In laboratory research and practical applications, the surface temperature of Li-ion battery or battery cell is commonly monitored for proper thermal management. Because heat is generated inside Li-ion battery during operation due to Joule heating, irreversible reaction heat, and entropic heat, it is expected that the internal temperature of a Li-ion battery cell would be higher than the outer surface temperature. Measuring internal temperature can thus provide more accurate information and it is a better indicator of the health and safety state of a Li-ion battery. In addition, the internal temperature data will provide a critical source of validation for the latest electrochemical-thermal coupled models for lowtemperature performance as well as battery safety.