Battery experts said last week Tesla Motors' proposed 300-mile, seven-seat
electric vehicle (EV) will serve as a stiff test of EV technology, calling on
engineers to employ the best batteries and lightest, strongest materials
available.
Model S, which
will roll out in 2011, is raising hopes in the electric car community because
it 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, $365
million of which will be applied to the Model S production engineering and
assembly.
Still, the challenge facing Model S
engineers appears to be a formidable one. By comparison, Tesla's earlier
vehicle, the Roadster, offered 244 miles of range for a $101,500 price tag.
Similarly, Mitsubishi Motors' new i-MiEV small electric
car will travel 100 miles between charges and cost about $47,000.
Those vast
differences might sound imposing, but battery experts said they're not
insurmountable.
"It's do-able," said David Swan, president of DHS Engineering, in reference to Tesla's
technological goal. "But they're going to have to stretch everything to get
there."
Indeed, battery experts said last
week that Tesla will need more than the current state-of-the-art lithium-ion
battery technology to make the goals work. They suggested that more
advancements in battery technology might be needed, along with the application
of stronger, lighter-weight body materials to keep the weight down.
"If they are willing to make an
aluminum-intensive body or a fiberglass body, and maybe compromise on other
high-mass components, they might be able to do it," said Donald Sadoway, the John
F. Elliott Professor of Materials Chemistry at MIT. "But it's going to be tough
to do that and keep the cost down."
Sadoway said current
state-of-the-art lithium battery technology would be hard-pressed to supply
enough energy for a big car to go 300 miles on a charge. According to Sadoway's
rule of thumb for EVs, an electric sedan would need approximately 1 W-hr/kg of
specific energy to go one mile, assuming that the battery comprises about 30 percent
of the vehicle's mass. Using that rule, today's best batteries might supply
enough energy for a four-door sedan to go 175 miles between charges. An EV-maker
could, however, boost that range by packing more batteries on board, therefore
losing some valuable volume elsewhere in the car, possibly in the trunk.
"If you're willing to put 40 percent of
the mass of the car into the battery, it brings you closer," Sadoway said. "My
hunch is that they have a very spiffy, advanced version of lithium-ion combined
with an abnormally-high mass fraction of the vehicle in the battery."
Experts acknowledged, however, that
a big battery would present a big cost challenge. Swan estimates that the
300-mile Tesla would need at least a 600-lb, 50-60 kW-hr battery, which could
significantly raise costs. (An unofficial Design
News survey of a group of five battery experts in 2008 indicated that
state-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 battery
could cost $25,000 or more.)
Such challenges come into sharper
focus 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 to
the $500/kW cost estimate, would run only about $8,000.
Still, battery experts said they
are optimistic about the Model S, especially if Tesla is ultimately willing to
reduce the vehicle's stated range and possibly even subsidize its cost for
consumers in the beginning. (However, Tesla's strategies are unknown at this point because the start-up car manufacturer is keeping a tight lid on its
plans for the Model S. Responding to e-mail requests from Design News, the
company said it is not yet ready for media interviews on the Model S.)
"Even with the 300-mile range, it's
not impossible," said Swan, who owns three EVs and has designed batteries for
the legendary 254-mph
White Lightning electric vehicle during the 1990s. "But all they have to do
is say it's a 200-mile vehicle and everything would get a lot easier."
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