Abstract: The limitation of traction battery systems in performance, cost, life, and safety constitutes the bottleneck for
the diffusion of electric vehicles. This paper analyzes dozens of electric vehicles in the phase of R & D and several major
commercialized electric vehicles, identifies and reviews four key technologies for the traction battery system, the assembly
of cells into the battery, thermal management, electric energy management, and safety. Underlying these key technologies,
two fundamental academic issues are specified: 1) the generation, transfer, and removal of heat in the stacked composite
system comprising cells and heat conduction plates/flow fields; 2) the modeling, identification, and control of the battery
system comprising a multitude of non-linear, time-varying cells connected in parallel and series. Further development
tendency for traction battery systems are viewed, such as the smart cell and the integration with the grid and internet.

Key words: electric vehicles, traction battery system, assembling, battery management system, battery thermal management, battery energy management, battery safety