#Method of lifespan uniformity study for the battery pack in the

Abstract—Battery un-uniformity is a critical problem for the battery pack in the automotive powertrain application. Life model and thermal model is combined with the ECM to study the divergence of the un-uniformity decay rate between the cells in the battery pack. The result shows different trends of decay in the different working condition which could be guides in the cell selection during the battery pack design and assemble phase. Copyright 2010 EVS25
LR
(2)
Lactual-available cycles in the actual working condition LR-available cycles in the nominal test condition TR-surface temperature of the battery cell in the nominal test condition (K) R-molar gas constant Ea-activation energy T-surface temperature of the battery cell in the actual working condition (K) f(IR) -current in the nominal test condition f(DODR) -DOD in the nominal test condition
Wang Jiayuan1, Sun Zechang1, Wei Xuezhe1 and Dai Haifeng1
1
School of Automotive Studies, Tongji University, NO.4800 CaoAn Rd., Shanghai, 201804, China, Email: stfdwjy@
Keywords—battery uniformity, ECM, lifespan decay, automotive
1.
Introduction
performance fade of the dedicated cells, which leads to more un-uniformity in the pack. The over-voltage could also cause more heat generation from the dedicated cells, which would impact the cells around them and even leads to thermal runaway. Thus, it is a complex problem for the battery management system (BMS) in estimation of the single cell status and controlling cells in the pack away from abuse. It is necessary to analyze the behavior of the cell uniformity during the application of the battery pack. Such information would be in the battery pack design and BMS algorithms adjust. The equivalent circuit model (ECM) has been widely used in the battery modeling and online status estimation [1] - [6]. The ECM is accurate in the description of a single cell. It is possible to use ECM of the cell to form a battery pack model in which all cells are connected in serial. The cell decays during the application, which would be reflected by the parameters change in the ECM. So a battery life model is added for adjust the cell parameters during the simulation. A vehicle powertrain model could be combined with the battery model for co-simulation to gain data for further study. The parameters of each cell could be set purposively to gain much clear
T 0 0 1 C SOC (k 1) ( ) SOC k T T R1C1 R1C1 0 u1 (k ) R1 (1 e ) I (k ) u1 (k 1) 0 e T T u2 ( k ) u2 (k 1) R2C2 0 0 e R (1 e R2C2 ) 2 U (k ) u (k ) u (k ) R I (k ) OCV ( SOC (k )) v (k ) 1 2 0
The battery or the battery pack is one of the key components in the EV or HEV. The battery pack consists of many cells, whose quantity may be from decades to thousands. However, the parameter of each cell is not in uniformity. The causes of cell un-uniformity could be categorized into two phases. One phase is intrinsic which happens during the process of the cell production; another is extrinsic which is caused by the use of the environment for the cell. In the cell production, there are several aspects that would cause the cell un-uniformity. They are the heterogeneous material, the uneven doping process and the flaw during the cell assembly. During the application of the cell, the different use of the environment for each cell would enlarge the intrinsic un-uniformity of the cells. The cell un-uniformity would cause many problems during the battery application. For example, some of the cells in the pack may be under-voltage or over-voltage during discharge or charge process while the voltage of the whole pack stays in the normal scope. The under-voltage and over-voltage could cause
© EVS-25 Shenzhen, China, Nov. 5-9, 2010 The 25th International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exposition
phenomena for partition the different cause and effect. The result would be helpful for the battery selection and battery pack design.
2.
Model development
L a c tu a l TR e T
1 [( E a E a ) ( f ( I ) f ( D O D ) f ( I R ) f ( D O D T TR T TR R
R
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)]
2.1. Cell model & battery pack model
© EVS-25 Shenzhen, China, Nov. 5-9, 2010 The 25th International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exposition
Method of lifespan uniformity study for the battery pack in the automotive application
The ECM model used in our study is presented in Figure 1. R0 represents the ohmic resistance; R1 and C1 represent the diffusion process during cell charge and discharge; R2 and C2 represent the mass transfer process during cell charge and discharge; C0 represent the capacity and SOC of the cell. Because SOC has a close relationship with the OCV, it could be described in the OCV-SOC function as the right part of Figure 1. The ECM could be transfer by state equations and an output equation as follow. T is the step time for calculation.
Then, the whole capacity released in the lifespan actual working condition has a relationship with it in the nominal testing condition as follows based on Eyring equation: