Thermal Performance of EV and HEV Battery Modules and Packs

Thermal issues associated with electric vehicle (EV) and hybrid electric vehicle (HEV) battery packs can significantly affect performance and life cycle. Temperature variations from module to module in a battery pack could result in reduced performance. As part of the U.S. Department of Energy's Hybrid Propulsion Systems Program, the National Renewable Energy Laboratory (NREL) works with automobile and battery manufacturers on thermal analysis and management of valve-regulated lead-acid battery packs for HEVs. We use fundamental heat transfer principles and finite element analysis tools to predict the temperature distributions in cells, modules, and packs. We use infrared photography and liquid crystal thermography to obtain thermal images of the surface of battery modules under HEV charge/discharge profiles. In this paper, we provide an overview of related literature on battery thermal management, along with non-proprietary results of some of our work. Introduction The performance and life-cycle costs of electric vehicles (EV) and hybrid electric vehicles (HEV) depend inherently on energy storage systems such as batteries. Battery pack performance directly affects the all-electric (zero-emission) range, power for acceleration, fuel economy, and charge acceptance during energy recovery from regenerative braking. Because the battery pack cost, durability, and life also affect the cost and reliability of the vehicle, any parameter that affects the battery pack must be optimized. Battery module temperature uniformity is one such parameter. Depending on the electrochemical couple used in the battery, the optimum operating range is different. Some systems such as leadacid, NiCd, Li ion, and NiMH batteries operate reasonably well near room temperature. Other systems, such as Li polymers, may need to operate at elevated temperatures, and systems such as NaS require operating temperatures around 330C. These high-temperature systems need active thermal management systems. Battery systems that operate at ambient temperatures can also benefit from battery thermal management systems. Generally, higher temperatures improve the battery's performance because of increased electrochemical reaction rates; however, the battery's lifetime decreases because elevated temperatures increase corrosion. If temperature uniformity can be obtained within and between modules, then, the pack can operate closer to its desired optimum operating temperature range. Another important impact of a battery pack’s operating temperature is the electrical balance among modules in the pack. The performance of a battery pack depends on the performance of individual modules. If the cells and modules in the pack are at different temperatures, each module will be charged/discharged slightly differently during each cycle. After several cycles, modules in the pack will become unbalanced, degrading the pack’s performance. HEV charging/discharge profiles are generally more aggressive than those of EVs, which results in greater heat generation. EV batteries are high specific energy; HEV batteries are high specific power. Thermal issues in an HEV pack, then, are of more concern than thermal issues in an EV pack and thus, a thermal management system is required for HEVs. To optimize the performance of a battery pack, the thermal management system should deliver (1) optimum operating temperature range for all modules, (2) small temperature variations within a module, and (3) small temperature variations among various modules. However, the thermal management system must be compact and lightweight, easily packaged in the vehicle, reliable, and low-cost. It must also allow easy module access for service and use minimum power for fans and pumps. In this paper we provide an overview of heat generation in battery modules and of related work on batteries and EV packs. We then discuss the results of a thermal analysis in battery modules and packs, and thermal imaging techniques that can assist in evaluating the thermal behavior of modules and packs.