Electronic Design

No Ultracaps In Electric Cars Yet, But Maybe There Should Be

One highlight of the January auto shows was General Motors' Chevy Volt, a hybrid concept vehicle targeting a 40-mile all-electric range using lithium-ion batteries and a small generator driven by a gasoline engine. It was interesting to note that ultracapacitors did not figure in the design. (The Tesla Motors Roadster doesn't use ultracaps either.)

Ever since many-Farad ultracaps became available several years ago, they've been touted as a complement to batteries for energy, rather than power, storage. Essentially, the idea is that it's quicker and easier to put energy into and extract energy from a cap than a chemical battery for regenerative braking and that first acceleration from a standstill.

"The battery pack in the Chevy Volt is located beneath the center tunnel and consists of a high-voltage stack of cells. Lithium batteries have very high specific energy, higher than nickel-metal-hydride and far greater than lead-acid. A plug-in hybrid is an energy-rich environment that many argue would not benefit from power-dense ultracapacitors," says John M. Miller, Maxwell Technologies' VP for advanced transportation applications.

"Maxwell Technologies differs and believes that augmenting lithium batteries with ultracapacitors offers an excellent opportunity to push the plug-in hybrid vehicle battery warranty to 15 years. This premise is based on the fact that lithium batteries, or any battery for that matter, has finite energy throughput and after some number of Wh-cycles will need replacing," he continues.

"Ultracapacitors offer the opportunity to displace the wear mechanisms of charge depleting (read this as hybrid electric vehicle cycles) from the plug-in battery pack by passing all the dynamic cycles to the ultracapacitor and using the lithium to deliver the deep energy discharges mandated by charge depleting cycles (read this as battery-electric cycles)," he notes. "Ultracapacitors provide the dynamic power buffer to lithium's deep energy reservoir, thereby expanding cycle life capability into the 15-year realm."

Maxwell Technologies

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