Friday, April 10, 2009

Battery longevity, the Achilles heel for the electric vehicle?

With the push for electric cars the emphasis seems to be primarily on range. And that would be a mistake. Battery cost is also very important, but most critical factor is battery longevity. It is potentially the Achilles heel for electric vehicles. The battery of choice for EV applications is the lithium ion battery, a true breakthrough in lightweight electric energy storage. With 3 to 4 times the energy density of lead acid and NiCad batteries, and 2.5 times nickel metal hydrides, lithium ions are the first batteries that can offer acceptable range with a reasonable weight. The initial version of GM’s EV-1 used lead acid batteries with a weight of 1,310 pounds (595 kg) and “fuel” capacity of 18.7 kWh. By comparison the Chevy Volt’s li ion battery pack weighs around 400 pounds (180 kg) and has a capacity of 16 kWh. Li ions have been around for nearly 20 years, powering laptops and then cell phones. The revolution came in 2004 and 2005 when manufacturers found ways to vastly increase the discharge rate without destroying the battery. Hobbyists caught on and soon electric model planes were flying with 60 amp bursts of power from a 5 ounce battery pack. DeWalt power tools began using the durable, American made, A123 cells. Higher discharge rates were what the auto industry needed and they took note. At that time, startup company Tesla Motors and established GM began development work on their EV projects: a pure EV from Tesla and a plug-in hybrid from GM. Shortly after, the whole automotive world followed suit. Li ions have a very low self discharge rate, about 5% per month, far better than the leaky NiMhs, and they have extremely low internal resistance. But they do have drawbacks. They are very expensive, they don’t like to be kept at full charge and they don’t like heat. Both GM and Tesla are using liquid cooling to keep the cell packs at reasonable temperatures. GM is only using about half the capacity of their battery pack and not charging above the 70% level in order to achieve a 10 year/150,000 mile life. They plan to warrant it at that level. GM is estimating their cost for a pack at around $10,000. Tesla rates the range of their Roadster from full charge to full discharge, which if used in practice, will seriously degrade the battery. They are estimating the pack will retain 70% of its capacity after 5 years/50,000 miles. That represents a range reduction from 221 to 155 miles. But it is warranted for only 3 years/36,000 miles. They have indicated the estimated cost of a replacement pack at slightly under $36,000. To soften the blow they are offering a buy-ahead program for $12,000 where you pay now and take delivery after 7 years. Based on the fact the Tesla battery has a little over three times the capacity of the Volt’s (53 vs. 16 kWh), the $36,000 figure for the Tesla probably more accurately reflects their true cost. So be prepared. Another entrant in the market Fisker Automotive, a plug in hybrid like the Volt, but far flashier (and more expensive), has not released warranty information, though it plans to begin selling its cars by the end of 2009. All EV manufacturers are, or will be using battery technology that is less than five years old and, at least in the case of GM, are planning a warranted lifetime double that. That’s a heavy risk for a very expensive part. Warranty issues could cripple even the largest of the car companies.

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