TransForum Vol. 10, No. 1

Charging Ahead: Taking PHEVs Farther on a Single Battery Charge

Ultracapacitors will dramatically boost the power of lithium-ion batteries, enabling plug-in vehicles to travel much further on a single charge.

Every six months, we’re reminded to change the batteries in our household appliances: smoke alarms, flashlights and radios. But what if you had to change the battery in your plugin hybrid electric vehicle (PHEV) just as often?

Fortunately, researchers at Argonne may have found a way to exponentially increase the calendar and cycle lifetimes of lithium-ion batteries. Electric double-layer capacitors— typically referred to as ultracapacitors—have an energy density thousands of times greater than conventional capacitors and a power density hundreds of times greater than lithium-ion batteries.

In an electric vehicle drivetrain, energy density provides sustained speed, while power density facilitates acceleration. “Energy density is what allows you to run a marathon; power density is what enables you to sprint,” said Ted Bohn, an automotive engineer in Argonne’s Center for Transportation Research.

“Ultracapacitors aren’t of much use just by themselves,” he added, “but when you couple them with lithium batteries, they dramatically boost the performance of the entire vehicle.”

When an electric vehicle merely needs to maintain a particular speed, it requires little of the battery’s power density. However, when the car needs to accelerate from a standstill to a cruising velocity, today’s lithium-ion batteries must strain to provide the necessary “oomph.”

“Ultracapacitors give an electric vehicle the initial boost it needs to get going,” Bohn said.

A PHEV or pure electric vehicle needs a battery with sufficient power density to accelerate the vehicle quickly. A vehicle that uses an energy-dense battery that lacks sufficient power density will fail prematurely, possibly in a matter of months if driven aggressively. By using the same potentially lower cost energy-dense battery, in combination with ultracapacitors, the vehicle will have sufficient performance and the batteries should last 10 years or more.

Today’s hybrid cars recharge their batteries by transforming kinetic energy from the wheels into potential electrical energy as the driver brakes. Conventional lithium-ion batteries, however, absorb this energy slowly and inefficiently. By contrast, ultracapacitors, because of their immense internal surface area, sort of soaking up reclaimed energy like a sponge.

“By integrating the entire system,” Bohn said, “we can drive down the cost. When we can put these various electronic elements together, we’ll transform an $8,000 battery into a $4,000 allelectric drivetrain system.”

Funding for this project was provided by the U.S. Department of Energy’s Vehicle Technologies Program under Lee Slezak.

March 2010