How Electric Cars Work

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by Marshall Brain

 

 

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Cite This! Close  Please copy/paste the following text to properly cite this HowStuffWorks article:

Brain, Marshall.  "How Electric Cars Work."  27 March 2002.  HowStuffWorks.com.  25 January 2009.

Inside this Article

  1. Introduction to How Electric Cars Work
  2. An Electric Car Example
  3. Inside an Electric Car
  1. Electric-car Motors and Batteries
  2. Battery Problems
  3. Charging an Electric Car
  4. See more »
    1. The Magna-Charge System
    2. Doing a Conversion
    3. Lots More Information
    4. See all Hybrid Cars articles

How the Chevrolet Volt Works

How the Chevrolet Volt Works Play Video

Inside an Electric Car

The heart of an electric car is the combination of:

electric car
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.


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:

electric car
The 300-volt, 50-kilowatt controller for this electric car is the box marked "U.S. Electricar."

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:

Electric Car
The potentiometers hook to the gas pedal and send a signal to the controller.

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.

Electric Car Battery
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.

Electric car wires
The heavy wires entering and leaving the controller


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.

 

electric car motor
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.


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.

Electric-car Motors and Batteries

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.

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).

 

In the next section we'll look at more problems with battery technology.

Battery Problems

­ 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.

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.

Of course, any car that uses batteries needs a way to charge them

Charging an Electric Car

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.

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.

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.

 

electric car
2008 HowStuffWorks
Opening the gas filler door reveals the charging plug.

 

 

electric car
2008 HowStuffWorks
Close-up of the plug

 

 

electric car
Photo courtesy Jon Mauney
Plug the car in anywhere to recharge.

 

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.

In the next section we'll look at the Magna-Charge system

The Magna-Charge System

The Magna-Charge system consists of two parts:

  • A charging station mounted to the wall of the house

Electric Car Charging
Photo courtesy Jon Mauney

 

  • A charging system in the trunk of the car

Electric Car Charging

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:

Electric Car Charging
Photo courtesy Jon Mauney

The paddle fits into a slot hidden behind the license plate of the car.

Electric Car Charging
Photo courtesy Jon Mauney

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. 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.

In the next section, we'll walk through a conversion step by step.

 

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