[1] Collaborat Innovat Ctr Elect Vehicles Beijing, Beijing 100081, Peoples R China
[2] Beijing Inst Technol, Natl Engn Res Ctr Elect Vehicles, Beijing 100081, Peoples R China
[3] Univ Witwatersrand, ZA-2000 Johannesburg, South Africa
[4] Rhein Westfal TH Aachen, D-52062 Aachen, Germany