Toyota Motor has said it would stick with nickel as the preferred battery material following three years of secret tests on Prius hybrids equipped with lithium ion packs.

The company told Bloomberg News it had just finished road tests of 126 Priuses in the US Japan and Europe begun in 2006. Further details were not released.

Automakers including Toyota are introducing models powered completely or in part by lithium-ion batteries, which can hold twice the energy of nickel packs. But Toyota said while lithium batteries performed well in its tests and yielded "small" gains in fuel economy due to lighter weight, nickel would remain the material of choice for conventional, mass market hybrids as lithium's benefits didn't justify the higher cost.

Bloomberg noted that the tests appear to be among the most thorough done by companies planning to introduce the batteries.

Menahem Anderman, president of consulting firm Advanced Automotive Batteries in California, told the news agency: ""We now know that a lithium-ion battery can work; that's not really the question. Cost is critical, and we still don't know enough about long-term durability."

Toyota has sold more than 2m hybrid cars and light trucks worldwide since introducing the Prius in 1997, almost all using nickel metal. This year the company plans to deliver test fleets in the US, Japan and Europe consisting of plug-in Priuses that can run 12 miles (19km) 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.