An electric car is a
car powered by an electric motor which is
controlled by several components which
make it function much like a standard
gasoline powered car. The only difference
is when you step on the accelerator the
electronic "brain" tells the motor how
fast to revolve. Instead of ann explosion
making pistons turn and electric car uses
clean electromagnetic forces created by
electrical current.
The
controller of a typical electric
car
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In most cases,
electric cars are created by converting a
gasoline-powered car. When you drive an
electric car, often the only thing that
clues you in to its true nature is the
fact that it is nearly silent.
Under the hood,
there are a lot of differences between
gasoline and electric cars:
- The gasoline
engine is replaced by an electric
motor.
- The electric
motor gets its power from a
controller.
- The controller
gets its power from rechargeable
batteries.
This
example Electric Car is owned by
Jon
Mauney.
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This electric
vehicle was originally a gasoline-powered
1994 Geo Prism. Here are the modifications
that turned it into an electric
car:
- The gasoline
engine, along with the muffler,
catalytic converter, tailpipe and gas
tank, were all removed.
- The clutch
assembly was removed. The existing
manual transmission was left in place,
and it was pinned in second
gear.
- A new AC
electric motor was bolted to the
transmission with an adapter
plate.
- An electric
controller was added to control the AC
motor.
The
50-kW controller takes in 300
volts DC and produces 240 volts
AC, three-phase. The box that
says "U.S. Electricar" is the
controller.
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- A battery tray
was installed in the floor of the
car.
- Fifty 12-volt
lead-acid batteries were placed in the
battery tray (two sets of 25 to create
300 volts DC).
- Electric motors
were added to power things that used to
get their power from the engine: the
water pump, power steering pump, air
conditioner.
- A vacuum pump
was added for the power brakes (which
used engine vacuum when the car had an
engine).
The
vacuum pump is left of
center.
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- The shifter for
the manual transmission was replaced
with a switch, disguised as an
automatic transmission shifter, to
control forward and
reverse.
An
automatic transmission shifter is
used to select forward and
reverse. It contains a small
switch, which sends a signal to
the
controller.
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- A small electric
water heater was added to provide
heat.
The
water
heater
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- A charger was
added so that the batteries could be
recharged. This particular car actually
has two charging systems -- one from a
normal 120-volt or 240-volt wall
outlet, and the other from a
magna-charge inductive charging
paddle.
The
120/240-volt charging
system
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The
Magna-Charge inductive paddle
charging
system
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- The gas gauge
was replaced with a volt
meter.
The
"gas gauge" in an electric car is
either a simple volt meter or a
more sophisticated computer that
tracks the flow of amps to and
from the battery
pack.
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Everything else
about the car is stock. When you get in to
drive the car, you put the key in the
ignition and turn it to the "on" position
to turn the car on. You shift into "Drive"
with the shifter, push on the accelerator
pedal and go. It performs like a normal
gasoline car. Here are some interesting
statistics:
- The range of
this car is about 50 miles (80
km).
- The 0-to-60 mph
time is about 15 seconds.
- It takes about
12 kilowatt-hours of electricity to
charge the car after a 50-mile
trip.
- The batteries
weigh about 1,100 pounds (500
kg).
- The batteries
last three to four years.
To compare the cost
per mile of gasoline cars to this electric
car, here's an example. Electricity in
North Carolina is about 8 cents per
kilowatt-hour right now (4 cents if you
use time-of-use billing and recharge at
night). That means that for a full
recharge, it costs $1 (or 50 cents with
time-of-use billing). The cost per mile is
therefore 2 cents per mile, or 1 cent with
time-of-use. If gasoline costs $1.20 per
gallon and a car gets 30 miles to the
gallon, then the cost per mile is 4 cents
per mile for gasoline.
Clearly, the "fuel"
for electric vehicles costs a lot less per
mile than it does for gasoline vehicles.
And for many, the 50-mile range is not a
limitation -- the average person living in
a city or suburb seldom drives more than
30 or 40 miles per day.
To be completely
fair, however, we should also include the
cost of battery replacement. Bbatteries
are the weak link in electric cars at the
moment. Battery replacement for this car
runs about $2,000. The batteries will last
20,000 miles or so, for about 10 cents per
mile. You can see why there is so much
excitement around fuel cells right now --
fuel cells solve the battery problem. More
details on fuel cells can be found in this
section.
The
Controller
The heart of an electric car is the
combination of:
- The electric
motor
- The motor's
controller
- The
batteries
A
simple DC controller connected to
the batteries and the DC motor.
If the driver floors the
accelerator pedal, the controller
delivers the full 96 volts from
the batteries to the motor. If
the driver take his/her foot off
the accelerator, the controller
delivers zero volts to the motor.
For any setting in between, the
controller "chops" the 96 volts
thousands of times per second to
create an average voltage
somewhere between 0 and 96
volts.
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The controller takes
power from the batteries and delivers it
to the motor. The accelerator pedal hooks
to a pair of potentiometers
(variable resistors), and these
potentiometers provide the signal that
tells the controller how much power it is
supposed to deliver. The controller can
deliver zero power (when the car is
stopped), full power (when the driver
floors the accelerator pedal), or any
power level in between.
The controller
normally dominates the scene when you open
the hood, as you can see here:
The
300-volt, 50-kilowatt controller
for this electric car is the box
marked "U.S.
Electricar."
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In this car, the
controller takes in 300 volts DC from the
battery pack. It converts it into a
maximum of 240 volts AC, three-phase, to
send to the motor. It does this using very
large transistors that rapidly turn the
batteries' voltage on and off to create a
sine wave.
When you push on the
gas pedal, a cable from the pedal connects
to these two potentiometers:
The
potentiometers hook to the gas
pedal and send a signal to the
controller.
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The signal from the
potentiometers tells the controller how
much power to deliver to the electric
car's motor. There are two potentiometers
for safety's sake. The controller reads
both potentiometers and makes sure that
their signals are equal. If they are not,
then the controller does not operate. This
arrangement guards against a situation
where a potentiometer fails in the full-on
position.
Heavy
cables (on the left) connect the
battery pack to the controller.
In the middle is a very large
on/off switch. The bundle of
small wires on the right carries
signals from thermometers located
between the batteries, as well as
power for fans that keep the
batteries cool and
ventilated.
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The
heavy wires entering and leaving
the
controller
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The controller's job
in a DC electric car is easy to
understand. Let's assume that the battery
pack contains 12 12-volt batteries, wired
in series to create 144 volts. The
controller takes in 144 volts DC, and
delivers it to the motor in a controlled
way.
The very simplest DC
controller would be a big on/off switch
wired to the accelerator pedal. When you
push the pedal, it would turn the switch
on, and when you take your foot off the
pedal, it would turn it off. As the
driver, you would have to push and release
the accelerator to pulse the motor on and
off to maintain a given speed.
Obviously, that sort
of on/off approach would work but it would
be a pain to drive, so the controller does
the pulsing for you. The controller
reads the setting of the accelerator pedal
from the potentiometers and regulates the
power accordingly. Let's say that you have
the accelerator pushed halfway down. The
controller reads that setting from the
potentiometer and rapidly switches the
power to the motor on and off so that it
is on half the time and off half the time.
If you have the accelerator pedal 25
percent of the way down, the controller
pulses the power so it is on 25 percent of
the time and off 75 percent of the
time.
Most controllers
pulse the power more than 15,000 times per
second, in order to keep the pulsation
outside the range of human hearing. The
pulsed current causes the motor housing to
vibrate at that frequency, so by pulsing
at more than 15,000 cycles per second, the
controller and motor are silent to human
ears.
An
AC controller hooks to an AC
motor. Using six sets of power
transistors, the controller takes
in 300 volts DC and produces 240
volts AC,
3-phase
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In an AC controller,
the job is a little more complicated, but
it is the same idea. The controller
creates three pseudo-sine waves. It does
this by taking the DC voltage from the
batteries and pulsing it on and off. In an
AC controller, there is the additional
need to reverse the polarity of the
voltage 60 times a second. Therefore, you
actually need six sets of transistors in
an AC controller, while you need only one
set in a DC controller. In the AC
controller, for each phase you need one
set of transistors to pulse the voltage
and another set to reverse the polarity.
You replicate that three times for the
three phases -- six total sets of
transistors.
Most DC controllers
used in electric cars come from the
electric forklift industry. The Hughes AC
controller seen in the photo above is the
same sort of AC controller used in the
GM/Saturn EV-1 electric vehicle. It can
deliver a maximum of 50,000 watts to the
motor.
The
Motor
Electric cars can use AC or DC
motors:
- If the motor is
a DC motor, then it may run on
anything from 96 to 192 volts. Many of
the DC motors used in electric cars
come from the electric forklift
industry.
- If it is an
AC motor, then it probably is a
three-phase AC motor running at 240
volts AC with a 300 volt battery
pack.
DC installations
tend to be simpler and less expensive. A
typical motor will be in the 20,000-watt
to 30,000-watt range. A typical controller
will be in the 40,000-watt to 60,000-watt
range (for example, a 96-volt controller
will deliver a maximum of 400 or 600
amps). DC motors have the nice feature
that you can overdrive them (up to
a factor of 10-to-1) for short periods of
time. That is, a 20,000 watt motor will
accept 100,000 watts for a short period of
time and deliver 5 times its rated
horsepower. This is great for short bursts
of acceleration. The only limitation is
heat build-up in the motor. Too much
overdriving and the motor heats up to the
point where it self-destructs.
AC installations
allow the use of almost any industrial
three-phase AC motor, and that can make
finding a motor with a specific size,
shape or power rating easier. AC motors
and controllers often have a regen
feature. During braking, the motor turns
into a generator and delivers power back
to the batteries.
The
Batteries
Right now, the weak link in any electric
car is the batteries. There are at least
six significant problems with current
lead-acid battery technology:
- They are heavy
(a typical lead-acid battery pack
weighs 1,000 pounds or
more).
- They are bulky
(the car we are examining here has 50
lead-acid batteries, each measuring
roughly 6" x 8" by 6").
- They have a
limited capacity (a typical lead-acid
battery pack might hold 12 to 15
kilowatt-hours of electricity, giving a
car a range of only 50 miles or
so).
- They are slow to
charge (typical recharge times for a
lead-acid pack range between four to 10
hours for full charge, depending on the
battery technology and the
charger).
- They have a
short life (three to four years,
perhaps 200 full charge/discharge
cycles).
- They are
expensive (perhaps $2,000 for the
battery pack shown in the sample
car).
You can replace
lead-acid batteries with NiMH batteries.
The range of the car will double and the
batteries will last 10 years (thousands of
charge/discharge cycles), but the cost of
the batteries today is 10 to 15 times
greater than lead-acid. In other words, an
NiMH battery pack will cost $20,000 to
$30,000 (today) instead of $2,000. Prices
for advanced batteries fall as they become
mainstream, so over the next several years
it is likely that NiMH and lithium-ion
battery packs will become competitive with
lead-acid battery prices. Electric cars
will have significantly better range at
that point.
When you look at the
problems associated with batteries, you
gain a different perspective on gasoline.
Two gallons of gasoline, which weighs 15
pounds, costs $3.00 and takes 30 seconds
to pour into the tank, is equivalent to
1,000 pounds of lead-acid batteries that
cost $2,000 and take four hours to
recharge.
The problems with
battery technology explain why there is so
much excitement around fuel cells today.
Compared to batteries, fuel cells will be
smaller, much lighter and instantly
rechargeable. When powered by pure
hydrogen, fuel cells have none of the
environmental problems associated with
gasoline. It is very likely that the car
of the future will be an electric car that
gets its electricity from a fuel cell.
There is still a lot of research and
development that will have to occur,
however, before inexpensive, reliable fuel
cells can power automobiles.
Accessory
Battery
Just about any electric car has one other
battery on board. This is the normal
12-volt lead-acid battery that every car
has. The 12-volt battery provides power
for accessories -- things like headlights,
radios, fans, computers, air bags, wipers,
power windows and instruments inside the
car. Since all of these devices are
readily available and standardized at 12
volts, it makes sense from an economic
standpoint for an electric car to use
them.
Therefore, an
electric car has a normal 12-volt
lead-acid battery to power all of the
accessories. To keep the battery charged,
an electric car needs a DC-to-DC
converter. This converter takes in the
DC power from the main battery array (at,
for example, 300 volts DC) and converts it
down to 12 volts to recharge the accessory
battery. When the car is on, the
accessories get their power from the
DC-to-DC converter. When the car is off,
they get their power from the 12-volt
battery as in any gasoline-powered
vehicle.
The DC-to-DC
converter is normally a separate box under
the hood, but sometimes this box is built
into the controller.
The
Charging System
Any electric car that uses batteries needs
a charging system to recharge the
batteries. The charging system has two
goals:
- To pump
electricity into the batteries as
quickly as the batteries will
allow
- To monitor the
batteries and avoid damaging them
during the charging process
Charging
Current
When
lead-acid batteries are at a low
state of charge, nearly all the
charging current is absorbed by
the chemical reaction. Once the
state of charge reaches a certain
point, at about 80 percent of
capacity, more and more energy
goes into heat and electrolysis
of the water. The resulting
bubbling of electrolyte is
informally called "boiling." For
the charging system to minimize
the boiling, the charging current
must cut back for the last 20
percent of the charging process.
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The most sophisticated charging systems
monitor battery voltage, current flow and
battery temperature to minimize charging
time. The charger sends as much current as
it can without raising battery temperature
too much. Less sophisticated chargers
might monitor voltage or amperage only and
make certain assumptions about average
battery characteristics. A charger like
this might apply maximum current to the
batteries up through 80 percent of their
capacity, and then cut the current back to
some preset level for the final 20 percent
to avoid overheating the
batteries.
Jon Mauney's
electric car actually has two different
charging systems. One system accepts
120-volt or 240-volt power from a normal
electrical outlet. The other is the
Magna-Charge inductive charging system
popularized by the GM/Saturn EV-1 vehicle.
Let's look at each of these systems
separately.
Normal
Household Power
The normal household charging system has
the advantage of convenience -- anywhere
you can find an outlet, you can recharge.
The disadvantage is charging
time.
A normal household
120-volt outlet typically has a 15-amp
circuit breaker, meaning that the maximum
amount of energy that the car can consume
is approximately 1,500 watts, or 1.5
kilowatt-hours per hour. Since the battery
pack in Jon's car normally needs 12 to 15
kilowatt-hours for a full recharge, it can
take 10 to 12 hours to fully charge the
vehicle using this technique.
By using a 240-volt
circuit (such as the outlet for an
electric dryer), the car might be able to
receive 240 volts at 30 amps, or 6.6
kilowatt-hours per hour. This arrangement
allows significantly faster charging, and
can fully recharge the battery pack in
four to five hours.
In Jon's car, the
gas filler spout has been removed and
replaced by a charging plug. Simply
plugging into the wall with a heavy-duty
extension cord starts the charging
process.
Opening
the gas filler door reveals the
charging
plug.
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Close-up
of the
plug
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Photo
courtesy Jon
Mauney
Plug
the car in anywhere to
recharge.
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In this car, the
charger is built into the controller. In
most home-brew cars, the charger is a
separate box located under the hood, or
could even be a free-standing unit that is
separate from the car.
The
Magna-Charge System
The Magna-Charge system consists of two
parts:
- A charging
station mounted to the wall of the
house
Photo
courtesy Jon
Mauney
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- A charging
system in the trunk of the
car
The charging station
is hard-wired to a 240-volt 40-amp circuit
through the house's circuit
panel.
The charging system
sends electricity to the car using this
inductive paddle:
Photo
courtesy Jon
Mauney
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The paddle fits into
a slot hidden behind the license plate of
the car.
Photo
courtesy Jon
Mauney
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The paddle acts as
one half of a transformer. The other half
is inside the car, positioned around the
slot behind the license plate. When you
insert the paddle, it forms a complete
transformer with the slot, and power
transfers to the car.
One advantage of the
inductive system is that there are no
exposed electrical contacts. You can touch
the paddle or drop the paddle into a
puddle of water and there is no hazard.
The other advantage is the ability to pump
a significant amount of current into the
car very quickly because the charging
station is hard-wired to a dedicated
240-volt circuit.
The competing
high-power charge connector is generally
referred to as the "Avcon plug" and
it is used by Ford and others. It features
copper-to-copper contacts instead of the
inductive paddle, and has an elaborate
mechanical interconnect that keeps the
contacts covered until the connector is
mated with the receptacle on the vehicle.
Pairing this connector with GFCI
protection makes it safe in any kind of
weather.
Equalization
Jon Mauney points out the
following:
- An important
feature of the charging process is
"equalization." An EV has a string of
batteries (somewhere between 10 and 25
modules, each containing three to six
cells). The batteries are closely
matched, but they are not identical.
Therefore they have slight differences
in capacity and internal resistance.
All batteries in a string necessarily
put out the same current (laws of
electricity), but the weaker batteries
have to "work harder" to produce the
current, so they're at a slightly lower
state of charge at the end of the
drive. Therefore, the weaker batteries
need more recharge to get back to full
charge.
Since the
batteries are in series, they also get
exactly the same amount of recharge,
leaving the weak battery even weaker
(relatively) than it was before. Over
time, this results in one battery going
bad long before the rest of the pack.
The weakest-link effect means that this
battery determines the range of the
vehicle, and the usability of the car
drops off.
The common
solution to the problem is
"equalization charge." You
gently overcharge the batteries to make
sure that the weakest cells are brought
up to full charge. The trick is to keep
the batteries equalized without
damaging the strongest batteries with
overcharging. There are more complex
solutions that scan the batteries,
measure individual voltages, and send
extra charging current through the
weakest module.
Doing
a Conversion
A majority of the electric cars on the
road today are "home brew" conversion
vehicles. People with an interest in
electric cars convert existing
gasoline-powered cars to electric in their
backyards and garages. There are many Web
sites that talk about the phenomenon and
show you how to do it, where to get parts,
etc.
A typical conversion
uses a DC controller and a DC
motor. The person doing the conversion
decides what voltage the system will run
at -- typically anything between 96 volts
and 192 volts. The voltage decision
controls how many batteries the car will
need, and what sort of motor and
controller the car will use. The most
common motors and controllers used in home
conversions come from the electric
forklift industry.
Usually, the person
doing the conversion has a "donor vehicle"
that will act as the platform for the
conversion. Almost always, the donor
vehicle is a normal gasoline-powered car
that gets converted to electric. Most
donor vehicles have a manual
transmission.
The person doing the
conversion has a lot of choices when it
comes to battery technology. The vast
majority of home conversions use lead-acid
batteries, and there are several different
options:
- Marine
deep-cycle lead-acid batteries (These
are available everywhere, including
Wal-mart.)
- Golf-cart
batteries
- High-performance
sealed batteries
The batteries can
have a flooded, gelled or AGM (absorbed
glass mat) electrolyte. Flooded batteries
tend to have the lowest cost but also the
lowest peak power.
Once the decisions
about the motor, controller and batteries
are made, the conversion can start. Here
are the steps:
- Remove the
engine, gas tank, exhaust system,
clutch and perhaps the radiator from
the donor vehicle. Some controllers
have water-cooled transistors, while
some are air-cooled.
- Attach an
adapter plate to the transmission and
mount the motor. The motor normally
requires custom mounting
brackets.
- Usually, the
electric motor needs a reduction gear
for maximum efficiency. The easiest way
to create the gear reduction is to pin
the existing manual transmission in
first or second gear. It would save
weight to create a custom reduction
gear, but normally it is too
expensive.
- Mount the
controller.
- Find space for,
and build brackets to safely hold, all
the batteries. Install the batteries.
Sealed batteries have the advantage
that they can be turned on their sides
and fitted into all sorts of nooks and
crannies.
- Wire the
batteries and motor to the controller
with #00 gauge welding cable.
- If the car has
power steering, wire up and mount an
electric motor for the power steering
pump.
- If the car has
air conditioning, wire up and mount an
electric motor for the A/C
compressor.
- Install a small
electric water heater for heat and
plumb it into the existing heater core,
or use a small ceramic electric space
heater.
- If the car has
power brakes, install a vacuum pump to
operate the brake booster.
- Install a
charging system.
- Install a
DC-to-DC converter to power the
accessory battery.
- Install some
sort of volt meter to be able to detect
state of charge in the battery pack.
This volt meter replaces the gas
gauge.
- Install
potentiometers, hook them to the
accelerator pedal and connect to the
controller.
- Most home-brew
electric cars using DC motors use the
reverse gear built into the manual
transmission. AC motors with advanced
controllers simply run the motor in
reverse and need a simple switch that
sends a reverse signal to the
controller. Depending on the
conversion, you may need to install
some sort of reverse switch and wire to
the controller.
- Install a large
relay (also known as a
contactor) that can connect and
disconnect the car's battery pack to
and from the controller. This relay is
how you turn the car "on" when you want
to drive it. You need a relay that can
carry hundreds of amps and that can
break 96 to 300 volts DC without
holding an arc.
- Rewire the
ignition switch so that it can turn on
all the new equipment, including the
contactor.
Once everything is
installed and tested, the new electric car
is ready to go!
A typical
conversion, if it is using all new parts,
costs between $5,000 and $10,000 (not
counting the cost of the donor vehicle or
labor). The costs break down like
this:
- Batteries -
$1,000 to $2,000
- Motor - $1,000
to $2,000
- Controller -
$1,000 to $2,000
- Adapter plate -
$500 to $1,000
- Other (motors,
wiring, switches, etc.) - $500 to
$1,000
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