Lifetime modeling and model-based lifetime optimization of Li-ion batteries for use in electric two-wheelers

Electric vehicles (EVs) and hybrid electric vehicles (HEVs) have received increased attention over the last years as a result of the growing environmental awareness. In the context of battery electric vehicles (BEVs), the transition from lead-acid to Li-ion batteries increased user acceptance of BEVs as the driving range could be extended. Compared to lead-acid batteries, Li-Ion batteries offer a higher specific energy and power density, which result in the extension of the range. However, battery lifetime remains a key issue. In the field of e-mobility, China is one of the pioneering countries due to its growing and flexible market and its expertise in battery production. Although conventional four-wheel EVs are mostly being studied, the market for electric two-wheelers (E2W) should not be neglected. Due to the partial ban of conventional two-wheelers, E2Ws already shape the cityscape in China’s metropolises such as Shanghai and Beijing. This study deals with the lifetime prediction of Li-ion batteries and the applicability of such models to optimize the operation and charging strategy. A model-based optimal control is therefore designed. The focus is on typical application scenarios for E2Ws in Shanghai. The results, however, can also be generalized and transferred to EVs as well as various application cases and driving conditions. A realistic driving behavior must first be derived to reflect the typical use of an E2W. Since most of the existing standardized driving cycles are designed for four-wheeled vehicles and exhaust emission tests, a data acquisition campaign is conducted to capture the local conditions in Shanghai. On the basis of a user behavior survey, a typical use case is derived for a daily commute to and from work. Fleet data is collected to represent typical driving cycles. Based on this, a typical driving cycle for E2W is designed in Shanghai, which distinguishes between the two dominant power classes of E2W in Shanghai. A comparison between the derived driving cycle and other cycles highlights the special characteristics of E2Ws in Shanghai due to their lower power. The next step is to characterize and model the E2W. The powertrain consists mainly of three components: electric motor, power electronics and battery. Models of the electric motor and power control in combination with vehicle-to-street characteristics are required to convert the recorded typical driving cycles to equivalent power profiles. This is illustrated by implementing a permanent magnet synchronous machine (PMSM) model in addition to a static force conversion which neglects motor dynamics. Different degrees of abstraction are required, as optimization often requires a balance between accuracy and complexity. More complex models are required for simulation and verification. The main focus of this study is to investigate and model a Li-ion battery from a system perspective. The growth of the so-called Solid-Electrolyte Interphase (SEI) layer is discussed as a dominant aging process. In addition, different model representations are compared regarding their applicability in simulation and optimization. Although the term Li-ion battery is commonly used, different electrode compositions exist within Li-ion batteries and cell chemistry specific effects need to be considered. Since there is not much data available on different cell chemistries in the literature, an aging campaign is initiated to investigate the characteristics of a lithium manganese oxide (LMO) cell.