19-12-2012, 06:10 PM
ELECTRIC VEHICLE BATTERY SYSTEMS
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INTRODUCTION
Road vehicles emit significant air-borne pollution, including 18% of
America’s suspended particulates, 27% of the volatile organic compounds,
28% of Pb, 32% of nitrogen oxides, and 62% of CO. Vehicles
also release 25% of America’s energy-related CO2, the principle greenhouse
gas. World pollution numbers continue to grow even more
rapidly as millions of people gain access to public and personal
transportation.
Electrification of our energy economy and the rise of automotive
transportation are two of the most significant technological revolutions
of the twentieth century. Exemplifying this massive change in the
lifestyle due to growth in fossil energy supplies. From negligible energy
markets in the 1900, electrical generation now accounts for 34% of the
primary energy consumption in the United States, while transportation
consumes 27% of the energy supply. Increased fossil fuel use has
financed energy expansions: coal and natural gas provide more than
65% of the energy used to generate the nation’s electricity, while refined
crude oil fuels virtually all the 250 million vehicles now cruising the
U.S. roadways. Renewable energy, however, provides less than 2% of the
energy used in either market.
ELECTRIC VEHICLE OPERATION
The operation of an EV is similar to that of an internal combustion
vehicle. An ignition key or numeric keypad is used to power up the
vehicle’s instrumentation panels and electronic control module (ECM).
A gearshift placed in Drive or Reverse engages the vehicle. When the
brake pedal is released, the vehicle may creep in a fashion similar to an
internal combustion vehicle. When the driver pushes the accelerator
pedal, a signal is sent to the ECM, which in turn applies a current and
voltage from the battery system to the electric motor that is proportional
to the degree to which the accelerator pedal is depressed. The motor in
turn applies torque to the EV wheels. Because power/torque curves for
electric motors are much broader than those for internal combustion
(IC) engines, the acceleration of an EV can be much quicker. Most EVs
have a built-in feature called regenerative braking, which comes into
play when the accelerator pedal is released or the brake pedal is applied.
Electric Vehicle Components
The major components of the EV are an electric motor, an ECM, a traction
battery, a battery management system, a smart battery charger, a
cabling system, a regenerative braking system, a vehicle body, a frame,
EV fluids for cooling, braking, etc., and lubricants. It is important to look
at the individual functions of each of these components and how they
integrate to operate the vehicle.
Electronic Drive Systems
An EV is propelled by an electric motor. The traction motor is in turn
controlled by the engine controller or an electronic control module.
Electric motors may be understood through the principles of electromagnetism
and physics. In simple terms, an electrical conductor carrying
current in the presence of a magnetic field experiences a force
(torque) that is proportional to the product of the current and the
strength of the magnetic field. Conversely, a conductor that is moved
through a magnetic field experiences an induced current. In an electric
propulsion system, the electronic control module regulates the amount
of current and voltage that the electric motor receives. Operating voltages
can be as high as 360 V or higher. The controller takes a signal from
the vehicle’s accelerator pedal and controls the electric energy provided
to the motor, causing the torque to turn the wheels.
There are two major types of electric drive systems: alternating
current (AC) and direct current (DC). In the past, DC motors were commonly
used for variable-speed applications. Because of recent advances
in high-power electronics, however, AC motors are now more widely
used for these applications. DC motors are typically easier to control and
are less expensive, but they are often larger and heavier than AC motors.
At the same time, AC motors and controllers usually have a higher efficiency
over a large operational range, but, due to complex electronics,
the ECMs are more expensive. Today, both AC and DC technologies can
be found in commercial automobiles.
BATTERY BASICS
A battery cell consists of five major components: (1) electrodes—anode
and cathode; (2) separators; (3) terminals; (4) electrolyte; and (5) a case
or enclosure. Battery cells are grouped together into a single mechanical
and electrical unit called a battery module. These modules are electrically
connected to form a battery pack, which powers the electronic
drive systems.
There are two terminals per battery, one negative and one positive.
The electrolyte can be a liquid, gel, or solid material. Traditional batteries,
such as lead-acid (Pb-acid), nickel-cadmium (NiCd), and others have
used a liquid electrolyte. This electrolyte may either be acidic or alkaline,
depending on the type of battery. In many of the advanced batteries
under development today for EV applications, the electrolyte is a
gel, paste, or resin. Examples of these battery types are advanced sealed
Pb-acid, NiMH, and Lithium (Li)-ion batteries. Lithium-polymer batteries,
presently under development, have a solid electrolyte. In the most
basic terms, a battery is an electrochemical cell in which an electric
potential (voltage) is generated at the battery terminals by a difference
in potential between the positive and negative electrodes. When an electrical
load such as a motor is connected to the battery terminals, an electric
circuit is completed, and current is passed through the motor,
generating the torque. Outside the battery, current flows from the positive
terminal, through the motor, and returns to the negative terminal.
As the process continues, the battery delivers its stored energy from a
charged to a discharged state.
The Pb-Acid Battery
A flooded or wet battery is one that requires maintenance by periodic
replenishment of distilled water. The water is added into each cell of the
battery through the vent cap. Even today, some large uninterruptible
power supply applications use flooded lead-acid batteries as a backup
solution. Although they have large service lives of up to 20 years, they
have been known to be operational for a longer time (up to 40 years for
a Lucent Technologies round cell).
The design of flooded lead-acid battery comprises negative plates
made of lead (or a lead alloy) sandwiched between positive plates made
of lead (or a lead alloy) with calcium or antimony as an additive. The
insulator (termed as a separator) is a microporous material that allows
the chemical reaction to take place while preventing the electrodes from
shorting, owing to contact.