Simulation Of Cell To Cell Variations And Thermal Management In Lithium-ion Battery Packs

Lithium-ion battery packs are one of the most promising energy storage systemsin applications for new energy automobile, smart grid and so on, though great effortshave still to been done to solve the existing problems. Cell to cell variations arecritical factors that shorten the cycle life of battery packs, and thermal management iscrucial to guarantee safety in operation of battery packs. Intensive study of both ofthem will contribute to the applications of lithium-ion battery packs in various fields.An equivalent circuit model was built to simulate the discharge/charge behaviorof individual cells, based on the sample of a commercial LiFePO4battery cell. Theimpact of cell state of charge and temperature on the cell impedence characteristicshas taken into consideration, as well as the hysteresis effect of cell open circuitvoltage. Therefore, the model is advantageous not only for the intrinsic characteristicof fast-computing as an equivalent circuit model, but also for its high accuracy in awide range of temperatures and C-rates. This model was then used to build batterypack models to analyze the impact of cell to cell variations on battery systems.The impact of cell to cell variations on battery packs with different electricconnections was analyzed, and found an available electric connection method to avoidthe adverse effects of cell to cell variations. Simulation study of the series-parallel andthe parallel-series connected battery pack indicates that the former output less energyin a single discharge/charge cycle, however, its cycle life was observed longer thanthe latter. The trade off between high energy output and long cycle life showed thatthere is an alternative connection other than the two connections.A temperature maldistribution of individual cells and its impact on the spread ofcell to cell variations in a series-parallel connected battery pack were also clarified inthis work. The results indicate that lower bulk temperature and higher temperaturemaldistribution lead to more inhomogeneous depths of discharge of the cells in thepack, while higher bulk temperature and higher temperature maldistribution shortenthe cycle life of the battery pack. Typically, the uneven current distribution among theparallel battery units, caused by the temperature maldistribution, was observed toincrease the cell aging inhomogeneity in the pack. Furthermore, the target of thermalmanagement was discussed using this model. The results suggest that for a continuousdischarge-charge process, the bulk temperature of the pack should be maintained at 20°C while the temperature difference should be constrained within10°C as amaximum tolerance.In terms of thermal management, a shortcut computation method was proposedto rapidly estimate the flow and temperature profiles in a parallel airflow-cooled largebattery pack. The method coupled a flow resistance network model with a transientheat transfer model to calculate the temperature distribution as influenced by theuneven airflow distribution within and among battery modules in the pack. Themethod not only avoids computational complexity compared with the commonly usedcomputational fluid dynamics, but also keeps a high accuracy for estimation. Usingthis method, the effects of structure parameters on the airflow and temperaturedistributions were presented, and an example of collective parameter adjustment foracceptable temperature uniformity of a battery pack was given.