How Electric Cars Work

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Image Gallery: Electric Cars

 

  The Subaru R1e electric car can be charged overnight on household current. It has a range of 50 miles and a top speed of 62 miles per hour. See more electric car pictures.

STAN HONDA/AFP/Getty Images

Electric cars are something that show up in the news all the time. There are several reasons for the continuing interest in these vehicles:

• Electric cars create less pollution than gasoline-powered cars, so they are an environmentally friendly alternative to gasoline-powered vehicles (especially in cities).
• Any news story about hybrid cars usually talks about electric cars as well.
• Vehicles powered by fuel cells are electric cars, and fuel cells are getting a lot of attention right now in the news. An electric car is a car powered by an electric motor rather than a gasoline engine.

 

A typical electric car, this one has some particularly snazzy decals. This vehicle is owned by Jon Mauney.

The electric car that we will use for this discussion is shown here.
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:



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 heart of an electric car is the combination of:

• The electric motor
• The motor's controller
• The batteries

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.

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.

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.

Charging an Electric Car

Opening the gas filler door reveals the charging plug.

2008 HowStuffWorks

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

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.




The Magna-Charge system consists of two parts:

• A charging station mounted to the wall of the house

• 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.

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