A system model has been adapted to simulate the hybrid-pulse-power-characterization test. Simulation results improve upon traditional Ragone plots by capturing the complexity of pulse performance during hybrid-electric-vehicle (HEV) operation. Using this approach, system design (electrode thicknesses and porosities) is optimized for a specific power-to-energy ratio. Lithium-ion batteries are capable of satisfying Department of Energy goals for pulse power and energy densities in HEVs. We show that a 2.7 V electrochemical double-layer capacitor (EDLC) available today is unable to meet these goals. It would be necessary to increase the intrinsic capacitance by a factor of 3, or to increase the voltage window to 3.7 V. We also investigate an asymmetric hybrid supercapacitor (a lithium titanate spinel/activated carbon system). We show that this technology, which has a higher energy density than a traditional EDLC, may obtain 13 Wh/kg (without accounting for packaging weight) and has promise for meeting the demands of an HEV. (C) 2008 The Electrochemical Society.