Thermal management is the main issue affecting battery performance, lifetime, and safety. Lithium-ion batteries must operate within the required temperature limits (between 15° C and 40° C, with a 2° C temperature variation within the cell module). Battery operational temperature limits are quite different from the ambient air temperature or temperatures of other underhood components. Because hybrid and electric vehicles often have the electric drive motors distributed to the wheels, the batteries can be packaged anywhere in the vehicle — typically in very confined spaces with little cooling airflow. The engineering knowledge base for traditional battery cooling that has been built over the years no longer applies, and engineering organizations are struggling to find practical and cost-effective solutions.