Fill ‘er up: A Tesla roadster at an ordinary charging station in Portland, Oregon.
It can take hours, even days, to recharge an electric car. That’s one reason GM added a backup gas-powered generator to its electric car, the Volt. Another option is the DC fast charger, which would allow cars to recharge up to 80 percent of the battery’s capacity in just half an hour.
Tesla Motors’ first DC fast-charging station, halfway between San Francisco and Los Angeles, will allow drivers to add 150 miles of range to their electric cars in half an hour—provided they have one of the higher-priced battery packages. Adding those miles would ordinarily take from several hours to more than a day, depending on the size of the battery and whether an ordinary outlet or a higher-voltage one is used.
But the impact of such charging stations will be limited by their high cost, and by the fact that fast-charging still takes far longer than it does to fill up a tank of gas. Conventional vehicles will remain the best option for people who regularly drive long distances; and those who purchase electric vehicles for commuting won’t need fast chargers—they can charge overnight at home or during the day at work.
For now, at least, the biggest barrier to electric-vehicle adoption is simply the high cost of the cars, which can cost twice as much as comparable gas-powered vehicles.
Most electric vehicles don’t require any special charging equipment. They come with on-board chargers that convert AC power from ordinary wall outlets to the DC power needed by the battery. But charging from a conventional 120-volt outlet is slow. It takes nearly a full day to charge the Nissan Leaf this way, and nearly three days to charge the version of Tesla’s Model S with a 300-mile range. EV owners often install special 240-volt connectors in their garages to shorten charging times to several hours—and almost all public charging stations charge at this rate.
DC fast chargers bypass a vehicle’s on-board charger—converting grid AC power to DC power outside the car—and deliver electricity directly to the battery at a higher rate than the on-board chargers would allow. A communications link incorporated into the charging cord allows the car’s battery management system to control the rate of charge to avoid damage to the battery. The system will, for example, slow down charging if the battery overheats or will typically stop charging when it reaches 80 percent of capacity.
The charging capacity of DC fast chargers varies. Some only deliver 20 kilowatts, but some experimental chargers deliver well over 100 kilowatts (in comparison, most 240-volt outlets will deliver 3.3 kilowatts). A 50-kilowatt charger would be more than enough to charge a Nissan Leaf to 80 percent capacity within half an hour (it has a 24-kilowatt-hour battery pack and less than 100-mile range). Charging times will vary widely depending on the temperature outside for the Leaf, but less so for other cars that have better battery cooling systems (see “Are Air-Cooled Batteries Hurting Nissan Leaf Range?”). But the standards for DC fast chargers are evolving—not all electric vehicles have fast-charging outlets (GM’s Volt doesn’t have one, for example), and adapters may be required for those that do. Only a couple of hundred have been installed so far in the United States.
The cost of the technology could prove a major obstacle. The equipment can cost tens of thousands of dollars, and installation costs can be triple the equipment costs, bringing the total to over $100,000 in some cases. Ecototality, which has installed 7,000 conventional, non-fast charging stations, is working with the U.S. Department of Energy to study whether drivers will use fast-charging stations enough for station owners to recoup the capital cost.
Hear more from Tesla at EmTech MIT.