How Electric Cars Work
Electric cars are something that show up in the news all the time. There are several reasons for the continuing interest in these vehicles:
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In this article, you will learn about electric vehicles both from the manufactured and the home-brewed points of view. You will also learn about an innovative program for middle and high school students that lets teams of students build and race electric vehicles!
A Sample Car
An electric car is a car
powered by an electric
motor rather than a gasoline
engine.
From the outside, you would probably have no idea that a car is electric. In most cases, electric cars are created by converting a gasoline-powered car, and in that case it is impossible to tell. 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:
A gasoline engine, with its fuel lines, exhaust pipes, coolant hoses and intake manifold, tends to look like a plumbing project. An electric car is definitely a wiring project.
In order to get a feeling for how electric cars work in general, let's start by looking at a typical electric car to see how it comes together. The electric car that we will use for this discussion is shown here:
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This electric vehicle began its life as a normal, gasoline-powered 1994 Geo Prism. Here are the modifications that turned it into an electric car:
The clutch assembly was removed. The existing manual transmission was left in place, and it was pinned in second gear.
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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:
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. As discussed in The Batteries, batteries 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 later in the article.
The Controller
The heart of an electric car
is the combination of:
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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:
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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:
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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.
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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.
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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:
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:
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:
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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.
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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:
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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:
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The paddle fits into a slot hidden behind the license plate of the car.
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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:
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:
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:
The EV Challenge
The EV Challenge (http://www.evchallenge.org)
is an innovative program for middle and high school students
that centers around building electric-powered cars:
The EV Challenge gets a majority of its funding from corporate sponsors and government organizations, including Advanced Energy Corporation, CP&L/Progress Energy, Duke Power, Dominion Virginia Power, the NC Energy Office, the NC Department of Environment and Natural Resources, and the EPA.
Jon Mauney (whose car is featured at the beginning of this article) is on the steering committee for EV Challenge. According to Jon, CP&L started the EV Challenge program in North Carolina. The program then spread to South Carolina, Florida, Virginia, West Virginia, and Georgia, and is now spreading nationwide. Thousands of students have participated in the EV Challenge.
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