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Lithium-Ion Batteries: Advances and Applications
edited by G. Pistoia, Elsevier B.V.
ISBN 978-0-444-59513-3
http://dx.doi.org/10.1016/B978-0-444-59513-3.00008-X
The Voltec System:
Energy Storage and Electric Propulsion
Roland Matthé, Ulrich Eberle*
GENERAL MOTORS EUROPE, ADAM OPEL AG, RÜSSELSHEIM, GERMANY
*CORRESPONDING AUTHOR: ULRICH.EBERLE@DE.OPEL.COM.
Abstract: Vehicle electrification is progressing significantly and is changing the
architecture of future cars. This trend is a result of the need for higher vehicle efficiency
and the desire to diversify the energy sources used for transportation. Voltec vehicles such
as Chevrolet Volt and Opel Ampera are electric vehicles (EVs) with extended driving range.
They operate as an EV as long as there is useful energy in the battery. However, unlike a
pure battery EV, they do not suffer from lost vehicle utility when the battery is depleted.
Volt and Ampera can continue operation by using an internal combustion engine as
energy converter.
Within the framework of this chapter, in addition to the focus on the current Voltec
battery and propulsion system technologies, a brief history of the General Motors EV
activities is also provided.
Keywords: Battery, Chevrolet Volt, Opel Ampera, Propulsion system, Voltec.
8
The Voltec System—Energy Storage
and Electric Propulsion
Roland Matthé
1
, Ulrich Eberle
2,
*
1
GLOBAL BATTERY SYSTEMS, GME ELECTRICAL SYSTEMS, INFOTAINMENT &
ELECTRIFICATION, ADAM OPEL AG, RÜSSELSHEIM, HESSE, GERMANY,
2
HYDROGEN & ELECTRIC PROPULSION RESEARCH STRATEGY, GM ALTERNATIVE
PROPULSION CENTER, ADAM OPEL AG,RÜSSELSHEIM, HESSE, GERMANY
*
CORRESPONDING AUTHOR: ULRICH.EBERLE@DE.OPEL.COM
CHAPTER OUTLINE
1. Introduction ................................................................................................................................... 151
2. A Brief History of Electric Vehicles .............................................................................................. 152
3. Extended-Range Electric Vehicles ................................................................................................ 158
4. The Voltec Propulsion System...................................................................................................... 161
5. Voltec Drive Unit and Vehicle Operation Modes....................................................................... 164
5.1. Drive Unit Operation............................................................................................................. 164
5.2. Driver Selectable Modes ................................................................................................ ....... 165
6. Battery Operation Strategy.......................................................................................................... 165
7. Development and Validation Processes...................................................................................... 169
8. Vehicle Field Experience ................................................................................................ ............... 171
9. Summary ........................................................................................................................................ 173
Acknowledgments ............................................................................................................................. 174
Nomenclature ..................................................................................................................................... 175
References................................................................................................ ........................................... 176
1. Introduction
Today, the demand for individual mobility is still growing in many parts of the world,
particularly in China and India. Temporarily, crude oil prices have already reached values
substantially greater than US $100per barrel, depending on the global market condition and
the considered oil grade. In addition, the efforts to reduce greenhouse gas emissions to meet
regulatory targets initiated the search for low-carbon fuels and fuels from non-fossil-fuel-
based sources. This process accelerated the development of vehicles using electrified
Lithium-Ion Batteries: Advances and Applications. http://dx.doi.org/10.1016/B978-0-444-59513-3.00008-X 151
Ó2014 Elsevier B.V. All rights reserved.
propulsion systems. After having fallen into oblivion during the first decades of the twentieth
century, these technology programs had been restarted in the 1960s, when the development
of originally aerospace-related technologies enabled the creation of the world’s first fuel cell
electric vehicles (FCEV) and battery electric vehicles (BEV) equipped with high-power bat-
teries. During the 1990s, the aim of zero-emission transportation drove the development of
electric vehicles (EVs) like the GM EV1 or FCEVs like the various generations of GM
HydroGen1 to HydroGen4, as well as the purpose-built GM Sequel. Progress in power
electronics, electric motors and the lithium-ion batteries led eventually to cars based on the
Voltec system such as the Chevrolet Volt and the Opel Ampera, the first EVs with extended-
range (ER) capability in the North American (2010) and European (2011) markets. These
vehicles are utilizing a lithium-ion battery allowing 40–80 km of electric range where the
electric motors exclusively provide the full power and top speed capability. If the battery
reaches a well-defined low state of charge (SOC), a generator driven by an internal com-
bustion engine (ICE) starts to provide the required power for long-distance driving.
The Voltec vehicles are utilizing an electric air-conditioning and electric cabin heating
system. To optimize regenerative braking, the electric drive system can decelerate the
vehicle and blend this process with the hydraulic brake system when higher deceleration
is demanded.
Furthermore, test vehicles equipped with data loggers deliver important data for
development, verification and validation on public roads in the United States, Europe and
Dubai. Available real-world data confirms how significantly the Voltec propulsion
concept can replace gasoline as an energy carrier by electricity. Application of electric
energy from renewable sources is reducing the tank-to-wheels (TTW) greenhouse gas
emissions further substantially.
2. A Brief History of Electric Vehicles
In the late nineteenth and the early twentieth centuries, EVs (see Figure 8.1) played a
significant role in the emerging automotive market. The first vehicle that set a speed record
exceeding 100 km/h was the “La Jamais Contente,” an EV driven by Camille Jenatzy, a
Belgian race driver and vehicle constructor. At the time, Oldsmobile, since 1908 part of
General Motors, also manufactured EVs. EVs were easier to start and more comfortable,
therefore being the early luxury vehicles: inter alia, Thomas A. Edison and Clara Ford
owned EVs. In 1911, Charles F. Kettering, the founder and head of the GM R&D organi-
zation from 1920 to 1947, invented the electric starter for ICEs. Because of this seminal
breakthrough, which had first been applied in a Cadillac vehicle, the ICE-driven vehicles
(fueled by the more easily available gasoline, as well as providing greater range) started to
dominate the automotive markets globally.
In the 1930s, the last American company building electric road vehicles stopped
production. It took until 1964 when General Motors Research & Development integrated
a silver–zinc battery originating from the US space program and electric motors in a
Corvair-based EV, the Electrovair (see Figure 8.2(a)). In 1966, GM R&D developed the GM
Electrovan (Figure 8.2(b)), the world’s first fuel cell vehicle, with an alkaline fuel cell
152 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
converting liquid oxygen and liquid hydrogen into electric energy. To drive the wheels, an
AC induction motor was used. In 1969, General Motors presented a further experimental
car, the XP-883. This concept vehicle combined a two-door hatchback body style with a
two-cylinder opposed water-cooled engine, lead–acid batteries, a flywheel alternator and
a DC series-wound electric motor. The XP-883 became an ancestor of a vehicle concept
today known as plug-in hybrid EV. GM was also involved via its subsidiary Delco
Electronics (cofounded by Kettering) into the design, development and testing of the
Lunar Roving Vehicle which featured electric wheel hub motors and two 36-volt silver-
zinc batteries. Three of these vehicles were operated on the moon by NASA astronauts
within the framework of Apollo missions 15, 16 and 17.
FIGURE 8.1 (a) Oldsmobile electric vehicle; (b) GMC electric truck.
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 153
But since all these technology strains were not mature enough for commercial
application at that time, they fell into oblivion.
In 1987, the first “World Solar Challenge”, a solar-powered car race through Australia,
inspired General Motors, which at the time had just acquired Hughes Aircraft, to jointly
develop a competition entry with the technology company AeroVironment [1]. The
“Sunraycer” (see Figure 8.3(a)) won the race with an average speed of about 67 km/h.
Eventually, the success of “Sunraycer” convinced the GM engineers to develop the
two-seater concept EV “Impact” [1]. In 1990, the Impact was designed to feature a low
drag coefficient (c
w
value: 0.19), low vehicle mass and low rolling resistance tires.
The efficient propulsion system consisted of two AC induction motors (total rated power
of 85 kW; reduction gear ratio of 10.5:1) and a power inverter with 228 MOSFET tran-
sistors. The battery system consisting of 32 lead–acid batteries had a voltage of 320 V and
a capacity of 42.5 Ah, thus storing 13.6 kWh of energy. An acceleration time from 0 to
100 km/h in less than 9 s and a top speed of up to 128 km/h convinced all test drivers that
EVs do not need to be slow-moving “traffic obstructions”, but can accelerate easily on a
FIGURE 8.2 (a) Electrovair (1964); (b) GM Electrovan (1966). (For color version of this figure, the reader is referred to
the online version of this book.)
154 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
freeway ramp and provide the required performance in order to commute on highways.
The electric range of the Impact, although significantly depending on driving style and
weather conditions, exceeded 100 km.
The excellent impression on the public left by the GM Impact was one of the aspects
that influenced the US state of California to establish the zero-emission vehicle (ZEV)
mandate, requiring large manufacturers to sell a certain share of all vehicles as ZEVs.
Based on this concept car, the GM EV1 [1] (see Figure 8.3(b)) had been designed,
matching the original aerodynamic performance, but now using a production-optimized
AC induction motor with a side-mounted reduction gear set and differential. The newly
developed “insulated gate bipolar transistors” (IGBT) switched the DC current to
generate three-phase AC current for the induction motor. Integrated in the power inverter
housing were the inverter for the air-conditioning heat pump and the high-voltage heated
windshield. For charging, a high-frequency inductive coupler (the so-called “blade” of
Delco’s “Magne Charge” system; standardized at the time as SAE J1773) was inserted in a
slot in the car front. EV1 production started in 1996. More than 1000 vehicles have been
built and leased to customers in California, Arizona and New York for several years. The
second-generation GM EV1 eventually offered an optional nickel–metal hydride battery
pack with a total energy of 26 kWh.
FIGURE 8.3 (a) GM Sunraycer; (b) GM EV1. (For color version of this figure, the reader is referred to the online version
of this book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 155
In Europe, the drive system of the Impact propelled the Opel Impuls2, a conversion
vehicle based on the Opel Astra Caravan in 1991. A new, specifically developed AC in-
duction drive unit with IGBT inverter technology was used to build a small fleet of Impuls
vehicles (see Figure 8.4). The fleet served as an automotive test bed for the integration of
various advanced battery systems such as nickel–cadmium, nickel–metal hydride,
sodium–nickel chloride, sodium–sulfur, and sealed lead–acid.
Those battery systems allowed typically a range of up to 160 km under specific driving
conditions; but long charge times of 8–10 h, due the 3.3 kW restricted power level of
typical German 230 V single-phase outlets, demonstrated that BEVs at the time were not a
full replacement for vehicles with ICEs which can be refueled within minutes.
The fuel cell systems developed within the General Motors Fuel Cell Activities allowed
both the extension of the EV range and refueling within minutes. In the year 2000, the Opel
HydroGen1, based on the Opel Zafira, had been demonstrated to the public. Later, a small
fleet of again Opel Zafira-based HydroGen3 vehicles had been used in demonstration pro-
grams in Germany, United States, Korea and Japan. Starting in 2007, a fleet of 119 Chevrolet
Equinox FCEV, equipped with nickel–metal hydride power batteries, has been handed over
to various private and commercial customers in the United States and Germany in order to
gather experience for the next-generation fuel cell drive systems. As of mid-2012, this fleet
has accumulated more than 4 million km on public roads within the framework of GM’s
“Project Driveway”, with three cars counting each well over 110,000 km.
Progress in the technology of lithium-ion battery systems and the improvement of power
densities of electric drives enabled the concept of the so-called “extended-range electric
vehicles” (EREVs) [2,3]. A vehicle using a powertrain system of this type, such as Voltec,
operates as a high-performance EV for most trips and uses a generator in connection with an
ICE to provide energy for long-distance driving. Due to the smaller battery capacity
compared to a hypothetical BEV of the same total range, reasonable charging times of less
than 4 to 6 h, when using existing European 230 V infrastructure, are feasible.
FIGURE 8.4 Opel Impuls. (For color version of this figure, the reader is referred to the online version of this book.)
156 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
The “Chevrolet Volt” concept car [2] was presented to the public at the North
American International Auto Show 2007 in Detroit. Eventually, in 2010, the Chevrolet Volt
(see Figure 8.5) was introduced to the US market. About 1 year later, the sale of Opel
Ampera (also a Voltec-based vehicle [3]) started in Europe. In early 2013, the Cadillac ELR,
a two-door coupe using a modified and performance-optimized Voltec powertrain, was
unveiled as a 2014 production model to the general audience at the Detroit and Geneva
motor shows.
Partial electrification improves automotive fuel economy and is introduced step by step
in various vehicle classes. In 2008, the GM two-mode hybrid system had been introduced in
large vehicles such as the Chevrolet Tahoe or trucks like the Chevrolet Silverado.
The “eAssist” mild hybrid system powered by a 115-V lithium-ion battery system is
available since the sales start of the 2012 Buick Regal and Buick LaCrosse midsize
vehicles.
The overall GM electrification strategy ranges from lower levels of vehicle electrifi-
cation like stop-start systems over mild and full hybrid systems to plug-in vehicles [3,4].
This class includes EREVs based on Voltec systems, as well as mass production BEVs like
the Chevrolet Spark, and EV prototypes such as the GM EN-V two-wheeler or the Opel
Meriva MeRegioMobil (enabling bidirectional power flow [3]). Finally, FCEVs utilize the
FIGURE 8.5 (a) Chevrolet Volt; (b) the Voltec system. (For color version of this figure, the reader is referred to the
online version of this book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 157
high energy density of a chemical energy carrier, namely hydrogen, to power their electric
motor via a fuel cell as converter [5]. All these plug-in and fuel cell electric vehicle con-
cepts replace at least partially gasoline- or diesel-based fuels by potentially “renewable”
electricity or hydrogen. For both detailed and comprehensive information on energy
sources, tank-to-wheels and well-to-wheels efficiencies regarding various types of pow-
ertrain concepts, the authors recommend Refs [3,4].
3. Extended-Range Electric Vehicles
Voltec vehicles, such as the Chevrolet Volt and the Opel Ampera, are extended-range
electric vehicles (EREV). In the “charge-depletion” (CD) mode, these cars use electrical
energy from the battery system until the SOC reaches a certain defined level (Figure 8.6);
then the system passes on to the “extended-range” (ER) or “charge-sustaining” (CS)
mode: the ICE kicks in and drives the electrical generator to deliver electrical energy in
order to keep the SOC constant within a defined range. First-generation Voltec vehicles
such as the Chevrolet Volt vehicles (model year 2011) have a 54-kW electric generator
installed, driven by a 1.4-liter four-cylinder ICE. The combustion engine is switched off
when no electric power is required by the system, e.g. in case of deceleration, downhill
driving, stops or low load requirements. The Voltec system allows the selection of the
most efficient engine operation areas.
InpureEVmode(alsoknownasCDmode),theVoltec battery system provides power
for acceleration and driving up to a speed of 161 km/h. This top speed is electronically
limited. In the ER mode (also known as CS mode), the gasoline engine-powered
generator delivers up to 54 kW of electric power. If needed, the battery system pro-
vides the additional power to maintain the full acceleration capability of the 111-kW
drive system.
The EV range (see Table 8.1 and Figure 8.6) based on Voltec battery energy is
40–80 km, depending on the driving style, ambient temperature conditions and climate
comfort settings. Determining the range of the Opel Ampera using the New European
Driving Cycle (NEDC) (see Figure 8.6(b)), a value of 83 km is obtained. In combination
with the ER mode, the vehicle is able to travel up to more than 500 km before a gasoline
refill or recharge of the battery is required.
For electric recharging, a Voltec vehicle such as the Opel Ampera provides an onboard
charger module (OBCM) which can be connected to a 230-V source in Europe, either
using a wall-mounted charge cord with 16 A or to a household outlet via a transportable
cord set with 10 A (part of the vehicle standard equipment). With a level of 16 A, the
charging takes less than 4 h. Using the transportable cord set, the time will be less than 6 h
or 8 h at a user-selected reduced amperage.
The controls system of a Voltec vehicle has the purpose to efficiently manage the
energy distribution for propulsion, heating ventilation and air-conditioning, and the 12-V
system (see Figure 8.7). When driving in pure EV mode, energy from the battery system
(rechargeable energy storage system) or, on ER drives, energy from the fuel tank, has to be
158 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
managed to keep the electric energy storage systems within the allowed SOC range,
depending on the drive mode. The overall objective is the optimization of vehicle effi-
ciency. When the vehicle is plugged in via the charge cord to a power outlet, power is used
to recharge the battery and on request to heat up or cool down, respectively, the cabin by
operation of electric heater or electric air-condition system. The driver can select to
precondition the vehicle in certain cold or hot climates. By doing so, the amount of usable
energy stored in the battery can be optimized and therefore more energy is available for
propulsion purposes; consequently, the effective EV range is increased.
Time
Regenerative
braking
Engine will be off
at certain times
(b)
(a) ParkingParking Electric vehicle mode
(engine off)
= charge depletion
Extended range mode
(engine generator on)
= charge sustaining
Charge mode
electric on-board charger
connected to grid
160
120
Highway
cycle
80
40
0
0 200 400 600
Time (s)
800 1000 1200
City cycle
Velocity (km/h)
100%
State of charge
FIGURE 8.6 (a) Operation modes of extended-range electric vehicles, from Ref. [5]. (b) New European Driving Cycle.
(For color version of this figure, the reader is referred to the online version of this book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 159
unit
power
inverter
Battery energy recuperation
power
from
grid
AC
Coolant
heater
cabin
Low voltage
vehicle network
A/C system -
compressor motor
12-V Battery
FIGURE 8.7 Schematic energy flow diagram. (For color version of this figure, the reader is referred to the online
version of this book.)
Table 8.1 Opel Ampera Powertrain Specifications (Based on Voltec)
Traction motor PM synchronous motor
Maximum power 111 kW
Maximum torque 370 Nm
Internal combustion engine 1.4-l DOHC I-4 engine (63 kW)
Generator PM synchronous motor (54 kW)
Top speed 161 km/h
Acceleration time (0–100 km/h) 9 s
Energy content (battery) 16 kWh
Charging time <4 h @ 230 V, 16 A (wall box)
<6 h @ 230 V, 10 A (cord set)
Range
EV mode, real-world 40–80 km
EV mode, NEDC value 83 km
Total >500 km
Combined CO
2
emissions (CS/CD)
based on NEDC and European ECE
R101 regulation
27 g CO
2
/100 km
Combined fuel economy (CS/CD)
based on NEDC and European ECE
R101 regulation
1.2-l gasoline (premium)/100 km
DOHC, double overhead camshaft; PM, permanent magnet.
160 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
4. The Voltec Propulsion System
The main subsystems of the propulsion system are the electric drive unit, the Voltec
battery, the 1.4-l ICE, the OBCM, the auxiliary power module (APM) (HV-12-V DC/DC
converter), and the electrically driven air-conditioning and cabin heating system.
The electric drive unit (see Figure 8.8) comprises two electric motors, one planetary
gear set, the gear reduction final drive and three clutches. The traction motor is rated at
111 kW and 370 Nm and the electric generator can deliver up to 54 kW of high-voltage DC
power through the power inverter. In both cases, permanent-magnet synchronous mo-
tors are used; a Voltec vehicle requires circa 3 kg of rare-earth material.
The Voltec battery system [2] contains 288 lithium-ion pouch cells, each with a 15 Ah
capacity and a 3.8 V nominal voltage. Three Li-ion cells are connected in parallel to create
cell groups of 45 Ah. A total of 96 cell groups are connected in series and the resulting
battery’s nominal system voltage is 360 V (see Figure 8.9(a))[2]. The cell groups are in-
tegrated in nine modules assembled to three sections (see Figures 8.9 and 8.10). The
pouch cells were developed and are manufactured by LG Chem, using a manganese-
based cathode material [2]. Each cell is pressed on one side to a heat exchanger plate
which contains channels for the thermal fluid (see Figure 8.9(b)). Heat exchanger plates
and cells are stacked with plastic frames containing the fluid manifold. The thermal fluid
is pumped by an electric pump and distributed through a valve to the radiator or the
chiller connection to the air-conditioning fluid system. The battery system contains a
fluid heater to keep the battery temperature above the outside temperatures in cold
climates. This heating function, also provided during plug-in charging of the battery,
keeps the discharge and charge power level of the battery system sufficiently high to
improve vehicle acceleration and enable regenerative breaking. By doing so, a vehicle
start-up at very low ambient temperatures, down to 40 C, is possible.
The liquid-based thermal management system of the Voltec battery system is fully
integrated into the vehicle’s heating, ventilation, and air conditioning system (HVAC) via
its thermal interface, a coolant-in and coolant-out hose [2]. The coolant is normally
FIGURE 8.8 Voltec electric drive unit. (For color version of this figure, the reader isreferred to the online version of this
book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 161
Rechargeable energy storage system
Battery
disconnect unit
including heater
control
Voltage current
temperature
module
Manual service
disconnect
and fuse
On-board
charger module
Auxiliary power
module
Traction
inverter
Battery controller
VITM
Voltage temp. sub-module Voltage temp. sub-module
Voltage temp. sub-module Voltage temp. sub-module
Battery sections
FIGURE 8.10 High-voltage architecture and details of the battery system. (For color version of this figure, the reader is
referred to the online version of this book.)
FIGURE 8.9 (a) Voltec battery pack; (b) active thermal system. (For color version of this figure, the reader is referred to
the online version of this book.)
162 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
cooled by a low-temperature radiator that is especially dedicated to the battery. This
method ensures a good efficiency and minimum energy use for the thermal management
which is subsequently important for an excellent range in pure EV mode. At high ambient
temperatures, the coolant heated up in the battery system is cooled down in a two-step
process: (1) in the radiator to an intermediate temperature level and (2) further down to a
final temperature level via a chiller that is connected to the vehicle’s AC system. Thus, the
Voltec battery system can be cooled adequately at nearly all ambient temperatures and
the vehicle’s overall efficiency is optimized. In addition, at low temperatures, it is possible
to make use of the vehicle’s HVAC system for slightly heating up the battery system. This
increases the battery performance at the beginning of the trip and allows the battery
system to reach its optimum temperature earlier.
The front part of the battery system contains two main contactors, rated for maximum
operation current, which are used during drive operation. They connect the traction
power inverter module (TPIM) and the APM to the DC/DC converter providing 12 V po-
wer. Through the TPIM, power is distributed to the electrically driven air-conditioning
system and the electric coolant cabin heater (see Figures 8.7 and 8.10). Two smaller,
power-saving contactors are used during longer battery charging periods, connecting the
OBCM. The contactors are integrated within the battery disconnect unit (BDU) which also
contains the electric heater for the battery thermal fluid and the heater control device.
The front plate contains connectors for the TPIM, OBCM and APM modules, additionally
two signal connectors for the controller area network bus (CAN bus) and the 12-V power
supply. On top of the battery housing, a socket for the manual service disconnect (MSD) is
positioned. The MSD plug contains the main fuse of the system. All external connections of
the battery system are equipped with contacts for the high-voltage interlock loop. Pulling a
connector or the MSD will cause the contactors to open in order to prevent arcing.
The battery system’s total nominal energy is 16 kWh; approximately 10 kWh of the
stored energy is usable. The maximum discharge power (10 s) within the SOC operating
window exceeds 115 kW at standard temperature conditions. The total mass of the bat-
tery system amounts to 198 kg, including cables to the traction inverter and the rear
bracket.
In order to monitor the battery system voltage, all cell group voltages, the battery
current, selected cell temperatures and the battery thermal fluid temperature of the
“voltage current temperature module” (VITM) are used. The VITM is located between the
BDU and the first section. This module is connected by data bus with several “voltage
temperature sub-modules” (VTSM), placed on top of each section (see Figure 8.10). The
VTSM modules measure cell voltages: they contain a transistor and a resistor per each cell
to control the cell discharge. Cell groups with a higher voltage can be discharged to
equalize the voltages of all cell groups within the battery system. This equalization or cell
balancing is required to enable the successful long-term operation of the system.
Furthermore, every module contains cell temperature sensors connected to a VTSM.
The battery current is measured with a Hall effect sensor. The VITM signals are processed
and analyzed by a battery-state-estimation algorithm in order to obtain the SOC value.
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 163
Additionally, the VITM signals are used to determine the insulation resistance between
high-voltage conductors and the vehicle ground potential.
5. Voltec Drive Unit and Vehicle Operation Modes
5.1. Drive Unit Operation
The electric drive unit (see Figures 8.8 and 8.11) of Voltec vehicles can be operated in four
different modes [3], the two pure EV (or “charge depletion”, CS) modes with one-motor
(1) and two-motor operation (2) and the two ER (or “charge sustaining”, CS) modes in
series (3), respectively, combined operation (4).
(1) In EV operation at lower speed, with clutches C3 and C2 open and clutch C1 closed,
the drive operates in “one-motor EV mode” and the traction motor exclusively propels the
vehicle. (2) In battery-only operation at higher speeds, the “two-motor EV mode” is more
efficient: clutch C2 is closed and clutches C1 and C3 are open. The two electric motors
(i.e. traction motor and generator) operate on the planetary gear, where the second motor
is now counteracting the torque on the ring gear. The torque and the speed of the two
electric motors is determined by the linear relationship defined by the planetary gearset
and can be adjusted continuously. The two-motor EV mode reduces the speed of the
electric motors. When using this mode at higher speeds, the vehicle efficiency and
consequently the mileage is improved. Considering the U.S. Highway cycle (US06), the EV
range is extended by additional 1 - 2 miles.
(3) In ER mode at lower speeds, the so-called “one-motor series ER” mode is used:
clutches C1 and C3 are closed, while clutch C2 is open (see Figure 8.11). The generator
driven by the ICE is generating electric power for the traction motor, the APM, the electric
air-conditioning system and for sustaining the battery SOC. Using the series-drive
Traction motor
Planetary sun gear
Planetary ring gear
Planetary carrier
Final drive gearing
2.16 ratio
Axle differential
C2
C1
Generator
Inverter
C3
Battery pack
FIGURE 8.11 Voltec drive architecture with clutches and planetary gear set.
164 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
configuration, there is no mechanical power flow from the engine to the wheels; the
vehicle is exclusively driven by the electric traction motor. (4) At higher speeds in ER
operation, the “two-motor combined mode” is applied: clutches C3 and C2 are closed
and clutch C1 is open. During highway driving, efficiency is improved by 10 to 15 percent
compared to (3) since the efficiency loss of the electric motor operated at high speed and
the dual energy conversion of the series path from mechanical energy to electrical energy
and back can be avoided. Engine power and traction motor power are combined in the
planetary gear set and drive the vehicle in this output power-split configuration jointly
(see Figures 8.11 and 8.12).
5.2. Driver Selectable Modes
The driver of a Voltec vehicle such as the Opel Ampera can select one of four driving
modes. After starting up the vehicle, “normal mode” is the default setting. In this mode,
the vehicle is operating as pure EV until the standard SOC level for the transition into
“charge-sustaining” operation is reached: the engine will turn on, depending on the
amount of energy required by the generator. In case the vehicle is switched into “sport
mode” by the driver, the accelerator pedal characteristics are changed. “Sport mode”
should be selected when driving at higher speeds, e.g. on a German “autobahn”, in order
to ensure “maximum power availability”. By contrast, “mountain mode” will increase the
SOC level for the transition into “charge-sustaining” operation (see Figure 8.6).
“Mountain mode” should be selected before reaching mountain pass roads or long, steep
grades at higher speeds: the extra energy reserve eventually allows combining the power
of the generator (54 kW) and battery over an extended time period to the maximum
propulsion power of 111 kW. Finally, the “hold mode” should be selected during a long-
distance trip if the driver wants to preserve some energy to enable EV mode operation at
the destination. By starting the ICE, “hold” mode will keep the SOC level fixed at the value
of the point of time when the mode was selected. The vehicle is “artificially” forced to
enter the “CS mode”.
6. Battery Operation Strategy
Operation of a lithium-ion battery requires preventing overvoltage, overcurrent and
undervoltage conditions in order to avoid cell damage or degradation of battery life.
Based on the internal resistance, the open-circuit voltage, the upper/lower voltage limit
and the current limit, the charge and discharge power will change as a function of the
SOC level.
Typically, the discharge power is maximal at high SOC and decreases toward low SOC
[2]. The charge power behaves more or less vice versa. At low and medium SOC, a good
charge power capability is observed. Details can be seen in Figure 8.13.
Similar to the charge power at low temperature, the charge power at high SOC needs to
be controlled quite precisely. As a consequence, the battery cannot be operated over the
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 165
complete SOC range. Low SOC values have to be excluded due to insufficient discharge
power; at high SOC, the traction battery cannot be used for regenerative braking any
more. Therefore, the usable energy window is only a part of the total energy [2].
At a high SOC level, the charging power has to be reduced to avoid cell overvoltage (see
Figure 8.13). To optimize regenerative braking, the electric power of the motor and the
hydraulic power have to be managed by a process called brake blending.
At very low battery SOC, the discharge power has to be reduced to avoid “cell
undervoltage”. The low-end SOC level is defined in such a way that the discharge power is
FIGURE 8.12 (a) PureEV; (b) extended-range drive operation. (For color version of this figure, the reader is referred to
the online version of this book.)
166 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
sufficient to enable transient power values of 115 kW in order to allow consistent accel-
eration and provide sufficient energy for extended overtakings (see Figure 8.13).
The total energy throughput of lithium-ion batteries is greater when many “small”
charge–discharge cycles are performed compared to a few “large” charge–discharge cy-
cles (see Figure 8.14). “Small” cycles could be, inter alia, cycles from 20% to 25% SOC. An
example of a “large cycle” would be one from 100% SOC to 5% SOC.
Stress within the cathode or anode materials can be caused when the cathode or anode
is fully “lithiated” or “delithiated”. This effect, leading to battery durability issues, has to be
understood and carefully assessed when deciding on the SOC operating strategy.
At lower temperatures, the internal resistance of the cells increases. This effect is
leading to a reduced battery power. For a consistent vehicle operation close to the rated
power, battery temperatures above 0 C are needed (see Figure 8.15). Down to 30 C, the
battery should have at least sufficient power to crank the engine. By ensuring this feature,
the EREV concept allows complementing battery power with generator power at subzero
temperatures and at low SOC levels for an improved vehicle performance.
Lithium-ion batteries are electrochemical systems whose processes are related to
temperature: higher temperatures accelerate side reactions which cause a reduction of
battery capacity or an increase in battery resistance. Long-term exposure to temperature
values above 32 C should be minimized to meet a 10-year durability target (see
Figure 8.16). Each specific battery chemistry has a different sensitivity, but the general
rule applies to all systems.
The battery operation strategy needs to balance power, energy and temperature.
Therefore, voltage, current and temperature need to be carefully monitored and
80%
60%
40%
20%
0%
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
–20%
–40%
–60%
–80%
At low SOC, the
propulsion power is
not sufficient to keep
vehicle acceleration
consistent
At high SOC, not all
regenerative braking
can be used due to
low charge power limit
25 ºC charge
25 ºC discharge
–100%
–120%
–140%
State of charge (SOC) →
Low charge
regen.
power
Charge powerDischarge power
Low
discharge
power
FIGURE 8.13 Lithium-ion battery charge and discharge power. (For color version of this figure, the reader is referred
to the online version of this book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 167
Power vs. temperature
140%
0%
20%
40%
60%
100%
120%
80%
–40 ºC –30 ºC –20 ºC –10 ºC 0 ºC 10 ºC 20 ºC 30 ºC 40 ºC 50 ºC
FIGURE 8.15 Power as function of the lithium-ion battery temperature. (For color version of this figure, the reader is
referred to the online version of this book.)
0%
0%
20%
20%
40%
40%
60%
60%
80%
80%
100%
100%
Battery energy throughput vs. usable SOC window
Increased EV range
120%
140%
160%
180%
More EV miles
per battery life
FIGURE 8.14 Total batteryenergy throughput as function of the state-of-charge window size. (For color version of this
figure, the reader is referred to the online version of this book.)
168 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
controlled. A thermal system which enables heating and cooling makes an electric pro-
pulsion system viable for real-world operation in different geographic regions and
climate zones. The power management system allows implementing operating strategies
for slow degradation and long battery lifetimes, but the basis of any such strategy remains
the selection of a robust and safe lithium-ion cell chemistry.
7. Development and Validation Processes
Modeling and experimental testing are the starting points of the development and vali-
dation chain of modern automotive products (see Figure 8.17). Therefore, standardized
tests and modeling methods are applied and the modeling results have to be verified
experimentally and the respective procedures and methods validated. The Voltec pow-
ertrain and battery systems are composed of various novel technologies and designs
which required in many cases the development of completely new test procedures.
Where applicable, e.g. for electronic controllers, existing procedures were applied or used
after modifications. This section will focus on the development and validation processes
of the battery system.
The battery development process included a large number of cell tests. For instance, in
order to characterize the cells, the usable power was measured over SOC and tempera-
ture. Furthermore, early in the process, cell abuse tests were performed to qualify their
usage in a vehicle production program. Such abuse tests included overcharge, over-
discharge, short-circuit, nail-penetration, hot-box and crush procedures.
To determine the best operation range, a large number of different cycle tests with
variations of depth of discharge, power and temperature were performed. For cell tests,
hundreds of test channels had been operated over several years. A test channel is a
bidirectional DC power supply which can be programmed to charge and discharge the
25
20
0 ºC 10 ºC 20 ºC
Batter
y
temperature
Life in years
30 ºC 40 ºC 50 ºC
15
10
5
0
Battery life / years
FIGURE 8.16 Battery life is shorter at higher temperatures. (For color version of this figure, the reader is referred to
the online version of this book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 169
battery cell according to power-over-time profile; the device measures the actual voltage
and current data. The cells are placed in environmental chambers, controllable from
40 Cupto70
C. The results of the numerous cell cycle tests are the basis for the
mathematical battery life model. The battery life model allows anticipating with sufficient
precision how different styles of vehicle usage in different climatic conditions would
affect battery capacity and resistance variations over time.
On a statistically relevant scale, the cell tests are complemented by tests at the module
and pack level, using larger battery cyclers. Within the framework of a battery pack lifecycle
test, a full charge-depletion profile is applied, followed by a simulated charge-sustaining
profile and eventually an accelerated recharge profile. The resulting complete test pro-
file is then rerun continuously as an accelerated lifetime test. Applying these profiles under
the simulated conditions of the climate zones of Detroit, Los Angeles or Phoenix in envi-
ronmental chambers, a number of battery packs have already reached durability values of
about 320,000 km without failing to meet the respective capacity or power requirements.
To validate the robustness of a Voltec battery system against vibration and shock, on
proving-ground road conditions, test profiles were recorded using a comparable con-
ventional vehicle. The profile was simulated on a full-size battery shaker combined with
temperature variations from minimum to maximum values, as required by automotive
specifications. During the vibration and thermal cycles, an electric power profile
(including the maximum charge and discharge levels) was applied to identify potential
issues as early as possible in the vehicle development process.
The Voltec battery system is designed according to the International Protection Code
(or briefly IP Code) classification IP6k9k. Dunk tests confirmed the protection against
water intrusion. Modules were tested according to the procedures of UN38-03, including
FIGURE 8.17 Development and validation processes of key requirements. (For color version of this figure, the reader is
referred to the online version of this book.)
170 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
vibration, shock and short-circuit tests. These tests are required to obtain the approval for
battery system transports by air, road or sea.
On a vehicle level, functionality, electromagnetic compatibility and the water drive-
through tests were performed. An NCAP vehicle crash performance rating of 5 stars (both
in the US and Europe) was obtained through a careful development process using, inter alia,
computer-aided engineering tools; simulation results were validated by vehicle crash tests.
Road tests were important to refine noise and vibration quality and vehicle handling and to
verify the controls functions of all systems under real-world conditions. Series production of
Voltec vehicles started at the end of 2010. Using LG Chem cells, the battery packs are
manufactured at a GM facility in Brownstown Township, Michigan. The ICE range-extender
has been originally produced at the GM/Opel powertrain plant in Vienna-Aspern, Austria.
Finally, at the GM plant in Hamtramck, Michigan, the Voltec car assembly takes place.
8. Vehicle Field Experience
In 2008, battery packs were mounted on mule vehicles for early testing of the production-
intent Voltec system. After these successful initial tests, the first Chevrolet Volt prepro-
duction vehicles (“integration vehicles”) were built in the GM prototype workshop in
summer 2009 and were used for finishing the calibration of the control systems. Most of
those vehicles had been equipped with data loggers to allow root cause analysis and
monitoring of the battery systems over extended periods. In 2010, the captured-test fleet
vehicles had been produced and were added to the test programs. Data recorders were
also installed in these cars. Enabled by the use of data loggers, one decisive result was the
successful experimental verification of the required cell-balancing quality over the course
of extended vehicle usage periods.
Vehicles in North America, Europe and the United Arab Emirates were operated and
tested on public roads by various drivers in order to gather data on battery power and the
energy flow within subsystems. The thermal system demonstrated the capability to keep
the battery temperatures below 30 C during the summer in Dubai and above 0 C during
the wintertime in Michigan. In Europe, the vehicles were driven in the stop-and-go traffic
conditions of large urban agglomerations (e.g. the Rhein-Main area around Frankfurt)
and on German autobahn sections with high average vehicle speeds on long grades
(e.g. “Albaufstieg” near Stuttgart). Real-world driving profiles from the German town of
Wiesbaden to Koblenz and vice versa are given in Figures 8.18 and 8.19. For a comparison
with the official NEDC profile and values, see Figure 8.6(b) and Table 8.1.
Real-world data recorded from the Chevrolet Volt customers in the United States via
GM’s OnStar system show that about 65% of the fleet miles are driven completely in EV
mode; for exact numbers see Figure 8.20(a). The measured clear prevalence of the EV
mode in real-world Chevrolet Volt operation is in very good agreement with the results of
the US Department of Transport 2003 BTS Omnibus Household Survey which show that
68% of US households have an average daily commute of less than 30 miles and 78% have
commute of less than 40 miles (see Figure 8.20(b)).
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 171
From Wiesbaden to Koblenz – dynamic driving style
180
160
140
120
100
80
60
40
20
0
90
80
70
60
50
40
30
20
10
0
Speed
Distance
CD mode Distance
CS mode
Vehicle speed (km / h)
Distance driven
(
km
)
Time
(
hh:mm:ss
)
00:00:00
00:03:57
00:07:54
00:11:51
00:15:48
00:19:45
00:23:42
00:27:39
00:31:36
00:35:33
00:39:30
00:43:27
00:47:24
00:51:21
00:55:18
00:59:15
01:03:12
01:07:09
01:11:06
01:15:03
01:19:00
FIGURE 8.18 Recording of a 50-km range in charge-depletion mode, real-world data (from Wiesbaden to
Koblenz); dynamic driving style. (For color version of this figure, the reader is referred to the online version of
this book.)
From Koblenz to Wiesbaden – country road, relaxed driving style
180
160
140
120
100
80
60
40
20
0
Vehicle speed (km / h)
Speed
Distance CD mode
Distance CS mode
Time
(
hh:mm:ss
)
70
60
50
40
30
20
10
90
80
0
00:00:00
00:05:42
00:11:24
00:17:06
00:22:48
00:28:30
00:34:12
00:39:54
00:45:36
00:51:18
00:57:00
01:02:42
01:08:24
01:14:06
01:19:48
01:25:30
01:31:12
01:36:54
01:42:36
01:48:18
01:54:00
Distance driven
(
km
)
FIGURE 8.19 Recording of a 86-km range in charge-depletion mode, real-world data (from Koblenz to Wiesbaden);
relaxed driving style. (For color version of this figure, the reader is referred to the online version of this book.)
172 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
9. Summary
The Chevrolet Volt and the Opel Ampera are the first extended-range electric vehicles
available in the North American and European markets. These cars allow a daily
commuter to drive w40–80 km using electric energy. Since, as an example, about 80% of
all German commuters [5] and circa. 70% of US commuters drive less than 50 km/day, the
Voltec technology offers great potential for a reduction in crude oil consumption and CO
2
emissions. Without requiring heavy infrastructure investment, the vehicles can be either
recharged from standard 230-V outlets or through a wall box connected to 230-V AC grid.
The Voltec battery and drive system is designed, developed and validated to be
operated under all climatic and traffic conditions. Data from real-world vehicle
FIGURE 8.20 (a,b) Share of driving in EV mode (Chevrolet Volt fleet in the United States, status as of July 31, 2012).
(For color version of this figure, the reader is referred to the online version of this book.)
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 173
operation confirm that these cars perform as expected under all relevant conditions. A
large majority of customers (about 65%) use it predominantly as an EV, suitable for
daily use, replacing their conventional vehicles. Customer feedback shows that drivers
highly appreciate the driving quality of fully electrified vehicles such as Volt and
Ampera.
Beside compact cars, the recent progress of automotive electrification technologies
allows opportunities to electrify other vehicle segments, as well [3,5]. Although many of
the remaining physical limitations of the various electrified propulsion systems need to
be addressed over the coming years, BEV, FCEV, and EREV powertrains (e.g. Voltec)
provide the highest potential to reduce CO
2
emissions, especially if renewable energy
sources are used to produce the required electricity and/or hydrogen.
Concurrent with these advanced propulsion technologies, the electrification of more
conventional ICE powertrains will also increase as these engines will be complemented
by integrated mild hybrid or strong hybrid systems across all vehicles classes [3].In
addition, applications that are highly sensitive to running costs, such as long-haul trucks,
could also benefit significantly from hybridization.
At the current state of technology, the BEV has range and vehicle mass limitations
due to the low energy storage density of batteries, but it shows potential for com-
mercial success in such applications as city buses and small urban vehicles.
Nowadays, EREV technology allows the end customers to drive an average distance of
40–80 km/day on electricity without the need for a second vehicle or restrictions to
vehicle use. The Voltec technology is therefore a substantial enabler for the wide-
spread use of EVs.
Both EREVs and BEVs provide opportunities for load leveling through smart charging.
This makes them a complementary technology to solar and wind power generation. In the
longer term, however, load leveling by large-scale storage of hydrogen offers the greatest
potential [3,5]. Unfortunately, the deployment of a sufficient infrastructure remains a
challenge since high infrastructure and product development investment is required for all
future energy carrier options.
Ultimately, the degree of electrification across the different application areas is a
function of energy prices, technology progress, infrastructure availability, the regulatory
framework, vehicle performance and fun-to-drive characteristics, and, finally, the overall
customer value proposition.
Acknowledgments
This chapter is based on a presentation by R. Matthe
´for the conference “Elektrik/Elektronik in Hybrid-
und Elektrofahrzeugen und elektrisches Energiemanagement” 2012, Miesbach (Germany) and on a
presentation by U. Eberle, R. Matthe
´, N. A. Brinkman, V. Formanski and U. D. Grebe for the Vienna Motor
Symposium 2012. Important contributions by H. Mettlach and L. Turner to these presentations, as well as
the tireless efforts by the US and European Voltec-related engineering and business teams, are gratefully
acknowledged.
174 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
Nomenclature
AC Alternating current
ACEA Association of the European Automotive Industry
APM Auxiliary power module
BEV Battery electric vehicle
CAFE Corporate average fuel economy
CAN Controller area network, an automotive data bus standard
CD Charge-depletion mode (SOC is decreasing)
CNG Compressed natural gas
CNGV Compressed natural gas vehicle
CONCAWE The oil companies’ European association for environment, health and
safety in refining and distribution
CS Charge-sustaining mode (SOC is constant over a period)
DFMEA Design failure mode effect analysis
DC Direct current
DOHC Double overhead camshaft
ECE United Nations Economic Commission for Europe
EMC Electromagnetic compatibility
EPA Environmental Protection Agency
ER Extended-range
EREV Extended-range electric vehicle
EU European Union
EUCAR European Council for Automotive R&D
FTP Federal test procedure
FCEV Fuel cell electric vehicle
GM General Motors
GHG Greenhouse gas emissions
HV High voltage (above 60 V in automotive applications)
HVIL High-voltage interlock loop
HVAC Heating, ventilation and air-conditioning
HV AC High-voltage alternating current
HV DC High-voltage direct current
ICE Internal combustion engine
IGBT Insulated gate bipolar transistor
JRC Joint Research Center of the European Commission
LV Low voltage (here: less than 60 V)
NEDC New European Driving Cycle
MOSFET Metal-Oxide-Semiconductor Field-Effect Transistor
MSD Manual service disconnect
NCAP New Car Assessment Program
OBCM Onboard charger module
PHEV Plug-in hybrid electric vehicle
PM Permanent magnet
R&D Research and Development
Chapter 8 • The Voltec System—Energy Storage and Electric Propulsion 175
References
[1] (a) M. Shnayerson, The Car That Could, Random House, New York, 1996;
(b) B. Tuckey, Sunraycer, Chevron Publishing Group, Hornsby, Australia, 1989.
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SAE World Conference 2011, 2011-01-1373.
[3] (a) N. Brinkman, U. Eberle, V. Formanski, U. D. Grebe, R. Matthe
´, Vehicle Electrification – Quo Vadis?
Fortschritt-Berichte VDI, Reihe 12 (Verkehrstechnik/Fahrzeugtechnik), Nr. 749, vol. 1, pp. 186–215,
ISBN 978-3-18-374912-6.
(b) U. D. Grebe and L. T. Nitz, Electrification of GM Vehicles – A portfolio of solutions,
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(b) Argonne National Laboratory, GREET1_2011 (Greenhouse Gases, Regulated Emissions, and
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[5] (a) U. Eberle, R. von Helmolt, in: G. Pistoia (Ed.), Electric and Hybrid Vehicles, Elsevier, Amsterdam,
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2010. Also available in html format, retrieved in September 2012: http://www.e-auto-industrie.de/
energie/articles/295843/.
RESS Rechargeable energy storage system
SOC State of charge
TPIM Traction power inverter module
TTW Tank-to-wheels
USABC United States Advanced Battery Consortium
VITM Voltage current temperature module
VOLTEC GM propulsion system for extended-range vehicles
VTSM Voltage temperature sub-modules
WTW Well-to-wheels
ZEV Zero-emission vehicle
176 LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS
