Abstract: Electrical storage technologies (i.e., batteries) play a ubiquitous role in all facets of modern technologies for applications ranging from very small to very large scale, both stationary and mobile. In the past decade, Li-ion batteries are quickly emerging as the preferred electrical energy storage technology due to the intrinsic power and energy storage density compared to older battery chemistries. All electrochemical batteries are strongly linked to their thermal state: on one hand, their electrical characteristics are strongly dependent on temperature and, on the other hand, their thermal state is a result of both their environmental temperature, but also their electrical usage due to internal heat generation. Furthermore, their life (and potentially safety) is also strongly affected by their thermal state. Li-ion batteries, due to their high electrical power capability and density tend to be used aggressively in many applications, rendering the thermal issues more acute. Finally, Li-ion battery packs (like all packs) are made of many cells interconnected in various series/parallel arrangements in tightly confined spaces. Hence, thermal management solutions need to be implemented for two primary reasons: rejecting the heat generated inside the pack to the environment to avoid high (or unsafe) temperatures leading to premature (or catastrophic) failure and providing a good thermal uniformity among all the cells so that their electrical performance (and aging) in well matched in a pack. This thesis focuses on the thermal modeling of Li-ion packs and the development of passive thermal management solutions for such packs. The thesis first provides an extensive review of the current literature on Li-ion batteries electrical and thermal modeling and current approaches for thermal management solutions of Li-ion packs. This study then focuses on a particular current application using a small Li-ion pack, namely a contractor-grade 36v cordless drill. This particular application was chosen as it encapsulates many of the features of larger automotive packs and represent and leads to an aggressive usage pattern where battery life is always an issue. This pack was experimentally studied to establish typical usage patterns and to measure the thermal and electrical state of the stock pack during such usage. The study then developed and validated a FEM computational pack model in the stock configuration. This experimentally validated models was then used as a proxy to reality to numerically investigate multiple possible configurations of passive thermal management solutions using a high thermal conductivity, Graphite-based heat spreading material to both reduce temperature non-uniformities within the pack and decrease of overall pack temperature (better heat rejection) during aggressive use. Finally, a preliminary experimental validation of one of the promising configurations of heat spreaders was investigated. The work described in this thesis clearly demonstrates that passive heat spreading technology can be very beneficial to reduce thermal stress on batteries and lead to more thermally homogenous packs. Furthermore, this study demonstrated that the investigation of such solutions can be performed with validated thermal FEM models to speed up the development of actual solution and reduce experimental prototype building. Future work will include more configurations, but also experimental investigation of battery life for both thermally managed and unmanaged packs under similar (aggressive) usage patterns. Finally, the conclusions from this study conducted on a cordless power tool are probably equally applicable to large automotive battery packs where life and costs are critical.