各種電池都有其危險性與使用限制，擁有高放電密度的鋰電池也不例外，若製造過程封裝不當、內部安全裝置不完善或於不合適操作環境下，都可能使鋰電池內部溫度上升，造成隔離膜熔解進一步發生爆炸，因此鋰電池使用上的安全性問題、熱評估及良好的熱穩定性更顯得重要。本研究探討商用18650鋰離子電池內部，若存在短路點其溫度累積之過程，並進一步說明不同短路位置時，其嚴重性以及作熱評估。首先利用計算流體力學分析軟體SC/Tetra建立網格系統與執行熱流場計算，並藉由實驗設計一適用於網格獨立性之熱源，結果顯示其實驗與模擬值相差2.04K，證明建構之數值模型具有相當之可信度。
接著透過文獻確認適用於內短路時之熱源與其體積，再藉由實驗量測得到適用於內短路熱源之材料參數，最後將上述條件分別帶入數值模型以計算短路點於電池不同位置時之變化與影響。由計算結果分析可知，若內短路點分別設置於頂端部分時，因受限於上蓋造成對流效果不佳；而置於底部區時，因流體流速於電池底部產生停滯點，對流效果比頂端更差，因此內短路點位於底處所產生溫度為最高，換言之即此處為最危險之情況；至於中間區域所產生的溫度為最低，故此處為最安全區。
實際影響電池安全性尚有內短路阻值與其電量狀態(State of Charge，SOC)，故本文分別選取短路電阻值為0.01、0.16與1(Ω)進行模擬，分析結果發現倘若短路電阻與電池整體電池相同時，其發熱瓦數為最高。其原因為發生內短路期間，雖然短路瓦數與電池整體瓦數差不多，但由於短路點體積遠小於整體故使得單位體積發熱瓦數極高，因此到達熱失控的時機將視短路熱源而定。故電阻為0.16(Ω)時，不到0.1秒鐘時間，其電池內部最高溫即到達408K(熱失控點)；當電阻值為0.01與1(Ω)時，到達熱失控點時機分別為1秒與0.3秒。至於改變電池SOC參數的模擬分析發現，在20%<SOC<80%之操作區，其內部最高溫會隨SOC減少而降低。
除此之外，本研究並針對電池不同擺放位置，對其熱傳行為作探討，結果顯示相較於內短路點位置、阻值與SOC對電池之影響，在發生內短路1秒鐘內，改變電池之放置位置其自然對流效應無法將熱量有效帶走，導致放置方式對電池的溫度場分佈無任何影響。故綜合歸納來說，若以安全性的考量下，發生短路時任何擺放方式皆無太大益處，唯有短路點位置、阻值及SOC才是增加電池安全性之主要因素。

Each type of batteries has its own risk and limitation for applications, certainly the high-energy-density lithium-ion battery is not an exception. Usually, these hazardous accidents result from the increasing temperature inside the lithium battery to reach the melting point of separator; hereafter a series of chemical reactions is trigged successively to evolve an explosion. This thermal-runaway phenomenon is mainly attributed to the improper packaging in manufacturing process, inadequate internal safety device, and unsuitable operation environment. Therefore, the safety issue, heat management, and thermal stability for the lithium battery has attracted growing attentions and motivated this research. In this study, a commercial 18650-type lithium-ion battery has been numerically investigated on how the temperature distribution is developed after an internal short circuit (ISCr) is initiated. Besides, its hazard and thermal evaluation are analyzed and described for different ISCr locations.
The commercial CFD software SC/tetra is adopted to construct the grid system and perform the thermal/fluid calculation. To validate this code, an experiment with the designed heat source inside battery is executed to obtain the thermal data for comparing with numerical calculation. The outcomes illustrates that the numerical tool is reliable because a small temperature deviation between experiment and calculation is found as 2.04K. Besides, it is essential to obtain the appropriate input conditions for an accurate simulation. Thus, a systematic scheme for evaluating the heat generation, the ISCr volume, and the material properties are established via a combined effort of experimental measurement and comprehensive literature survey. Finally, incorporated with the correct inputs, the numerical model can be used to predict the thermal behavior under ISCr condition for different ISCr locations, ISCr resistances, and SOCs (states of charge).
There are three ISCr positions, top, middle and bottom portions, are studied here. The numerical results demonstrate that the highest temperature appears at the case of bottom ISCr location while the lowest one is found for ISCr located at the middle portion. The maximum temperature occurs because the fluid is trapped at the bottom-side where stagnation point and poor convection exist. Accordingly, the bottom part is considered as the most dangerous location and the middle portion as the safest location. Besides, the safety of lithium battery is influenced by resistance of ISCr and SOC. Here, the resistances of 0.01, 0.16 and 1(Ω) are chosen for simulating the thermal propagation inside the battery. It indicates that the dissipation power has the peak value when the ISCr resistance (Rs) equals to the internal resistance of battery (Ri). It follows that thermal runaway is mainly dependent on the ISCr power since the energy density of ISCr is much larger than that of battery, thank to the smaller volume of ISCr. For this reason, CFD calculations predict the battery temperature reaches 408K (i.e. thermal runaway state) within 0.1 second when the value of Rs is 0.16Ω. Similarly, the thermal runway state is attained within 1 and 3 seconds for the cases of Rs=0.01Ω and 1Ω, respectively. Furthermore, simulations for various SOCs show that the highest temperature inside the battery reduces for a decreasing SOC.

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