磷酸鋰鐵電池模型與電池組之散熱分析

Abstract

磷酸鋰鐵電池由於適合大功率放電和安全性較高等特性，已經越來越廣泛地運用在純電動車以及混合動力電動車上。一般來說，鋰電池在放電過程中溫度會持續上升，如果超過其理想操作溫度範圍，會導致鋰電池的壽命受到影響。鋰電池操作時的溫度控制不僅僅是維持鋰電池的溫度在合理範圍內，同時也要顧慮到電池溫度一致性的問題，因為電池的任何不一致性皆會影響電池組的放電效率。然而，若想從實驗來獲取鋰電池放電之相關結果，往往需付出大量的時間與成本。有鑑於此，本研究之目的為建立一精準的鋰電池模型，來加速電池放電特性如放電電位、電量與熱生成等的分析效率，進而提供電池組一些散熱策略的建議。 本文針對兩種不同形式的磷酸鋰鐵電池模型做分析，分別是含有材料特性、物理背景意義的Newman模型和經類比電池系統特性的等效電路模型。將兩者模擬結果與實驗進行比對之後，等效電路模型產生較為精準的模擬結果。因此，後續電池組之散熱模型係以單電池等效電路模型為基礎開發。電池組之散熱分析包括：自然對流條件下改變電池間距；強制對流條件下固定總流率，改變風扇數量；及透過改變風扇及出風口位置進行流向控制。 對電池組而言，如何維持各個電池的狀態一致性是最重要的。因此，根據模擬結果發現，若要增加電池組溫度的一致性，應適當地減少電池彼此間之距離、適當地增加風扇數量以及視放電速率來進行對流流向與風扇位置之配置。

Lithium iron phosphate (LFP) batteries are becoming widely used in electric vehicles (EV) and hybrid electric vehicles (HEV) owing to its capability of high power output and its safety characteristics. Generally, the temperature of LFP batteries rises while discharging and if it goes beyond the appropriate temperature range, it will lead to a shorter cycle life. Besides, for a battery pack, we not only need to sustain temperature in an appropriate range, but we also need to ensure the consistency among batteries, which hugely influences the discharge rate of a battery pack. But, it is time consuming and expensive to optimize battery pack design through trial-and-error tests and experiments. Hence, this study aims to develop an accurate model to facilitate the analysis of battery pack performance and further, to provide a few insight into thermal management of a battery pack. This study analyzes two different types of LFP battery models. One is the Newman model, which incorporates the properties of material and physical processes occur in batteries. Another is the equivalent circuit model, ECM, which resembles the physical processes of a battery with equivalent circuits such that it reduces the complexity of analysis. In comparison to experimental data, we find that the ECM model simulates more accurate results than Newman model. Therefore, the battery pack model is developed on the basis of the ECM, single cell model. The thermal management strategies analyzed include changing the adjacent battery distance under natural convection, adjusting the number of fans under at fixed total flow rate (force convection) and varying the locations of inlet and outlet for flow arrangement. In conclusion, for a battery pack, the most important thing is to maintain consistency among individual cells. Based on our simulation results, we find that reducing the adjacent battery distance and increasing the number of fans are feasible strategies to achieve that. Apart from that the aforementioned strategies, we also learn that flow arrangement via exchanging the location of inlet and outlet may achieve better performance in certain cases.