電池單元性能差異對電動車電池組可靠度分析之影響

Abstract

近年電動車(electric vehicle, EV)產業發展快速，佔電動車整車成本將近一半之電動車電池系統相關研究也成為趨勢。電池在經由多個充放電循環後，其電容量會逐漸降低，致使電動車失效風險隨之上升，最終導致發生失效。本研究旨在建立一套可靠度工程(reliability engineering)分析方法，依鋰離子電池單元(battery cell)之健康狀態(state of health, SOH)定義電池性能，藉以探討並預估電動車電池組(battery pack)因性能退化導致可靠度隨充放電循環次數降低之趨勢。傳統可靠度 分析方法僅考慮各電池單元健康狀態是否符合一定要求來決定電池模組之系統可靠度，未必符合多數電池管理系統之實際運作，是以本研究引入可靠度工程中 kout-of-n之系統可靠度概念，考量電池模組所有電池單元中有數個健康狀態差異過大即會引起失效，探討電池模組與電池組之可靠度，也探討相關參數變化對電池組壽命分布及可靠度之影響。經案例分析比對，本研究發現傳統系統可靠度分析 方法會高估電動車電池組之可靠度，而本研究所提方法分析結果則趨近實際狀況，可避免錯估可靠度導致電池失效之風險。針對相關參數之探討，研究結果顯示，如電池管理系統能順利運作，溫度對電池組壽命分布與可靠度影響並不大，影響較大的是電池組內各單元老化後之健康狀態差異過大，若單元健康狀態較為平均，則電池組的壽命與可靠度都會明顯提升。

With the rapid development of electric vehicle (EV), researches related to EV battery systems have flourished in recent years, partly because the battery system accounts for almost half of the cost of an EV. For the battery system, its capacity degraded gradually along with charge-and-discharge cycles and hence increases the risk of the EV. The reliability issue of the battery system thus arises. The present study proposes an analytical model based on reliability engineering for studying the capacity degradation and quantitative reliability of EV battery packs. The model, in turn, is constructed in consideration of states of health (SOH) of battery cells and their uncertainties. As compared to the traditional system reliability model that judges the pack’s reliability purely based on considering if or not a battery cell’s SOH exceeds a prescribed threshold, the proposed model adopts the k-out-of-n system reliability concept and considers that SOHs of battery cells within the pack do not differ too much to assure the pack’s reliability. This study also adjusts a few parametric values in the model to investigate their influences on battery pack’s life and reliability. Through numerical case studies, it is found the proposed model results in a more conservative system reliability prediction of a battery pack than the traditional model, and the capacity degradation trend of the battery pack coincides with a real case. In consideration of an effective battery management system that avoids fierce temperature changes, the influence of temperature is not very obvious in this study. The uneven performance of battery cells reflected by discrepancies of their SOHs is found crucial to the battery pack. If the performance of each battery cell is balanced, the battery pack’s life and reliability would increase significantly.