Toyota Motor Corp., the biggest seller of hybrid autos, is sticking with nickel as the preferred battery material for most of the vehicles after three years of secretly testing Prius hatchbacks with lithium-ion packs.

Toyota last month ended road tests of 126 Priuses in the U.S., Japan and Europe that began in 2006, Jana Hartline, a company spokeswoman said. Details of the program, in which the cars' nickel metal hydride batteries were replaced with more expensive lithium models, weren't released.

Automakers are introducing models all or partly powered by lithium-ion batteries holding twice the energy of nickel packs. While Toyota's lithium version performed well and gave small fuel-economy gains because of lighter weight, nickel is favored for conventional, mass-market hybrids for its cost, said Kazuo Tojima, the carmaker's senior staff engineer for batteries.

Lithium's durability, stability and safety are assured, the Toyota City, Japan-based company's tests showed, Tojima said.

The tests appear to be among the most thorough done by companies planning to introduce the batteries, said Menahem Anderman, president of consulting firm Advanced Automotive Batteries in Oregon House, Calif.

"We now know that a lithium-ion battery can work; that's not really the question," he said. "Cost is critical, and we still don't know enough about long-term durability."

Toyota has sold more than 2 million hybrid cars and light trucks worldwide since introducing the Prius in Japan in 1997, almost all using nickel. The automaker hasn't announced plans to sell standard hybrids with lithium batteries in the U.S.

This year, Toyota will begin delivering test fleets in the U.S., Japan and Europe consisting of plug-in Priuses that can run 12 miles (19 kilometers) solely on lithium-ion battery power after charging at an electrical outlet. The car is being shown this week at the Frankfurt Motor Show.

The company also plans to sell a small electric car for urban commuters, powered solely by lithium packs, by 2012.

Nissan Motor Co. will begin offering battery-electric Leaf compact cars next year, which the Yokohama-based company says will travel 100 miles on a fully charged lithium pack. Mitsubishi Motors Corp. is selling in Japan the i-MiEV, a $51,000 electric minicar that also travels as far as 100 miles using only lithium-ion batteries.

Toyota doesn't expect battery-powered cars to succeed in the mass market until 2020 because batteries are too costly and capacity limits their range.

"Electric vehicles of today are less costly than in 1990s, but if you compare them with the other vehicles out there they are still too expensive," Executive Vice President Takeshi Uchiyamada said at news conference at the Frankfurt show. "Unless there is a very big breakthrough in battery costs, I don't think electric vehicles can take a large market share."