Thermal-runaway (TRA) is one of the failure modes in batteries. Much research has been conducted to find the exact cause of this issue and how to prevent it. Despite all these efforts, a proper model which can predict the thermal-runaway does not yet exist. In Part I of the present study, this phenomenon is considered theoretically and as a result, a general set of governing equations are presented by which the thermal behavior of batteries can be obtained. Since the proposed system of equations is general, it can be used for investigating the thermal-runaway in any kind of battery systems. In a the second part of this paper (Part II), the present model will be applied to lead-acid batteries. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3592486] All rights reserved.