On-board Aging Estimation using Half-cell Voltage Curves for LiFePO₄ Cathode-based Lithium-Ion Battery for Electric Vehicle Application

The aim of this work is the design of an algorithm for on-board determination of the actual capacity of a LiFePO₄ cathode-based lithium-ion battery in electric vehicle applications. The presented approach is based on the detection of the predominant aging mechanisms (in terms of loss of lithium and loss of active material in both electrodes) through the determination of the single electrode voltage curves. The information related to the characteristic length and position of the voltage plateaus which can be gathered during the battery operation can be used to obtain the actual aging state of the cells. The length of the plateaus depends on the respective position that the voltage curves of the single electrodes have in relation to each other. Relating the change of the plateau characteristics with the possible aging mechanism allows the determination of the actual battery aging state in terms of total cell capacity. The work presents a possible implementation of an algorithm for capacity determination based on the described methodology. The algorithm is validated with various differently aged LiFePO₄ cells. Furthermore, the work discusses the ability of the method to detect the actual battery capacity when only part of the quasi-OCV curve is measured. Achieved accuracy and existing limitations are described and discussed in detail.