The present status of the technology of ultracapacitor cells and modules was reviewed utilizing test data for a 45V Ness module consisting of eighteen 3500F cells. It was found that the modules function as one would expect based on the cell characteristics, but the packaging factors for both weight and volume of available modules are much lower than would be acceptable in most vehicle applications. It is projected that packaging factors of .85 for weight and .7 for volume can be expected in the future. The cell-to-cell voltage variability of the 45V module during repeated PSFUDS cycle testing was found to be small with balancing circuits in place, but its self-discharge was faster than expected based on the self-discharge characteristics of the single cells. Redesign of the balancing circuits is expected to greatly reduce the self-discharge of the module. Applications of ultracapacitors in 42V systems with lead acid batteries and in mild hybrid vehicles were reviewed. It was concluded that the available 45V module with minor improvements in packaging could be used in the 42V applications. Conceptual designs of energy storage units for higher voltage (150V) mild hybrid-electric powertrains and computer (Advisor) simulations of vehicles using those designs indicated that the current technology (carbon/carbon cells) can be used to develop mild hybrid passenger cars of all classes. It may be necessary to store less than the optimum energy (Wh) using the present technology, but projected improvements in the ultracapacitor technology by 2009 could be utilized to increase the stored energy by a factor of two or reduce the size of the capacitor unit such that both its weight and volume would be less than a unit using nickel metal hydride batteries. The simulations show that mild hybrid passenger cars using capacitors have higher fuel economy by 10-15% than corresponding vehicles using nickel metal hybrid batteries.
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