Abstract: Efficient regenerative braking of electric vehicles (EVs) can enhance the efficiency of an energy storage system (ESS) and reduce the system cost. To ensure swift braking energy recovery, it is paramount to know the upper limit of the regenerative energy during braking. Therefore, this paper, based on 14 typical urban driving cycles, proposes the concept and principle of confidence interval of “probability event” and “likelihood energy” proportion of braking. The critical speeds of EVs for braking energy recovery are defined and studied through case studies. First, high-probability critical braking speed and high-energy critical braking speed are obtained, compared, and analyzed, according to statistical analysis and calculations of the braking randomness and likelihood energy in the urban driving cycles of EVs. Subsequently, a new optimized ESS concept is proposed under the frame of a battery/ultra-capacitor (UC) hybrid energy storage system (HESS) combined with two critical speeds. The battery/UC HESS with 9 UCs can achieve better regenerative braking performances and discharging performances, which indicates that a minimal amount of UCs can be used as auxiliary power source to optimize the ESS. After that, the efficiency regenerative braking model, including the longitudinal dynamics, motor, drivetrain, tire, and wheel slip models, is established. Finally, parameters optimization and performance verification of the optimized HESS are implemented and analyzed using a specific EV. Research results emphasize the significance of the critical speeds of EVs for regenerative braking.

Key words: Electric vehicle , · Regenerative braking , · Critical speed , · Hybrid energy storage system , · Driving cycle , · Confdence interval