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Lithium-ion batteries power next generation of electric vehicles

By Margery Conner, Technical Editor -- EDN, 5/22/2007

This year’s Advanced Automotive Battery Conference, which took place May 16 through 18 in Long Beach, CA, offered four days’ worth of technical papers analyzing the challenges inherent in adapting lithium-ion batteries, the favored battery technology for consumer-electronics portables. These batteries find use in automobiles, including EVs (electric vehicles) and PHEVs (plug-in hybrid electric vehicles). PHEVs differ from the more common HEVs (hybrid electric vehicles), such as the Toyota Prius, in that the engine in PHEVs acts only as a backup if the user exceeds the mileage range. The PHEV battery requires daily charging for its primary power.

At the conference, JB Straubel, chief technology officer of Tesla Motors, presented “Development of Advanced Li-ion Battery Pack for EV and PHEV Applications,” a paper explaining the advantages of using many small, individually fused, 18650-form-factor lithium-ion cells, rather than one large, prismatic battery, to make a battery pack. “It’s easy to jump to the panacea of the new technology and forget the lessons [of] the past,” said Straubel. “And we do have 10 years of a learning curve on lithium-ion [18650] cells.” Lithium-ion batteries can cause thermal-runaway problems; Straubel pointed out that you can more easily handle catastrophic failures with small individual cells than with one large cell. Tesla claims impressive numbers for its pack: well-to-wheel efficiency of more than 135 mpg; energy storage of 56 kWhr; total energy density of 125 Whr/kg, fully tested, including vents and cooling; and an overall energy density for the entire car of 50 Whr/kg.

Tesla engineers employed many pre-emptive procedures in designing for lithium-ion-battery safety, which Straubel passed on to attendees at the conference. For example, they warn, don’t accept vendors’ claims that they meet all safety standards; ask each vendor about its record and how long it has been in business. Also, assume that thermal runaway will occur, but design to prevent it. Include pack monitoring and protection and emphasize fuse design. Perform extensive testing, again emphasizing fuse testing. Finally, you should get third-party reviews from consultants, professors, and others.

Straubel did not specify Tesla’s battery-pack secret for the car, which goes form 0 to 60 mph in 4 seconds and goes more than 200 miles between charges. However, he did reveal that Tesla is forming a subsidiary, Tesla Energy, in response to the many inquiries that the company claims it receives about its lithium-ion-battery-pack design. Despite the PHEV-inclusive title of Straubel’s paper, Tesla’s current road map calls only for EVs, leaving the door open for Tesla Energy to serve as a battery-pack designer for next-generation PHEV vendors.