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Posted on May 15, 2008 by  & 

Lithium ion cells are showing their power

Lithium ion batteries have a relatively short lifespan and in extreme circumstances large lithium batteries can catch fire or even explode whereas this is usually impossible with laminar versions. It is worth tracking the large types, however, because they tend to have the big funding leading to the major advances that then percolate down to the thin film versions.
The new Roadster made by Tesla Motors of San Carlos, California, is an all-electric sports car with a chassis based on the Lotus Elise. Its 185-kilowatt engine is powered by nearly 7000 finger-sized lithium ion batteries packed into its trunk.
Tesla Motors claim, "The battery pack of the electric vehicle is one of the largest and technically most advanced lithium ion battery packs in the world. It is capable of delivering enough power to accelerate the Tesla Roadster from 0 to 60 mph in about 4 seconds. Meanwhile, the battery stores enough energy for the vehicle to travel more than 200 miles (based on EPA city/highway cycle) without recharging, something no production electric vehicle in history can claim."
It has not been possible in the past to achieve this specification for an electric vehicle as it would need to carry more than a thousand kilograms of nickel metal hydride batteries, which would make the vehicle too large and heavy to achieve the acceleration and handling performance.
Due to their high energy density, lithium ion batteries have become the technology of choice for laptops, cell phones, and many other portable applications. Because they have all this energy stored in a small space there have been dangerous incidents with laptops leading to recalls by the manufacturers.
The Tesla designed battery below, circulates cooling fluid to extend the battery packs life whilst adding safety benefits. Surface area is essential to cooling batteries since the surface is where heat is removed - with the 7000 small cells which are slightly bigger than a AA battery, because of their small size, each cell is able to quickly redistribute heat within and shed heat to the ambient environment making it essentially isothermal. This cooling architecture avoids "hot spots" which can lead to failures in large battery modules.
Normally Large lithium ion batteries have a short life partly because the cathode, which is usually made from layers of lithium cobalt dioxide, wears out quickly.
When the battery is being charged, positively charged lithium ions migrate from the cathode across a separator screen into the porous graphite anode, which becomes replete with lithium atoms.

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Posted on: May 15, 2008

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