People who drive on city streets and need travel only a few miles can use battery-only power. Those who have a longer commute at high speeds -- or who drive aggressively with the air conditioning on -- may want a smaller battery to improve the vehicle's overall efficiency, Rousseau said.
For different types of trips, renting a different type of hybrid vehicle may be most efficient, he said.
Another Argonne researcher told the conference a highly efficient diesel engine was much more cost-effective for highway driving than a hybrid vehicle with a lithium battery, based on the current cost of fuel and electricity. But the hybrid would beat the diesel vehicle in city driving.
Another variable to consider is the cost of making batteries.
With nickel hydride batteries now in vogue in such hybrids as Toyota's Prius, nickel prices have risen sharply. That has made lithium-ion batteries, which are a more promising technology because they can hold more power in a more compact space and have other advantages, more enticing as an alternative. Of course, lithium is likely to rise in price if it is used for vehicle batteries as well as laptop batteries, Rousseau said.
"There is no single silver bullet," he said on the sidelines of the conference. "There is not one technology that will be best for everybody. Our goal is to understand how people drive, and depending on how they drive, what is the impact of one technology or another, from a fuel efficiency point of view, and a cost point of view."
(Editing by Carol Bishopric)