In the previous chapters, vehicle and battery requirements have been discussed, in connection with the design constraints in terms of cell selection and battery management and control. Now it is time to discuss usage and durability of the battery in electric vehicles. The durability of the components is of key importance in order to design economically and environmentally sustainable vehicles. Many components are designed to last as long as the vehicle itself, which is about five to 15 years depending on the type of vehicle and market. Since the battery is the most costly component in an electric vehicle, it is important to design it optimally for the intended application with respect both to usage and durability. Moreover, the battery is possibly the most sensitive component. In the following sections, ageing and durability of batteries will be discussed, from the role of usage conditions for durability to the details of degradation mechanisms of the active and non-active materials within the cell.
The goal is to provide knowledge of how batteries age and degrade, including the operating parameters affecting the degradation, in order to enable the designer to postpone the EOL as far as possible. To understand the correlation between utilisation and degradation is specifically important for HEVs where energy management can be adjusted in a different way compared to EVs; the HEV has two energy sources to be optimised for battery durability and fuel savings.
Battery cells start to degrade as soon as they are assembled. There are several factors influencing the life of a battery: e.g. temperature, current rates and direction, SOC ranges, mechanical effects like vibrations – all having a negative impact on battery life. The usage history will also have a considerable effect on durability, in the same manner as it influences the state functions. Strong path dependency and a complex matrix have to be understood in order to get a full picture of the degradation and to predict durability and hence the remaining life of the battery. To know the possible degradation mechanisms helps in designing a battery, and a corresponding management system, both for performance and life.