Details are scarce, but it turns out that Toyota has been testing Priuses with lithium-ion batteries in them for years. Not many vehicles were involved – Bloomberg says only 126 such Priuses were on the roads in U.S., Japan and Europe – but the tests began in 2006. Toyota spokeswoman Jana Hartline confirmed that the tests ended last month. A Toyota battery engineer said that the li-ion packs gave fuel economy a small boost, but that, unsurprisingly, the company still prefers NiMH packs in the production models. Either way, Toyota says that the "durability, stability and safety are assured" when it comes to li-ion batteries. One analyst said that Toyota's tests, "appear to be among the most thorough done by companies planning to introduce [li-ion] batteries."
These Priuses are not the plug-in Priuses that are getting a very public testing around the world. We wonder what sorts of secrets are happening with that program.