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Tesla Motors says its dell laptop battery all-electric Model S sport sedan will join the Roadster in late 2011 and be sold initially with battery packs offering 165 and 230 miles of range.

But Tesla has also said that the $57,400 base-price car (which is eligible for a $7,500 federal tax credit) would have a third option a year after launch: a battery pack that will give it an impressive range of 300 miles between charges.312-0068 Battery, 6Y270 Battery, 1X793 Battery, C1295 Battery But just how big would that pack have to be?

In an interview, J.B. Straubel, Tesla’s chief technical officer, said that the size of the pack has not been finalized. But, he said, a range of 85 to 95 kilowatt-hours is possible. “We hesitate to print a number,” Mr. Straubel said. “It would be the biggest pack on the market, 312-0068 Batteryand we’re designing and building it ourselves.”

Mr. Straubel seems unfazed by the prospects of building — to size, weight and cost — an unprecedented battery pack. “Saying it can’t be done is like saying there’s never been a gas tank that big,” he said. The pack is within “technical reality,” given the experimental high-density cells Tesla is now working with, Mr. Straubel explained, adding that these cells could, with today’s technology, give the Roadster a range of 280 to 300 miles. “When we announced the Roadster, the skeptics said we would never get over 200 miles,” he said. The Roadster has an official range of 244 miles.

Needless to say, there are some skeptics about the 300-mile battery pack,312-0068 Battery, 6Y270 Battery, 1X793 Battery, C1295 Battery at least with today’s technology. Dick DeVogelaere is the vehicle chief engineer for Magna International on the Ford battery car project, scheduled for 2011 production. “It would take a pickup truck to haul that battery around,” 6Y270 Battery,he said. “It would probably weigh in excess of 1,000 pounds, maybe 1,200.”

Mr. DeVogelaere also predicted that the cost would be three times that of a standard pack. “It’s all about mass,” he said. “If Tesla has a way to make it very light, maybe they can get there. But they would need some major breakthroughs — it would be difficult based on existing technology.”

Also expressing some doubts is Paul Wilbur, president and chief executive of Aptera, which is building a three-wheeled aerodynamic electric car for release later this year (initially in California only). “That’s a whopper pack,” he said. “Ours is about 20 kilowatt-hours. Even if a pack like that was 50 cents a watt-hour, it would be a $45,000 battery, and unbelievably expensive and unbelievably heavy — maybe 1,800 pounds.”1X793 Battery, He also said it would have very long recharge times even with 220-volt service.

But Mr. Wilbur said that a technical breakthrough could “throw everything I just said out the window,” and he cited a new $100 million factory General Electric is building in upstate New York to make sodium-nickel-chloride batteries (initially for locomotives and other heavy-duty applications) with what Mr. Wilbur said is “higher energy density than anything in the lithium family.”

Charles Gassenheimer, chairman and chief executive of battery maker Ener1, said that if Tesla continued to use its small, laptop-size battery cells, some 12,000 would be required for an 85-kilowatt-hour battery pack. 312-0068 Battery, 6Y270 Battery, 1X793 Battery, C1295 BatteryThe Roadster uses fewer than 7,000.

Mr. Straubel declined to put a price on the forthcoming battery pack, but he said,C1295 Battery “It will definitely not be some off-the-wall price, not $100,000. It will be in line with overall vehicle pricing.” In a Design News article last month Mr. Straubel said he thinks the cost can be reduced to $300 a kilowatt-hour, putting the pack “in the ballpark” of $18,000.