The goal is to demonstrate that recharging can be accomplished routinely in under 30 minutes without severely reducing the operating lifetime of the batteries or causing other problems. In the year since the MIT Electric Vehicle Team started working on the project, new and established companies have begun to offer commercial rapid-recharging systems, and Japan has officially adopted a new standard for the connectors for such systems and has begun installing the systems in more than 100 locations. The Nissan Leaf, a pure electric five-passenger car to be introduced in the U.S. later this year, is already capable of rapid recharging in 30 minutes in places that have the necessary “Level III” charging system. (So far, there is just one such station in the U.S., in Portland, Oregon).
Next week, Lennon Rodgers, a doctoral student in mechanical engineering and a member of the MIT Electric Vehicle Team, will present a paper on the team’s rapid-charging tests at the 12th International Conference on Advanced Vehicle and Tire Technologies in Montreal. The paper was co-authored by fellow team members Radu Gogoana ’10, a master’s student in mechanical engineering, Paul Karplus (an undergraduate at Stanford) and Michael Nawrot ’11.
Rapid charging, also known as Level III, requires much higher voltages and current than what is supplied by conventional household circuits. The Japanese rapid-charging standard, called CHAdeMO, provides DC power at up to 500 volts with a current of 125 amps. Typical chargers operate on standard AC power, using either 110 volt household current (Level I), which generally can recharge an electric car’s batteries overnight, or special systems (similar to those needed for electric stoves or clothes dryers) that use 220 volts (Level II), which can cut the charging time in half. “Rapid charging” systems typically refer to those that can charge the batteries to at least 80 percent of capacity within 30 minutes.
Because of the large power requirements, this is not something you’d ever do in your home garage. Rather, this fast-recharge technology might be installed in central recharging stations comparable to today’s gas stations, where the cost of the necessary infrastructure could be warranted and where a fast turnaround is necessary. In many cases, rapid charging systems can provide a 50 percent charge — typically enough to travel 50 miles — in under 5 minutes, comparable to the time it takes to fill a gas tank.
While rapid charging — largely being promoted by companies with long commercial experience with recharging industrial fork lifts and similar vehicles — is beginning to attract attention, there has been relatively little testing on the effects of repeated rapid charging on battery life and performance. “Is it damaging over time? That’s the issue we wanted to study,” says Rodgers. That’s the kind of data the MIT team was collecting in an attempt to prove the potential for this technology.
Rodgers says that the chemistry used in lithium-ion batteries made by the MIT spinoff company A123 Systems is the best suited for rapid charging, and the company’s website declares that the batteries are capable of being fully recharged in 15 minutes. These batteries, based on research carried out at MIT in the lab of Yet-Ming Chiang, professor of materials science and engineering, have been selected for several planned new electric vehicles including cars from Fisker Automotive and buses and trucks from Daimler and Navistar.
A time-lapse video showing members of MIT’s Electric Vehicle Team working on the conversion of a car and a motorcycle from conventional gasoline power to all-electric operation. The conversions used batteries donated by MIT spinoff company A123 Systems.
Video: Melanie Gonick; still images/footage: MIT EVT
In the team’s tests, they ran one of these battery cells through 1,500 charge and discharge cycles, using an automated system. After 1,500 cycles, the battery had lost less than 10 percent of its initial power capacity, Rodgers says. The team used a fan to prevent overheating, which by stressing the chemical and mechanical components can lead to degradation.
To test a rapid-charging system under realistic conditions, the team converted a motorcycle to all-electric operation, and then performed a successful rapid-charging test, reaching more than 80 percent charge within 10 minutes.
The MIT EV Team has also completed the conversion of a 2010 Mercury Milan hybrid (donated by Ford) into a pure electric vehicle. The initial conversion was successfully completed last summer, and this summer they have been making major improvements — reinstalling the 8,000 lithium ion phosphate battery cells provided by A123, rewiring the system with a new control system, adding a powerful cooling system for the batteries, and making changes to make the car street-legal. They hope to use the car for testing of rapid charging technology, although they are still looking for funding to get the necessary equipment. Commercial rapid-charging systems can cost tens of thousands of dollars.
Kristen Helsel, vice president of EV Solutions for Aerovironment, a company that makes charging systems for electric vehicles, says that it’s unlikely anyone will start installing rapid-charging stations in the U.S. in substantial numbers until the country adopts an official standard, and “I don’t expect it in the near term. There are still multiple designs under consideration.” But a rapid charging capability is going to be crucial for widespread acceptance of electric vehicles, no matter what their driving range is on a full charge, because people will always want the possibility of being able to go farther, she says. In the meantime, research such as that being carried out by the MIT EV Team can play a useful role, she says. “Better batteries are coming, and because of that the ability to charge at any level is going to be a constantly evolving thing. We need to continually evolve the technology, and to better understand the effects of different things” on battery life and other factors, she says. “There’s all sorts of good work that needs to be done.”
For car designers, Rodgers says, there is a tradeoff they need to consider: They can include larger battery packs that provide a longer driving range, but are more difficult to recharge rapidly, or smaller packs that give a shorter range but cost less and can more easily be charged rapidly.
In addition to analyzing battery performance, the team analyzed the impact that rapid charging of electric vehicles might have on the electric grid. They concluded that spikes of usage that might present problems for the grid could be eliminated by using an intermediate battery system. Instead of directly charging the vehicle from the grid, a large battery pack — perhaps using batteries recycled from other cars — could be slowly charged using a “trickle charge” from the grid, thus using low-cost, off-peak power, and then rapidly transfer its charge to the vehicle’s batteries.