Electrochemical capacitors have attractive performance features for gas-electric hybrid vehicle applications, namely high-power density and essentially unlimited cycle life. A capacitor system can efficiently capture and store the 5-10s of braking energy generated during vehicle stopping and then release it during Subsequent vehicle acceleration. Even with their exceptional power performance, these types of capacitors have less than 100% cycle efficiency-some energy is always lost as heat. Thus, high-rate cycling of electrochemical capacitors may create thermal management problems. the magnitude increasing with cycle rate. This paper examines energy loss of capacitors Undergoing periodic constant-current charge/discharge cycling from one-half rated voltage to rated voltage. Cycle efficiencies of five commercial, large-cell capacitors are reported as a function of cycle rate and compared with series-RC equivalent circuit model results. Three of the capacitors were well represented by such the simple model with good correlation over a span covering almost two orders Of Magnitude of charge/discharge times. The other two capacitors were not well represented by this simple model, which was due to their substantially greater porous electrode behavior. Consequently, the series-RC model appears adequate for investigating thermal management issues of some, but not all large ECs and demonstrates that caution is needed when selecting capacitor electrical models to perform thermal management engineering. (c) 2006 Elsevier Ltd. All rights reserved.