Challenges Loom on Tesla's Model S Project

Battery experts said last week Tesla Motors' proposed 300-mile, seven-seatelectric vehicle (EV) will serve as a stiff test of EV technology, calling onengineers to employ the best batteries and lightest, strongest materialsavailable.

Model S, whichwill roll out in 2011, is raising hopes in the electric car community becauseit offers a huge increase in driving range for a proposed cost of $49,900. Moreover,expectations for the vehicle soared even higher this week as Tesla received$465 million in low-interest loans from the U.S. Dept. of Energy, $365million of which will be applied to the Model S production engineering andassembly.

Still, the challenge facing Model Sengineers appears to be a formidable one. By comparison, Tesla's earliervehicle, the Roadster, offered 244 miles of range for a $101,500 price tag.Similarly, Mitsubishi Motors' new i-MiEV small electriccar will travel 100 miles between charges and cost about $47,000.

Those vastdifferences might sound imposing, but battery experts said they're notinsurmountable.  

"It's do-able," said David Swan, president of DHS Engineering, in reference to Tesla'stechnological goal. "But they're going to have to stretch everything to getthere." 

Indeed, battery experts said lastweek that Tesla will need more than the current state-of-the-art lithium-ionbattery technology to make the goals work. They suggested that moreadvancements in battery technology might be needed, along with the applicationof stronger, lighter-weight body materials to keep the weight down.

"If they are willing to make analuminum-intensive body or a fiberglass body, and maybe compromise on otherhigh-mass components, they might be able to do it," said Donald Sadoway, the JohnF. Elliott Professor of Materials Chemistry at MIT. "But it's going to be toughto do that and keep the cost down."

Sadoway said currentstate-of-the-art lithium battery technology would be hard-pressed to supplyenough energy for a big car to go 300 miles on a charge. According to Sadoway'srule of thumb for EVs, an electric sedan would need approximately 1 W-hr/kg ofspecific energy to go one mile, assuming that the battery comprises about 30 percentof the vehicle's mass. Using that rule, today's best batteries might supplyenough energy for a four-door sedan to go 175 miles between charges. An EV-makercould, however, boost that range by packing more batteries on board, thereforelosing some valuable volume elsewhere in the car, possibly in the trunk.

"If you're willing to put 40 percent ofthe mass of the car into the battery, it brings you closer," Sadoway said. "Myhunch is that they have a very spiffy, advanced version of lithium-ion combinedwith an abnormally-high mass fraction of the vehicle in the battery."

Experts acknowledged, however, thata big battery would present a big cost challenge. Swan estimates that the300-mile Tesla would need at least a 600-lb, 50-60 kW-hr battery, which couldsignificantly raise costs. (An unofficial DesignNews survey of a group of five battery experts in 2008 indicated thatstate-of-the-art lithium-ion battery technology would cost between $500 and$1,000 per kW-hr. Making such cost assumptions, a 50 kW-hr batterycould cost $25,000 or more.)

Such challenges come into sharperfocus when comparing Tesla's proposed vehicle to Mitsubishi Motors' new i-MiEV.The i-MiEV, which rolls out in July, costs $47,000 and offers a 100-mile range.The small four-seater reportedly employs a 16 kW-hr battery, which according tothe $500/kW cost estimate, would run only about $8,000.

Still, battery experts said theyare optimistic about the Model S, especially if Tesla is ultimately willing toreduce the vehicle's stated range and possibly even subsidize its cost forconsumers in the beginning. (However, Tesla's strategies are unknown at this point because the start-up car manufacturer is keeping a tight lid on itsplans for the Model S. Responding to e-mail requests from Design News, thecompany said it is not yet ready for media interviews on the Model S.)

"Even with the 300-mile range, it'snot impossible," said Swan, who owns three EVs and has designed batteries forthe legendary 254-mphWhite Lightning electric vehicle during the 1990s. "But all they have to dois say it's a 200-mile vehicle and everything would get a lot easier."

Challenges Loom on Tesla