In this paper, we propose a Haar wavelet with Mallet prolongation energy management strategy, a pure electric city bus produced by CENS Energy Tech Co, Ltd with a hybrid energy storage system that combines a lithium battery with an ultracapacitor is tested and modeled under a MATLAB-CRUISE joint simulation environment based on the experiments. In verifying its validity, the proposed method is compared with the common rule-based control strategy from the charging and discharging characteristics of the battery, the feedback efficiency of the ultracapacitor, and the overall system efficiency during China City Road Cycle. The results show in the proposed method that state of charge of the battery is more stable avoiding from the peak current shock, the battery life has been protected effectively, the ultracapacitor has been utilized more fully, and the vehicle performance is better for the tracking performance and control flexibility than the rule-based control strategy. KEYWORDS energy flow analysis, Haar wavelet, hybrid energy storage system (HESS), Mallat prolongation 1 | INTRODUCTIONHigh-energy consumption and traffic pollution have become serious problems in China. Electric vehicles, which emit no pollutants or environmentally harmful gases, offer a promising solution but are hampered by short driving range and limited battery life. To overcome this problem, in recent years, many researchers have proposed hybrid storage energy systems (HESS) with additional energy storage devices, such as fuel cells and ultracapacitors. 1,2 Under the conditions of the identified vehicle type and road conditions, the management strategy for controlling the power flow is regarded as research hotspot, the power flow between the elements of HESS, and drive system is equally worthy of further study. 3 Although there are numerous results and related literatures in researching vehicle energy management methods, these are relatively more dispersed and HESS is rarely mentioned together. Because of the cost functions being expressed as the vehicle's performance indicators, the optimal control is the most commonly used method in the energy management analysis, in which the driving conditions are known. However, this method is not suitable for real-time vehicle control, only providing a method to solve the offline problem. 4,5 Dynamic programming (DP) is a typical control method in optimal control; in the operation process, the overall decision making is gradually shifted. This multiple-step optimal control problem is converted into several single-step ones. This greatly simplifies the overall solution process. In the study of Kobayashi and Miyatake, 6 DP is adopted to reduce the fuel consumption for parallel hybrid electric vehicles (PHEVs) with HESS; similarly, the fuel economy, battery life, and cost for PHEV with HESS are compared with sole energy storage while DP is used in the study of Ou and Sun. 7 Dynamic programming is very conventional to solve the optimal control problem. However, the more dimensions of the problem, the longer ca...