[1] Xidian Univ, Sch Electromech Engn, Key Lab Elect Equipment Struct Design, Minist Educ, Xian 710071, Peoples R China
[2] Northwestern Polytech Univ, Sch Marine Sci & Technol, Xian 710072, Peoples R China
[3] Tianjin Inst Power Sources, Natl Key Lab Sci & Technol Power Sources, Tianjin 300384, Peoples R China