Researches On Plug-in Hybrid Electric City Bus Control Strategy With Consideration Of Battery Degration Characteristics

Energy crisis and environmental pollution are two major problems for the currentautomobile industry, and new energy vehicles are considered to be an effective way tosolve them. Among the extensive researches on new energy automobiles, plug-in hybridelectric vehicles, combining both the advantages of pure electric vehicle as energyconservation and environmental protection and traditional vehicles with long mileage, is afeasible solutions to solve the above problems under current conditions. In this paper, anenergy management strategy of plug-in hybrid city bus system optimized on completeoperating period is explored, taking the battery performance attenuation as crucialconsideration.According to the characteristics of power battery in PHEVs, impact on economy ofPHEVs from attenuation of battery performance data from battery cycle life experimentshave been analytically calculated. And an appropriate working range of SOC for PHEVs isselected considering the influence on battery cycle life from all-electric range (AER) ofPHEVs and different SOC ranges synthetically. Aiming at the immeasurability of the actualcapacity of the battery, which is pivotal for the estimation of SOC and the design of energymanagement strategy, LIBSVM model is adopted for on-line prediction of SOH based onhistorical cycles and real-time data from HCU.A modified equivalent circuit model is built based on the analysis of internal reactionprocess, and the parameters of it is simplified on the basis of the application of battery onPHEVs as well as experiments data and then verified by simulation throughMatlab/Simulink. For the high demand of instantaneity and accuracy of battery SOCestimation and the quantity of interference on the acquisition of signals, adaptive extendedkalman filter algorithm is applied here to predict battery SOC. Taking the increasingly poorestimation of SOC due to the performance decay resulting from growing number of PHEVscycles into account, the SOH parameters are introduced into the battery equivalent circuitmodel, through which adjust the model parameters based on SOH to achieve theself-adaption for aging cells and then the precise prediction of SOC in all working time.In line with the driving characteristics of PHEVs a PHEVs energy managementstrategy based on Pontryagin’s minimum principle has been proposed, which obtain thereal-time optimal control vector by searching co-state variables to realize the best economyin single cycle. Analyzing the relationship between the optimal co-state variables andbattery parameters and considering influence on the economy of the vehicle from batterycell performance degradation, the co-state variables are adjusted at the time of SOHchanging, to preventing PHEVs working in CS mode due to the aged battery and running atoptimal CD mode in each cycle, which achieve the optimal economy of PHEVs in the whole range of battery life.A hybrid control unit (HCU) based on rapid prototyping technology and a batterymanagement system based on dual CAN network architecture are developed, verifiedcombining with engine control unit (ECU), motor control unit (MCU), AutomatedMechanical Transmission control unit (TCU) on the bench for stability, coordination aswell as the availability of the above energy management strategy. The PHEV roadexperiments have been finished for further certification of this research.