Modular multilevel converters for battery electric vehicles: Variable dc voltage control to optimize battery lifetime

Nowadays, battery electric vehicles are spreading and the customer expectation is increasing not only in terms of performances and vehicle range but also in terms of reliability and lifetime of the battery onboard. In recent years, several innovative solutions for the traction drives have been proposed with the aim to increase the efficiency and the performance. Particularly interesting is the solution based on the double star chopper cell converter integrating the battery in the converter modules and implementing also the battery management system functionalities. For this converter, different modulation techniques have been proposed but all of them keeps the dc bus voltage constant implying the use of all the battery modules in any working moment. In this paper, the possibility to reduce the battery usage, and, therefore, their ageing, by controlling the dc bus voltage as function of the vehicle speed is analyzed. The proposed control technique is implemented in a numerical model and the obtained results show the possibility to increase, in a significant way, the battery lifetime with positive effects also on the vehicle range.