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The first electric vehicles (EVs) from mainstream manufacturers are hitting car showrooms. There will be EVs from Japanese manufacturers Nissan and Mitsubishi on the road from early next year. General Motors and Ford are following fast behind, with the Volt and Focus Electric due in early 2012. Within the next few years, it’s probable that every major automotive manufacturer will have an electric offering, whether full EV or hybrid.
The common technology of the various marques and models is the battery, which is the key enabler of the EV in more ways than one. It determines the range of the vehicle, is a strong determining factor in the price, and is a rapidly developing technology that automotive engineers must embrace.
The development path of battery technology, and the long-term viability of the automotive industry, are begrudgingly entwined.
The Nissan Leaf, the Mitsubishi i-Miev, the Volt and the Focus Electric all use lithium-manganese oxide spinel polymer cells in a pouch casing. The i-Miev’s battery pack produces 16kW/h and a range of 80 miles, the Leaf 24kW/h and a range of 100 miles. The Volt’s total energy is 16kW/h with a range of 40 miles on electric, and the Focus Electric’s is 23kW/h with a range of 100 miles.
Dr Allan Paterson, senior electrochemist at Scottish battery firm Axeon, says the lithium manganese pouch battery is a well-established technology that is “reasonably” specced and priced. Packaging is a major consideration in the design of battery packs and pouch cells are fast becoming a favourite in automotive applications.
They use a flat polymer electrolyte, are smaller and lighter than cylindrical or prismatic cells, and have a large surface area to enable heat dissipation. However, the cells have low mechanical stability, and require robust packaging.
A major difference between the Leaf and i-Miev, and the Volt and Focus Electric, is that the latter pair will have a liquid “thermal management system” to ensure battery longevity and usability in extremely hot or cold weather. The reason the Focus Electric and Volt have active thermal management systems reveals another of the major design considerations of EV batteries – heat.
“Effective thermal management is key,” says Paterson. “If you don’t dissipate heat you will severely damage the cell. In bigger cars like the Volt and Focus Electric the cells are used more aggressively, so more thermal management is needed to deal with increased internal resistance.”
Axeon develops battery packs for automotive firms according to automotive firm requirements. The battery maker is not tied to any particular technology so has a wide range of chemistries at its disposal when designing a pack. Even so, it is often difficult to meet client expectations. “People want to produce a huge amount of power through different temperature variations, but there are a lot of trade-offs with battery technology,” says Paterson.
“Packs can be optimised for either performance or range, and a key consideration is always cost. The lithium-ion cells are not cheap.”
R&D in battery technology is therefore focused on the material used in the positive electrode because this is where about 40% of the cost of a battery cell lies. Paterson expects nanotechnology to play a huge role in the next generation of batteries. Innovative nanostructures will make it possible to increase power density by increasing surface area and enable more durable, faster-charging battery cells, he adds.
There are further performance gains from the development of the battery management systems, the electronics and software that control the use of the pack. Ways of “shuttling” around the charge in the pack to maximise the lifespan of cells is something that will be developed for battery packs soon, says Paterson.
Eventually, new lithium-ion chemistries will provide the quantum leap needed in battery technology to move from range-extended hybrids to a market of full EVs only. Paterson says: “The fundamental R&D going into the batteries is showing maturity. We know where the end point is for chemistries like rechargeable metal air, and lithium sulphur.
“These technologies will provide a quantum leap. Instead of an increase of a few percent a year in energy density, these technologies give you a tenfold increase, giving you the range people expect from a tank of petrol."