A Rule Based Power Split Strategy for Battery/Ultracapacitor Energy Storage Systems in Hybrid Electric Vehicles

Abstract

This paper presents a simple and efficient rule based power split strategy for a combined battery/ultracapacitor energy storage system having electrochemical characteristics in hybrid electric vehicles. For this purpose, a novel rule based controller with three stages is introduced. The first stage is determination of the operation modes (i.e. either charge or discharge commands) of the energy sources based on the direction of the power request (i.e. either in traction or regen request) and the charge/discharge states (i.e. energy sources either in charging or discharging phase). In the second stage, new weighting parameters used in rule tables are formulated based on the state of charge levels (SOC) of the energy sources to ensure the charge sustainability (i.e. SOC within limits). In the last stage, the power split rules are defined in rule tables based on the operation modes, states of the energy sources and the weighting parameters. The performance of the proposed strategy has been tested on a hybrid electric city bus developed in MATLAB/Simulink and compared with alternative rule based power split strategy through comprehensive simulation studies under different drive cycle conditions. For comparison purpose, two different case studies have been conducted. First, the results have been compared with the battery only system and then the effectiveness of the proposed strategy has been compared with that of alternative one. The simulation results showed that the proposed strategy significantly reduces the frequency of high C-rate current drawn from the battery and provides a more smooth change in battery current during sudden acceleration events when comparing with alternative strategy.