Electric Bicycle
How
They Work
Electric bikes are standard bicycles with an added battery-powered
electric motor. E-bikes can push you along without pedaling
or you can pedal at the same time. The average bicyclist can
expect to ride 5-10 mph faster on an electric bike using the
same effort as on a standard bike. With an electric bicycle
you get that great cruising feeling immediately and continuously
as opposed to a standard bike where you have to work to get
to that point. Power, when activated by a switch (power-on-demand)
or in response to pedaling (ped-elec), gives an immediate
push. When you release the switch or stop pedaling, the motor
coasts or freewheels (like when you stop pedaling a standard
bicycle). Although all electric bikes are designed to work
with your pedaling, power-on-demand allows you to break the
rule. Many systems offer a variable speed control and others
are simply one speed. Certain electric bicycles will not deliver
motor power unless it senses pedaling. And its "power
output to pedal pressure" ratio is usually adjustable.
Electric bikes come in two basic designs: adaptive and purpose-built.
The adaptive type starts with a bicycle and adds a drive system
to it. A purpose-built is designed from the ground up. Adaptives
allow the rider to choose the bike or use an already owned
bicycle and will require installation. Purpose-builts often
generate interesting designs and features. Both designs contribute
to the ease of riding a bicycle. E-bikes can be powered by
one of four battery types: Nickel Cadmium, Nickel Metal Hydride,
Lead Acid or Lithium Ion.
Why
an Electric Bicycle?
Electric bicycles are a great alternative to driving a car
for short errands and pleasure riding. Their environmentally
friendly nature adds to their appeal. Electric bicycles are
quite popular these days because they allow an individual
to ride a bicycle with or without pedaling.
Electric bicycles are considered to be regular bikes for
legislative purposes. They do not have to be registered
as a motor vehicle and do not require you to buy insurance.
They are practical, both for personal transit and transporting
moderate loads. Factors such as improved batteries, decreasing
traffic congestion, monetary benefits (cheaper than owning
a car) and new bicycle infrastructure (bike lanes, bike
racks, etc.) are attracting users. To meet the demand, many
manufacturers offer e-bikes.
Safety
Electric bikes can keep you out of trouble and let you get
out of trouble more quickly. This is because the extra power
gives you extra acceleration. You are now more comparable
to a car. The faster you can accelerate, the faster you can
avoid trouble and get out of trouble.
Faster
Travel
A car is made to travel at high speeds, but in heavy traffic
and city driving the average speed drops dramatically. The
problem is of course congestion. During such conditions an
electric bicycle can maintain a higher average speed than
a regular bicycle and still take advantage of access to areas
that cars and motorcycles cannot reach. This can thus result
in a faster time from start to finish than if you drive your
car.
Personal
Fitness
It is true that a regular bike will give you a better workout
if used the same amount of time and under the same conditions.
However, surveys have shown that electric bike owners ride
their electric bikes at least twice as much as they would
if it were a regular bike. This is because an electric bike
is more enjoyable when carrying heavy loads, climbing hills,
riding into strong winds, etc., and therefore owners tend
to use it more often and try more difficult rides. Thus, even
though the electric motor provides a lot of the effort, the
fact that you are riding for a longer time and going on more
strenuous rides, can mean that the electric bike in fact gives
you more exercise.
Climbing
Hills
This is probably one of the primary advantages of an electric
bike. An electric bike with sufficient power can essentially
flatten a hill, thus increasing your average speed and vastly
improving your endurance for long rides.
Other
Considerations
Recently in the United States, legislation was passed that
set the maximum speed of an electric bike at 20 mph without
pedaling. However, with pedaling you can attain speeds of
well over 20 mph. There are many companies that manufacture
and sell electric bikes so specifications vary, but typically,
the bikes are powered by 150-400 watt motors. Also, the bikes
can cover a distance of between 10 and 25 miles on level pavement
before needing a recharge. Depending on the type and size,
recharging the battery can take between 3 and 6 hours.
Electric
Bike Performance
Electric bike performance depends on many factors as does
any other bicycle. The factors include: Terrain, speed, wind,
pulling extra objects, correct tire inflation, battery size
and type, weight of the rider and baggage and the drive system
efficiency. The speed you go makes a big difference in how
far you go. Riding at a lower speed will allow you to ride
farther. E-bike motors can peak at several hundred watts,
but most operate continuously in the range of 200-400 watts.
Electric bicycles will make a big difference in getting you
down the road and up the hill.
Cost
The initial cost for an electric bike is more than a standard
bike, but the increase in cost can be recouped if you decide
to sell it, since the evidence points to the fact that an
electric bike should have a much better resale value. The
cost of repairs is also comparable. Electricity is cheap so
that is not much of a problem. The real cost difference is
in the battery depreciation and replacement. Therefore, putting
all the cost differences together leads to the fact that an
electric bike costs more to run- from approximately $0.07-$0.13
per mile as opposed to $0.05 per mile for a non-electric bike.
However, you should really compare the running cost of an
electric bike to that of a car, when the electric bike replaces
car mileage. Depending on the weight of the car, your driving
style and the size of the engine, a car can cost you about
$0.90-$1.27 per mile to operate and thus, an electric bike
can save you a lot of money.
Batteries
Nickel
Cadmium Battery (1)
Since their development and practical application in 1961,
Nickel Cadmium batteries have been widely used due to their
reliability proven through years of use and their ease of
use in a variety of applications such as for rough use.
NiCad rechargeable batteries can be recycled, and it is
important to do so because of the toxic metal cadmium contained
in the batteries. There is growing environmental concern
over the presence of the toxic metal Cd in the battery.
Although they cost 2 to 3 times more than equivalent lead
acid batteries, nickel-cadmium batteries are widely used
in portable power applications, e.g. electric tools and
TV cameras: they are the major battery type in the IRIS
system e.g. BB521, BB590. The main advantages are a higher
energy density, especially at high discharge rates, and
a longer cycle life, typically 500 to 2000 cycles, as opposed
to the 200 to 700 cycles for automobile and sealed lead
acid batteries. Unlike lead-acid cells, individual NiCad
cells can be deeply discharged and left in this condition
without harm. It is, however, bad practice to deeply discharge
a battery of series-connected cells due to possible damage
through voltage reversal of weaker cells.
Unfortunately, NiCad cells tend to lose their charge fairly
rapidly, especially above 35°C. Furthermore, if partially
discharged to a given level and then recharged on a regular
basis, some types of cell suffer a temporary loss of capacity
called the memory effect. This effect is reversible and
full capacity can be restored by several deep discharge/charge
cycles at a low rate and some modern chargers include a
facility to do this at intervals (called charger-analyzers).
Another operational problem of NiCad batteries is that the
voltage varies very little during discharge making it difficult
to estimate the state of charge.
Nickel
Metal Hydride Battery (2)
The nickel-metal hydride cell chemistry is a hybrid of the
proven positive electrode chemistry of the sealed nickel-cadmium
cell with the energy storage features of metal alloys developed
for advanced hydrogen energy storage concepts. This heritage
in a positive-limited cell design results in batteries providing
enhanced capacities while retaining the well-characterized
electrical and physical design features of the sealed nickel-cadmium
cell design.
Nickel-metal hydride cells, with the exception of the negative
electrode, use the same general types of components as the
sealed nickel-cadmium cell. The basic concept of the nickel-metal
hydride cell negative electrode emanated from research on
the storage of hydrogen for use as an alternative energy
source in the 1970s. Certain metallic alloys were observed
to form hydrides that could capture (and release) hydrogen
in volumes up to nearly a thousand times their own volume.
By careful selection of the alloy constituents and proportions,
the thermodynamics could be balanced to permit the absorption
and release process to proceed at room temperatures and
pressures.
Two general classes of metallic alloys have been identified
as possessing characteristics desirable for battery cell
use. These are rare earth/nickel alloys generally based
around LaNi5 (the so-called AB5 class
of alloys) and alloys consisting primarily of titanium and
zirconium (designated as AB2 alloys). In both
cases, some fraction of the base metals is often replaced
with other metallic elements. The AB5 formulation
appears to offer the best set of features for commercial
nickel-metal hydride cell applications.
The metal hydride electrode has a theoretical capacity
approximately 40 percent higher than the cadmium electrode
in a nickel-cadmium couple. As a result, nickel-metal hydride
cells provide energy densities that are 20-40 percent higher
than the equivalent nickel-cadmium cell.
The nickel-metal hydride positive electrode design draws
heavily on experience with nickel-cadmium electrodes. Electrodes
that are economical and rugged exhibiting excellent high-rate
performance, long cycle life, and good capacity include
pasted and sintered-type positive electrodes.
The balance between the positive and negative electrodes
is adjusted so that the cell is always positive-limited.
This means that the negative electrode possesses a greater
capacity than the positive. The positive will reach full
capacity first as the cell is charged. It then will generate
oxygen gas that diffuses to the negative electrode where
it is recombined. This oxygen cycle is a highly efficient
way of handling moderate overcharge currents.
The electrolyte used in the nickel-metal hydride cell is
alkaline, a dilute solution of potassium hydroxide containing
other minor constituents to enhance cell performance.
The baseline material for the separator, which provides
electrical isolation between the electrodes while still
allowing efficient ionic diffusion between them, is a nylon
blend similar to that currently used in many nickel-cadmium
cells.
The nickel-metal hydride couple lends itself to the wound
construction which is similar to that used by present-day
cylindrical nickel-cadmium cells. The basic components consist
of the positive and negative electrodes insulated by separators.
The sandwiched electrodes are wound together and inserted
into a metallic can that is sealed after injection of a
small amount of electrolyte.
In variation of this design, nickel-metal hydride cells
are also being produced in prismatic versions. The prismatic
cells may fit more easily into volume-critical applications.
The general internal construction of the prismatic cell
is similar to the cylindrical cell except the single positive
and negative electrodes are now replaced by multiple electrode
sets. Thus the trade-off for improved packaging in select
applications is increased complexity in cell assembly with
the corresponding increases in production cost.
Both cylindrical and prismatic nickel-metal hydride cells
are typically two-piece sealed designs with metallic cases
and tops that are electrically insulated from each other.
The case serves as the negative terminal for the cell while
the top is the positive terminal. Some finished cell designs
may use a plastic insulating wrapper shrunk over the case
to provide electrical isolation between cells in typical
battery applications.
Nickel-metal hydride cells contain a resealable safety
vent built into the top. The nickel-metal hydride cell is
designed so the oxygen recombination cycle described earlier
is capable of recombining gases formed during overcharge
under normal operating conditions, thus maintaining pressure
equilibrium within the cell. However, in cases of charger
failure or improper cell/charger design for the operating
environment, it is possible that oxygen, or even hydrogen,
will be generated faster than it can be recombined. In such
cases the safety vent will open to reduce the pressure and
prevent cell rupture. The vent reseals once the pressure
is relieved.
Lead
Acid Battery (3)
Lead acid batteries are usually more economical and have
a high tolerance for abuse.At the positive electrode, lead
dioxide (PbO2) is converted to lead sulfate (PbSO4)
and at the negative electrode, sponge metallic lead (Pb)
is also converted to lead sulfate (PbSO4). The
electrolyte is a dilute mixture of sulfuric acid that provides
the sulfate ion for the discharge reactions. Lead acid has
become an accepted high-performance power source for clean
applications. The sealed lead cell consists of positive
and negative electrodes and their accompanying separators
that are wound in a spiral pattern. The electrodes consist
of pure lead grids pasted with mixtures of lead oxides.
These oxides are converted to the proper active materials
when the cell receives its first charge in a process called
formation. The pure lead supporting grids allow the flexibility
needed for winding the plate and also give excellent corrosion
resistance to prolong cell life. The separator consists
of a fibrous glass mat. The cell works as a starved electrolyte
system where the quantity of electrolyte is limited to the
amount that is either absorbed in the plated or wets the
fibers in the separator. The result if open gas paths between
the plates that allow gases evolved during overcharge to
diffuse from one plate to the other where they are recombined.
This recombination provides a closed system reducing venting
of gases under normal overcharge conditions. A resealing
safety vent is provided to handle pressure buildup during
abusive overcharges. Since the electrolyte is recycled,
the water loss that requires routine maintenance or limits
life is minimized. The sealed lead system has proven to
provide high performance and long life in a clean, compact
package.
Lithium
Ion Battery (4)
Current lithium battery use is primarily limited to small
electronic devices, like laptop computers and camcorders,
because of their high cost and safety concerns. Several
materials contribute to the high cost, but the most frequently
used cathode material -- lithium cobalt oxide -- is extremely
expensive.
Lithium battery research is currently underway at the US
Department of Energy's Sandia National Laboratories in New
Mexico to improve lithium ion battery materials and may
result in smaller, longer-lasting batteries for applications
as diverse as portable computers and electric vehicles.
“The research combines a new mixture of metals to
create the cathode portion of the lithium ion battery --
a high-tech, environmentally friendly electrical energy
storage device. For inorganic chemist Tim Boyle and chemical
engineer Jim Voigt, both in Sandia's Materials Processing
Department, building a better lithium ion battery is much
like baking a cake -- a matter of putting together the right
ingredients in the cathode. ‘We've tried various combinations
of lithium [a lightweight metal] with manganese, cobalt,
nickel, chromium, and aluminum and are making some breakthroughs,’
Boyle says. If the right combination of materials can be
found -- and the researchers think they are close -- lithium
ion rechargeable batteries may become economical enough
and have a long enough run time to be practical to power
electric cars or replace existing traditional lead-acid
batteries. A battery consists of three basic parts -- two
electrodes (a cathode and anode) separated by an electrolyte.
Lithium ion batteries use host materials for the electrodes
(for example, carbon as the anode and lithium cobalt oxide
as the cathode) to avoid using metallic lithium, thereby
improving safety. Electrochemical reactions at the electrodes
produce an electric current that powers an external circuit.
During charge and discharge of lithium ion rechargeable
batteries, lithium ions are shuttled between the cathode
and anode host materials in a ‘rocking horse’
fashion. Sandia has done extensive past work to improve
carbons for use as anodes. The cathode work builds on the
previous anode endeavors, says Dan Doughty, manager of Sandia's
Lithium Battery R&D Department. This is where Boyle
and Voigt's research could make a difference.
Two factors drive the quest for a better lithium ion
rechargeable battery, Boyle says. First, the batteries are
more ‘environmentally friendly.’ ‘Lithium
manganate is like sand. It has almost no environmental impact
-- unlike lead acid batteries that contain poisonous heavy
metal.’ Boyle says. ‘Also, the lithium battery
can be recharged -- meaning that it isn't thrown out, but
used over and over again.’
The second reason is that lithium batteries are lightweight
and provide more electricity than non-lithium batteries
of equal size and weight. As a result, they are ideal to
power portable electronics, a rapidly growing market. Also,
they might be used in electric cars, which require batteries
that are cheap, light, powerful, and long-lasting. The challenge,
then, is to find the right combination of cathode elements.
Boyle and Voigt are in a unique position to do this because
of a process they invented and patented three years ago
to combine elements.
Their system is a simple waterless process in which the
materials being combined are dissolved in methanol. The
solution is then dried in a vacuum, baked at 200 degrees
C in a box furnace for 24 hours, transferred to a tube furnace
where it is heated to 800 degrees C, and held for 24 hours
under a flowing oxygen atmosphere. The result is a homogenous
powder. Deciding which elements to combine is not a "hit
or miss" testing process, Boyle says. Before elements
are combined, computer models are developed showing the
structural integrity of the final material. After determining
via the computer modeling which combinations are best, the
solutions are mixed, powders processed and batteries tested.
The material's performance is tested by measuring the capacity
and useful life of the new cathode materials using electrochemical
methods. Also, X-ray diffraction is used to prove these
materials are phase pure. Boyle's experiments show that
cobalt, nickel, manganese, and other transition metals might
be the most effective combination of materials. The introduction
of the nickel to replace some of the cobalt would reduce
the cost of the final material while maintaining the high
capacity. The manganese allows for more flexibility in the
charge distribution and also would reduce costs because
it is replacing the expensive cobalt. Another advantage
of using manganese is that it is a benign material and therefore
environmentally less damaging than some of the other elements
used in lithium batteries. Sandia has long been a leader
in designing and building batteries for defense applications.
The lithium battery cathode development program is funded
by a Department of Energy Office of Basic Energy Science
initiative to develop novel, high performance battery materials.
‘They want us to find a higher capacity material for
the cathode that will give these batteries a longer life,’
Doughty says. ‘Boyle and Voigt's research fits right
into this goal.’
Work Cited
(1) “Nickel-Cadmium Batteries”.
Retrieved 19 June 2002 [online]
http://www.corrosion-doctors.org/secondaries/nickel.htm
(2) “Nickel-Metal Hydride Application
Manual”. Retrieved 19 June 2002 [online]
http://data.energizer.com/batteryinfo/
application/manuals/nickel_metal_hydride.htm
(3) “Introduction to Batteries: Lead-Acid
Batteries”. Retrieved 19 June 2002 [online]
http://www.hepi.com/basics/pb.htm
(4) “Sandia Research May Bring Smaller,
Longer Life Lithium Batteries Into Our Lives”. Retrieved
20 June 2002 [online]
http://www.sandia.gov/media/lithium.htm
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