電動車電池系統之可靠度分析

Abstract

近年來全球電動汽車市場快速增長，2021年的增長率即高達108%，而佔電動車整車成本將近五成之電池系統也自然成為研究要項之一。電動車的電池系統主要由動力電池系統(power battery system)和電池管理系統(battery management system)組成，本研究目的是建立一套可靠度工程(reliability engineering)分析方法，針對前述兩個系統進行故障樹分析，再分別考慮它們的失效模式，預估系統壽命、失效率與可靠度，並比較兩系統間之相對可靠度。動力電池系統其實就是俗稱的電池組(battery pack)，其係由數千顆電池單元(battery cell)先組成數個電池子系統(battery group)，再由子系統組合成電池組，其中包含一定比例的備用電池單元與子系統。本研究引入可靠度工程中 k-out-of-n 之系統可靠度概念，探討電池組在不同充放電循環次數和時間下之可靠度，並分別考量探討不同數量比例備用電池單元與備用電池子系統之設置對電池組可靠度的影響。電池管理系統則由數個電子元件所組成，負責控制和監測每顆電池單元的健康狀態，為整體電池系統中的重要角色。本研究將電池管理系統視為其所包含主要電子元件串聯在一起的系統，依美軍可靠度工程手冊所要求的失效率規格，評估電池管理系統之失效率，再經適當轉換，獲得電池管理系統在不同充放電循環次數下之可靠度。本研究最後比較前述兩個系統之可靠度、平均失效時間(MTTF)、失效時間標準差等不同度量。研究結果顯示，電池管理系統之平均壽命為動力電池系統之17.86倍，所以在此兩系統中，我們應該特別留意並加強動力電池系統的失效，以確保電動車整體電池系統的安全性與可靠度。針對參數影響的探討，本研究改變備用電池子系統的數量，從8%提升至10%，發現在同一可靠度要求下，電池組可增加100個充放電循環次數。

In recent years, the electric vehicle market has experienced rapid growth, with a staggering growth rate of 108% in 2021. The battery system, which accounts for nearly 50% of the overall cost of an electric vehicle, has naturally become a focal point of research. The battery system of an electric vehicle primarily consists of a power battery system and a battery management system. The objective of this study is to establish a reliability engineering analytical method for the battery system, primarily the above two mentioned systems it consists of. It begins with fault-tree analyses for both systems and then considers their failure modes to estimate system life, failure rate and reliability for each system. It also compares the relative reliabilities between the two systems. The power battery system, commonly referred to as the battery pack, is composed of several thousand battery cells, which are organized into multiple battery groups. These groups are then combined to form the battery pack, which includes a certain proportion of spare battery cells and groups. This research employs the k-out-of-n system reliability concept for both the battery group and the battery pack. It also investigates the reliability of the battery pack under different charging and discharging cycles, and examines the influence of different proportions of spare battery cells and battery groups on the reliability of the battery pack. The battery management system is comprised of several electronic components responsible for controlling and monitoring the health status of each battery cell. It plays a crucial role in the overall battery system. In this study, the battery management system is considered as a system where all electronic components are connected in series, following the reliability block diagram approach in reliability engineering. The failure rate of the battery management system is evaluated based on the failure rate requirements documented in the US Military Handbook. By appropriate transformation, the reliability of the battery management system under different charging and discharging cycles is obtained. Finally, this study compares the reliabilities, mean-time-to-failures (MTTFs), and standard deviations of failure time between the two aforementioned systems using different metrics. The result shows that the MTTF of the battery management system is 17.86 times longer than that of the power battery system. Therefore, for these two systems, particular attention should be given to strengthening the reliability of the power battery system to ensure the safety and reliability of the vehicle battery system. Regarding the investigation of parameter influences, this study focused on changing the quantity of spare battery groups from 8% to 10%. It was found that under the same reliability requirement, the battery pack could withstand an additional 100 charging and discharging cycles.