Lithium Ion is becoming the technology choice for HEV, PHEV, as well as many stationary applications. With such large battery packs it is important to understand how the individual cells and packs will behave in a variety of scenarios. One of the main drawbacks for Lithium Ion cell technology is the potential for thermal runaway. Heat management of these large battery packs requires data on the heat output of the individual cells during normal operation. This information can be readily obtained using isothermal calorimetry.