How Electric Cars Work
Electric cars create less pollution than gasoline-powered cars, so they
are an environmentally friendly alternative to gasoline-powered vehicles
(especially in cities).
The 50-kilowatt controller of a typical electric car
A Sample Car
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:
The gasoline engine is replaced by an electric motor.
The gasoline engine, along with the muffler, catalytic converter, tailpipe
and gas tank, were all removed.
The 50-kW controller takes in 300 volts DC and produces
A battery tray was installed in the floor of the car.
The vacuum pump is left of center.
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
A small electric water heater was added to provide heat.
The water heater
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
The Magna-Charge inductive paddle charging system
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.
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).
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 electric motor
The controller normally dominates the scene when you open the hood, as you can see here:
When you push on the gas pedal, a cable from the pedal connects to these two potentiometers:
The heavy wires entering and leaving the controller
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. See How the Power Grid Works for a discussion of 3-phase power. The controller additionally provides a charging system for the batteries, and a DC-to-DC converter to recharge the 12-volt accessory battery.
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.
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.
They are heavy (a typical lead-acid battery pack weighs 1,000 pounds
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.
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
To pump electricity into the batteries as quickly as the batteries will
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
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.
Close-up of the plug
Photo courtesy Jon Mauney
The Magna-Charge System
A charging station mounted to the wall of the house
Photo courtesy Jon Mauney
A charging system in the trunk of the car
The charging system sends electricity to the car using this inductive paddle:
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.
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.
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 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,
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.
Batteries - $1,000 to $2,000
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