Reliability Analysis of Lithium-Ion Battery-Packs Used in Electric Vehicles

因鋰電池具有高能量密度、良好的穩定性、充放電效率高、循環壽命長等優點，逐漸成為現今電動車儲能裝置使用的電池組，但國內少有鋰電池組量化可靠度及其與成本間相互關係的研究，是以本研究考慮鋰電池組中各電池單元健康狀態(State of Health)具有不確定的特性，將整個鋰電池組視為多狀態系統(Multi-State System)，且電池組內的電池單元在經多次充放電使用後會有健康狀態退化的現象，利用上述不確定特性結合機率學上的通用生成函數法(Universal Generating Function)，來評估不同配置鋰電池組的系統可靠度及其與成本間的關係，因為過度提升電池組可靠度會使成本相對提升，所以需要在這兩者間取得一個平衡。此外，本研究也應用模糊理論深化電池健康狀態判斷的不確定性，探討該不確定性對評估結果的影響。本研究透過數值分析發現，具有相同數量鋰電池單元的串並聯電池組成本相同，但使用較多並聯配置的電池組可靠度優於使用較多串聯配置的電池組，在成本相同考量下，吾人可藉此設計出可靠度較佳配置的鋰電池組。透過本研究所提方法可讓電動車設計工程師對電池組可靠度及其與成本間取捨提供一參考依據。The lithium-ion battery-pack has been considered a good candidate for using in electric vehicles as an energy storage device owing to its high power-density, good stability, high charge-discharge efficiency and high-cycle life. The battery-pack usually consists of many battery-cells. The state of charge of the battery-cell degenerates along with charge-discharge cycles, and it results in the reliability issue of the battery-pack. However, there was little study on the relationship between the reliability and cost of a lithium-ion battery-pack. It should be an interesting research topic because over-enhancing the reliability of the battery-pack makes the cost increase and it is necessary to make a tradeoff between them. In this study, the state of health of a single battery-cell is modeled as a multi-state system in consideration of its uncertainty, and its reliability can be evaluated. Accordingly, the lithium-ion battery-pack becomes a large system consisting of many battery-cells and its reliability can be evaluated as well. In addition, fuzzy set theory is applied to take uncertainty of the state discretization into consideration. Based on the above modeling and the probability theory, universal generating function of the battery-pack can be derived, and the relationship between battery-pack reliability and cost of the battery-pack for various kinds of assembly can be disclosed. From the result of numerical analysis, it is found that with the same cost for the same numbers of battery-cells, the assembly having more parallel cells is more reliable than that of having more series cells