General

4.2.1.1 Electrochemistry

Where required, readers should refer to one of the many specialized monographs on electro-
chemistry [Diard et al, 1996; Newman and Thomas-Alyea, 2004].

An electrochemical system is a heterogeneous system containing an electronic conductor
in contact with an ionic conductor, together forming an electrode. Every reversible electro-
chemical system is the seat of reciprocal transformations of chemical to electrical energy. A
storage cell, consisting of two electrochemical systems in contact, comprises two electrodes
separated by an electrolyte and connected by an external electronic conductor.

Chapter 4 · On-board energy storage systems 181

An electrode is a material that conducts electrons and ions, placed in contact with a dif-
ferent conducting medium, which releases or captures electrons and ions.

An electrolyte is a material that conducts ions exclusively, with the ability to exchange
ions between electrodes. An electrolyte must possess high conductivity and selectivity for
ions reacting at the electrodes, while blocking the mobility of electrons. Electrolytes can be
liquids (aqueous solutions, dissolved salts, solutions in an organic solvent), gels, or solids
(glass, ceramics, ionic solids, polymers).

In electrochemical systems, so-called oxidation-reduction reactions take place at the
interfaces between electrodes and electrolyte, together with a transformation of materials at
the atomic level through electron transfer. By convention, we refer to oxidation whenever
a species loses one or more electrons, and reduction whenever a species gains one or more
electrons. The balance of the transformation, corresponding to a redox half-reaction, involves
the presence of a species in two forms: the oxidizer (written Ox), or oxidized form, is the
species that undergoes reduction, and the reducer (written Red), or reduced form, undergoes
oxidation. The balance of a "simple" redox half-reaction is written as:

(4.1)

where n refers to the number of electrons, e~, exchanged between oxidizer and reducer. In
the redox pair thus formed, customarily written as Ox/Red, the degree of oxidation of the
oxidized form is greater than the reduced form, and the reduced form has more electrons than
the oxidized form.

A storage cell makes use of two redox pairs, Oxj/Redj and Ox2/Red2, located at the
electrodes on either side of an electrolyte. A spontaneous reaction takes place between the
reducer of one pair and the oxidizer of the other:

Ox Red reaction products energy (4.2)

The electrolyte is the intermediary through which the species can interact and move from

one electrode to the other during the reaction (discharge or charge), either directly as in
lithium batteries through the transport of Li+ (4.2.4.1) or in Ni-MH batteries through proton

transport (4.2.3.1), or by direct participation in the reaction, as H2S04 does in lead batteries
(4.2.2.1).

The oxidizing or reducing ability of a species is always measured against another spe-
cies on the scale of redox potentials. The oxidation-reduction potential associated with every
redox couple, or redox potential, E, is expressed in volts and can be used to predict the
reactivity of chemical species with one another. By convention, the standard potential, E°, is
measured under standard conditions of pressure (25 °C, 1 bar) in comparison to the proton/
hydrogen (H+/H2) couple of pure water, whose potential is zero. The lower the redox poten-
tial of a couple, the greater the reducing power of the couple. Conversely, the higher the
redox potential of a couple, the greater its oxidizing power.

Standard potentials (Table 4.2) of redox couples in a given solution have already been
determined and are used to classify various couples. Note that in the scale of potentials,
determined in an aqueous solution, lithium metal is the strongest reducer, i.e., the most read-
ily oxidized.

182 Hybrid vehicles

Table 4.2. Standard potentials of redox couples compared to the normal hydrogen electrode
[Robert and Alzieu, 2004a]

Redox pair E°(V)

Li/Li+ -3.04
Na/Na+ -2.71
H2/H20 -0.83
Ni(OH)2/Ni -0.72
S/S2" -0.48
Cd2+/Cd -0.40
-0.35
PbS04/Pb -0.13
Pb2+/Pb
H2/H+ 0
NiOOH/Ni(OH)2 0.45
02/H20 1.23
Pb02/PbS04 1.69

In the presence of two redox pairs, Oxj/Redj and Ox2/Red2, with respective potentials
Ej0 and E2°, where Ej0 > E2°, it is possible, using the so-called gamma rule (Figure 4.1), to
predict the direction of a spontaneous redox reaction between the strongest reducer (here pair

2) and the strongest oxidizer (pair 1). The partial reactions of type (4.1) involved, together

with the principle of conservation of charge and electroneutrality, result in the reduction of

the oxidizer of pair 1 and the oxidation of the reducer of pair 2.

Figure 4.1

Predicting the direction of a redox reaction by using the gamma rule.

The potential difference generated by the association of a reducer and an oxidizer
increases with the distance between the redox pairs on the potential scale.

If the two half-reactions take place at distinct interfaces, an electrochemical generator is
formed when two electrodes with a potential difference are connected to an external circuit.

Chapter 4 · On-board energy storage systems 183

The (spontaneous) discharge supplies electrical power to the external circuit, equal to the
product of this voltage and current. By convention, the electrode at the interface where oxi-
dation occurs is called the anode, while the electrode at the interface where reduction takes
place is called the cathode. Note, however, that a possible reaction may not take place, or
only very slowly, for kinetic reasons. On the other hand, spontaneous reactions can, unfortu-
nately, occur in an electrochemical generator when the two electrodes are disconnected from
an external circuit, leading to what are known as self-discharge losses. These are secondary
reactions.

4.2.1.2 Operation and Characteristics of a Storage Cell
[Berndt, 2003; Robert and Alzieu, 2004a]

A. Operation of a Storage Cell

By the term electrochemical generator, we generally distinguish two classes of systems,
depending on the nature of their reversibility: electrochemical storage cells are recharge-
able, or secondary, generators, and will be covered here; batteries are nonrechargeable, or
primary, generators, which provide an amount of electricity determined during their manu-
facture. However, it is worth noting that the recharging efficiency varies with the speed with
which the user intends to charge the system.

In discussing a vehicle system, we generally refer to a battery pack1, which is a complex
component consisting of storage cells, also known as elements or cells, connected in series or
parallel, and balanced by electronic components to maintain a uniform state of charge for the
entire unit. The cells are generally cooled and monitored by a battery management system.

A storage cell or battery element comprises electrochemical and technological compo-
nents (Figure 4.2):

- two electrodes (negative and positive) at a potential difference, which conduct elec-
trons and ions, and consisting of active materials deposited on a support, placed in
separate compartments and connected by an external circuit; these serve, alternately,
as anodes and cathodes;

- a porous separator preventing short-circuits between the electrodes (ordinarily a poly-
mer membrane that allows ions, but not electrons, to pass through);

- an electrolyte, an exclusively ionic conductor, which impregnates the entire system
(active materials and separator);

- two supports, current collectors (electronic conductors);
- a rigid or flexible enclosure to contain the system;
- optionally, an electrolyte filling system, a recombination compartment, or a pressure

maintenance system.

1. Etymologically, the term battery should be reserved for several storage cells combined in a serial or
parallel arrangement. However, it is currently used in French to refer to a storage cell by analogy with the
term "battery" in English, which refers to all electrochemical generators (with a distinction being made
between a "secondary" and "primary battery," depending on whether or not the system is rechargeable).

184 Hybrid vehicles

Figure 4.2

Cutaway drawing of a battery.

When the battery is operating, partial, simultaneous electrochemical oxidation and reduc-
tion reactions, distinct in space, take place at the electrode-electrolyte interface. During these
reactions, active material in the electrodes, gives up or absorbs electrons circulating in the
external circuit, while the electrolyte transfers material involved in the reactions in the form of
ions. The electric current, I, which, by definition, circulates in a closed system, from the posi-
tive to the negative pole of the generator, is electronic in nature in the external part of the circuit
and ionic inside the generator. We distinguish discharge and charge operations (Figure 4.3):

- When discharging (Figure 4.3a), reactions are spontaneous: the negative electrode
serves as the anode, where oxidation takes place, and provides electrons to the external
circuit, while the positive electrode serves as the cathode, where reduction takes place,
absorbing electrons that reach it from the external circuit. The current circulates from
the positive to the negative pole of the electrochemical cell.

- When charging (Figure 4.3b), electrical energy is supplied to the system from an exter-
nal source in order to force a reaction that is the reverse of the spontaneous discharge
reaction. Under these conditions, the negative electrode serves as the cathode, where
reduction takes place, and receives electrons from the external circuit, while the posi-
tive electrode serves as the anode, where oxidation takes place, and supplies electrons
to the external circuit. The current circulates from the positive to the negative pole of
the external source of electrical energy.

The IUPAC {International Union of Pure and Applied Chemistry) recommends that the
following nomenclature be used to refer to an electrochemical cell:

negative electrode/electrolyte/positive electrode
For example, lead starter batteries used in vehicles consist of Pb(s/H2S?4(aq)/Pb02(s)
storage cells when charged. This means that two solid lead electrodes reside in an aqueous
solution of sulfuric acid, which serves as the electrolyte.

Chapter 4 · On-board energy storage systems 185

Figure 4.3

Operation of a storage cell (a) when discharging, (b) when charging.

B. Construction of a Storage Cell

Rechargeable storage cells used in vehicle batteries exist in several formats (Figure 4.4):
cylindrical, rectangular (or prismatic), and flat, flexible, metal-plastic pouches known as
pouch cells.

The traditional cylindrical or rectangular formats are suitable for technologies using a
liquid electrolyte. In a cylindrical storage cell, the electrodes and separators are rolled into
a spiral and placed in a metal container. The container generally serves as the negative pole
of the storage cell. The cover, which contains a safety vent, is then the positive pole. In a
rectangular storage cell, successive layers of negative and positive electrodes interspersed
with separators are placed in a metal casing sealed with a welded cover. The body of the cas-
ing may be connected to a polarity, generally negative. Part of the cover includes the center
terminal, generally positive, and, often, a reversible safety vent. Finally, spiral rectangular
storage cells make use of spiral construction inside a rectangular container.

Flat, flexible storage cells were introduced more recently, primarily for use with a gel
or solid polymer electrolyte, to provide mechanical cohesion between the electrodes. Here,
the casing consists of a flexible metal-plastic film, since it only needs to keep the electrodes
and the polymer electrolyte in contact. The savings in weight and size are real and the risk of
liquid leaks nonexistent; however, the pouch must prevent water and oxygen from entering
in order to protect the electrode bundle.

Additionally, for some technologies, such as lead, Ni-MH, or Li-ion, several storage cells
can be assembled together inside the same one-piece housing (Figure 4.5).

186 Hybrid vehicles

Figure 4.4
Battery structure: (a) cylindrical, b) rectangular, (c) pouch cell.
Source: [Caillon, 2001] for (a) and (b); IFP Energies nouvelles for (c)

Figure 4.5
Example of several Ni-MH elements in a one-piece casing.

Chapter 4 · On-board energy storage systems 187

C. Electrical Characteristics of a Storage Cell

We can distinguish static parameters, associated with thermodynamic equilibrium (no cur-
rent circulates), and dynamic parameters, when the storage cell is in operation, in terms of
their thermodynamic and kinetic effects. Dynamic values can be significantly different from
static values. For a detailed review of the phenomena and physical laws underlying these
characteristics, we refer readers to fundamental works of electrochemistry, such as Diard
[1996] and Newman and Thomas-Alyea [2004], or the reference works by Caillon [2001]
and Winter and Brodd [2004].

a. Static Characteristics

From the point of view of thermodynamics, the energy per mole of a storage cell during
two electrochemical reactions, separate and coupled, is given by:

(4.3)

where n is the number of electrons exchanged, F is Faraday's constant (96,500 C/mole), and
E the voltage at the terminals of the storage cell, in volts, known as the electromotive force.

The theoretical voltage, Uth, at the terminals of an electrochemical generator is, by con-
vention, a positive value expressed in volts (V) as the potential difference between the posi-
tive and negative electrodes at equilibrium. Measured during the stationary (open system)
and relaxed (equilibrium) or quasi-stationary state (when very slowly charging or discharg-
ing the system over a period of many hours), the theoretical voltage depends on the nature of
the redox pair and the temperature. It should be pointed out that the voltage of a storage cell
can remain stable during discharge (in a lead battery, for example) or decrease, sometimes
significantly, with the change in potential at each terminal.

The rated voltage is the voltage reported by the manufacturer, determined under condi-
tions specific to that manufacturer.

The quantity of electric charge, Q, that the system can supply to the exterior without being
recharged is given in coulombs (C) or, more commonly, in amp-hours (1 Ah = 3,600 C). This
quantity depends on the nature of the redox pair, maximum and minimum voltage limits,
the amount of active material available, the state of charge, the age of the storage cell, and
finally, its temperature. Note that the capacity of the generator is less than or equal to the
minimum capacity between the negative and positive electrodes, depending on the equilib-
rium of the system.

The theoretical capacity, Cth, of a storage cell is defined as the maximum quantity of
electric charge that can be restored after a complete, continuous discharge that takes place
extremely slowly. Note that the nominal capacity of a storage cell, Cnom, is defined as the
quantity of electric charge restored during complete, continuous discharge at a current, tem-
perature, and voltage limit specified by the manufacturer.

The state of charge (SOC) of a storage cell indicates its available capacity, Cavail, com-
pared to its maximum capacity, Cmax [Cugnet, 2008]. The concept of maximum capacity is
used to address the diminished capacity of a used battery.

188 Hybrid vehicles

(4.4)

When new, the SOC can be expressed as a ratio or percentage of nominal capacity, which
varies between 100% when fully charged to 0% when discharged. Some publications refer
to depth of discharge (DOC), an alternate form of SOC, which is also expressed as a ratio or
percentage of nominal capacity, and ranges from 1-SOC to 100-SOC (%) respectively.

The state of health (SOH) of a storage cell quantifies the loss of performance resulting
from age. This change is reflected in the loss of capacity or the increase in resistance. It can
be defined as the ratio between maximum capacity and nominal capacity.

SOH (4.5)

The theoretical energy stored, customarily written Wth, is the product of the theoretical
voltage and the theoretical capacity:

(4.6)

It is expressed in joules (J) or, more commonly, in watt-hours (1 Wh = 3600 J). It depends
on the voltage of the electrochemical pair and the amount of active material available.

b. Dynamic Characteristics

Before addressing the characteristics of a storage cell not at equilibrium, we need to define
a variable that is commonly used in battery manufacture, the current rate, given in amps (A)
and expressed as a multiple or submultiple of capacity, C. C/n is the constant current needed
to discharge an element of nominal capacity C in n hours. For an element of 50 Ah, for exam­
ple, a current rate of C/10 corresponds to a current of 50/10 = 5 A, while a current rate of 2C
corresponds to a current of 50 x 2 = 100 A.

The practical voltage, Upracticai, of a storage cell varies whenever it operates and is
crossed by a non-zero current I, compared to the electromotive force, U0, of an open circuit
(Figure 4.6). The practical voltage is given by the following equation:

(4.7)

where R is the ohmic resistance of a storage cell, a function of temperature, T, and η(Ι, Τ) is
a term representing the overvoltages that depend, nonlinearly, on current and temperature.

Several distinct phenomena are the reason for the observed cumulative difference between
dynamic and static voltages, as shown in Figure 4.7 for a discharge pulse:

- the ohmic drop (Vohm) associated with the resistance of the electrolyte, the connec­
tors, and collectors;

- the voltage drop associated with the charge transfer resistance (Vct) created at the elec­
trodes as a result of the kinetics of redox reactions;

- the voltage drop associated with resistance to the transport of material in the vicinity
of the electrodes by diffusion delay (Vdiff); here, the interfacial concentration profiles
are a limiting factor in the presence of strong currents.

Chapter 4 · On-board energy storage systems 189

Figure 4.6
Change in the voltage of a storage cell as a function of the state of charge and
the discharge current.

Figure 4.7
Breakdown of the different components of cell voltage, Vcell, during a simu-
lated 70 A pulse in a 2.3 Ah LiFeP04/C Li-ion element at 50% SOC, 25 °C.
The energy dissipated when current is applied results in significant voltage changes
(which are even more pronounced when the current is high and applied continuously), which
have a direct impact on the energy output of the battery during use.

190 Hybrid vehicles

Practical capacity refers to the available capacity of a storage cell under non-standard
discharge conditions. It generally decreases whenever the discharge current is high, due to
the fall in practical voltage (Figure 4.6). But other factors also have a significant influence
on practical capacity, including the continuous or discontinuous nature of operation, tem-
perature, and age. Moreover, during storage, practical capacity can be reduced as a result of
self-discharge. We find that these losses increase with storage temperature because the kinet-
ics of secondary chemical reactions and the diffusion of materials, which are responsible for
self-discharge, are thermally activated.

The internal resistance (in ohms) of a storage cell is not limited to the ohmic resistance
of the electrodes and electrolyte, but also includes the resistance due to overvoltage at the
interface between electrodes and electrolyte. The internal resistance depends on the state
of charge of the storage cell and the method used to measure it. Generally, we calculate the
internal resistance from Ohm's law by measuring the cell's voltages after discharge, at two
levels of continuous current, successively, for several seconds.

The electric impedance of a storage cell is a complex variable relating current and volt-
age, measured by applying an alternating or transitory electric disturbance around its point of
operation. At high frequencies, electrical impedance corresponds to the ohmic resistance of
the components (electrodes and electrolyte).

The instantaneous power that a storage cell can supply or absorb, calculated as the prod-
uct of voltage and current, whether discharged or charged, is expressed in watts (W). A sim-
ple calculation shows that the maximum discharge power of a storage cell would be obtained
for a discharge where the voltage is equivalent to approximately half the no-load voltage.
But such operation can exceed the maximum operation supported by the internal connectors
and lead to dangerous overheating. The concept of maximum exchanged power is especially
critical for hybrid applications, for it involves the resistance of the battery, which will condi-
tion the alteration in instantaneous voltage, and the authorized minimum and maximum volt-
age ranges. These characteristics are dependent on the temperature, the state of charge, and
the state of health of the battery, and will also change overtime. As a result, any expression of
exchanged power must make use of all hypotheses to avoid a situation where the conditions
used fail to correspond to the actual use of the component.

c. Reduced Characteristics

When comparing different systems, we calculate the "reduced" characteristics of each stor-
age cell per unit volume (density calculation) and per mass unit (calculation of a specific
variable) [Fuhs, 2008]. In this case we speak of energy density (Wh/L) and specific energy
(Wh/kg), power density (W/L), and specific power (W/kg). The difficulty in making com-
parisons is to determine which masses and volumes we are referring to: the molar mass and
volume of the active material when designing an electrode, the actual mass and volume of
active materials, or the mass and volume of the complete system, including its technological
components. Additionally, test conditions (temperature, SOC, duration, etc.) must be speci-
fied in any comparison.

The sensitivity of power to operating parameters is illustrated in the case of a 202 V
Ni-MH pack. The change in specific power is simulated by means of an electrochemical

Chapter 4 · On-board energy storage systems 191

model as a function of pulse time (Figure 4.8) and temperature (Figure 4.9) for a high-current
HPPC 2 cycle. It appears that specific power decreases significantly when the SOC and tem-

perature decrease, or when the pulse duration increases.

Figure 4.8

Change over time of the specific power pulse during discharge and charge at
25 °C as a function of depth of discharge (for a 202 V Ni-MH pack).

Figure 4.9
Change of maximum specific power with temperature during discharge and
charge over a 10 s pulse period at different depths of discharge (for a 202 V
Ni-MH pack).

2. The Hybrid Pulse Power Characterization (HPPC) procedure, proposed in the FreedomCar Test
Manual [FreedomCAR Program Electrochemical Energy Storage Team, 2003] to determine the change
in the maximum power of battery elements, involves a test at the maximum current supported by the
battery.