Thermal characteristics and thermal behavior of lithium batteries are important both for the batteries meeting operating life requirements and for safety considerations. Sandia National Laboratories has a broad-based program that includes analysis, engineering and model development. We have determined thermal properties of lithium batteries using a variety of calorimetric methods for many years. We developed the capability to model temperature gradients and cooling rates of high-temperature primary lithium thermal batteries several years ago. Work is now under way to characterize the response of ambient-temperature rechargeable lithium-ion batteries to thermal abuse. Once the self-heating rates of lithium cells have been established over a range of temperatures, the thermal response can be estimated under a variety of conditions. We have extended this process to isolate the behavior of individual battery components and have begun to understand the chemical nature of the species responsible for heat evolution within the cells. This enhanced level of understanding will enable more accurate modeling of cell thermal behavior and will allow model-based design of safer, more abuse-tolerant lithium batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs) in the future. Progress toward this goal and key information still needed to reach it are discussed.