3-Level Flying Capacitor Inverter with Trapezoidal Wave AC Heating Capability for Automotive Batteries

Lithium-ion batteries (LIBs) in electric vehicles at low temperatures must be properly heated to avoid a decrease in their discharge capability. Internal ac heating techniques utilizing Joule heat generated by ac current flowing through an internal resistance of LIB have been proposed as an efficient and fast heating method. However, traditional internal ac heating techniques that use triangular and sinusoidal waves are not ideal due to their relatively low rms value at a given peak current, which results in a long heating time. This paper proposes a 3-level flying capacitor inverter for reducing heating time. The proposed PWM technique generates trapezoidal wave with a high rms value at a given peak current. Experimental results of a prototype for a 12-cell LIB demonstrated that the proposed trapezoidal wave heating reduced heating time to approximately one-third compared with traditional sinusoidal-wave heating.