Improved equivalent circuit model for high capacity lithium ferro phosphate battery

Abstract

Electric Vehicle (EV) gets the attention and interest of scientists due to its advantages of zero green house gaseous emissions and higher efficiency. Battery pack is utilised as energy storage element in EV. Strict handling on battery pack is important to ensure battery pack performs in safe and consistent manner under various load demand and driving state. Therefore, an efficient Battery Management System (BMS) which can perform State Of Charge (SOC) estimation, cell equalisation and temperature control, should be put as the primary concern. In this aspect, an accurate battery model is required to give high quality SOC estimation and battery management. Equivalent circuit model is widely used as the battery model since it can be easily connected to external circuit in a simulation platform. However, the existing battery models are generally built for low capacity battery and do not take into account on nonlinear capacity effect. In this thesis, equivalent circuit model for 18 Ah Lithium Ferro Phosphates (LiFePO4) battery is developed. LiFePO4 battery is a good energy storage element for EV since it has good thermal and chemical stabilities. The thesis studies the existing battery modelling technique and investigates the dynamic characteristics of 18 Ah LiFePO4 battery. A new battery modelling approach with consideration of nonlinear capacity effect has also been proposed for high capacity LiFePO4 battery. Moreover, a simplified methodology for battery modelling is proposed to improve existing battery model. Parameter extraction is discussed and the proposed battery model is validated from the experiment data. The comparison between experiment and simulation results shows that the proposed model is capable of predicting dynamic behaviours of the battery with minimum error.