Those plans have been scrapped. Speaking at the GoingGreen conference yesterday, Musk officially said that Tesla was only pursuing all-electric cars and not hybrids.
Why the reversal? During the early part of the summer, Tesla began to more seriously analyze the engineering challenges of making a plug-in series hybrid. In an all-electric car, the battery pack only has to be charged every 250 miles or so. In a series hybrid, the battery needs to be charged every 40 miles before getting recharged by the gas engine. Thus, the battery in a series hybrid goes through a lot more cycles in a year.
Lithium ion battery packs, which Tesla put in its Roadster, do not hold up well under intense cycling like that. “The battery you need for a plug-in (series) hybrid has to have a very high cycle life,” said Tesla Chief Marketing Officer Daryl Siry. “You need an entirely different chemistry.”