Experimental results and comparison with the previous method.
Abstract
Electric vehicles powered by lithiumion batteries take advantages for urban transportation. However, the safety of lithiumion battery needs to be improved. Selfinduced internal short circuit of lithiumion batteries is a serious problem which may cause battery thermal runaway. Accurate and fast identification of internal short circuit is critical, while difficult for lithiumion battery management system. In this study, the influences of the parameters of significance test on the performance of an algorithm for internal short circuit identification are evaluated experimentally. The designed identification is based on the meandifference model and the recursive least square algorithm. First, the identification method is presented. Then, two characteristic parameters are determined. Subsequently, the parameters of the significance calculation are optimized based on the measured data. Finally, the effectiveness of the method for the early stage internal short circuit detection is studied by an equivalent experiment. The results indicate that the detection time can be shortened significantly via a proper configuration of the parameters for the significance test.
Keywords
 lithiumion battery
 internal short circuit identification
 significance test
 meandifference model
 equivalent experiment
1. Introduction
Safety and energy saving as well as environmental protection are the key points for automobile industry development [1, 2]. Lithiumion batteries have been widely used due to their high energy density, long cycle life, low environmental pollution, no memory effect, and high charge and discharge rates [3, 4, 5]. With the rapid development of materials technology, the energy density of lithiumion batteries is gradually increased, causing the thermal runaway events accordingly. Its safety problem must be taken into account seriously [6, 7, 8, 9]. The safety of batteries may affect the confidence of the users of electric vehicles. Therefore, battery safety is one of the biggest obstacles for the application of electric vehicles. Internal short circuit is an important step leading to the thermal runaway. Thus an accurate and rapid identification for internal short circuit is necessary and needs to be solved [10].
The internal short circuit can be detected by comparing the output of the battery with a threshold or a predicted value via a mathematical model. Ikeuchi et al. measured the electricity output of the battery and compared with a threshold computed by the amperehour integration [11]. Ouyang et al. proposed three characteristic parameters according to the parameter effect and the depleting effect of an internal shortcircuited battery. The variations of the open circuit voltage (OCV) and the internal resistance were determined based on a meandifference model. Then, a significance criterion and a fault tolerance mechanism were used to judge whether the internal short circuit happened [12]. Feng et al. used a 3D electrochemicalthermal model to simulate the internal short circuit and regarded it as a parameter estimation problem [13]. The measured voltage and the temperature of the cell were input to a model to evaluated excessive depletion of the capacity and abnormal heat generation. The internal short circuit resistance was estimated by Seo et al. according to the SOC variation and the discrete amperehour integrals from an equivalent circuit model with internal short circuit branches [14]. Xia et al. employed a voltage correlation coefficient and a threshold to determine whether an internal short circuit occurred via capturing the initial abnormal voltage signal [15]. This method could eliminate the influence of the SOC. Zhang et al. designed a lowpass filter to estimate the leakage current and the resistance of the internal short circuit cell in real time [16]. Kang et al. developed a multifault diagnostic strategy based on an interleaved voltage measurement topology. An improved correlation coefficient method was employed to eliminate the inconsistency among the cells and the measurement errors [17]. Kong et al. used a principle of similar charging voltage to obtain the remaining charging electricity and used the average voltage to obtain the internal short circuit resistance and thus the internal short circuit was identified. However, the charging voltage does not have similarities in some cases, and the average voltage may not be correctly obtained [18]. Feng et al. analyzed the characteristics of internal short circuit using an electrochemicalthermal coupling model. This is significant for internal short circuit identification, but it also reveals the difficulty of internal short circuit identification [19].
The operation conditions of the lithiumion battery in an electric vehicle are very complicated and its safety is critical. Therefore, it is important to find a method which can detect the selfinduced internal short circuit in time. Our previous study proposed a method to detect the internal short circuit [12]. However, the detection time of this method is still not satisfactory. In this study, the identification method for the internal short circuit is modified, and the influences of the parameters of the significance test are evaluated. Then, the modified identification algorithm is validated via an equivalent experiment. First, the basic working principle and procedures for the internal short circuit detection are introduced based on the meandifference model. Then, the parameters for the internal short circuit identification are configured. Next, a replacement experiment is carried out, and the results of this method are computed. Subsequently, the results of a significance test are analyzed. The outcomes indicate that the detection time can be shortened significantly compared with those of the previous study. Finally, the possibility of the identification algorithm for earlystage internal short circuit detection is investigated. The results of this study can provide a reference for the practical applications so that the safety of lithiumion batteries can be improved.
2. Identification method
The mathematical model of the designed identification method is presented in this section. First, the equivalent circuit model is introduced. Then, the meandifference model and the recursive least square algorithm are described. Finally, the identification procedure is illuminated.
2.1 Equivalent circuit model
Figure 1(a)
shows the equivalent circuit model of a lithiumion battery with an internal short circuit resistor, where
Based on Figure 1(a) , the OCV can be expressed by
When the internal short circuit resistance approaches infinite, it can be taken as there is no internal short circuit. However, if it is less than a certain value, an internal short circuit branch may be considered to be generated in the battery cell. The parameter effect is the change of internal resistance and open circuit voltage due to the internal short circuit branch according to Eq. (1). The depleting effect is the drop of the open circuit voltage and the fluctuation of the internal resistance caused by the selfdischarge of the internal short circuit battery.
2.2 Meandifference model
The parameters of the equivalent circuit model for different cells inside a battery pack are not the same due to the discrepancies of material ingredients, manufacturing processes, and working conditions. The meandifference model includes a mean model for the pack and a difference model for each cell, which can be used to represent difference among the cells. The parameters of the mean model are obtained according to the average performance of the battery pack. Meanwhile, the difference model displayed in
Figure 1(b)
describes the difference of a specific cell against the mean model. In
Figure 1(b)
,
2.3 Recursive least square algorithm
For a battery pack in series layout, the total current and the terminal voltage can be measured. Accordingly,
After that,
The calculation process of the RLS algorithm with a forgetting factor can be expressed by Eqs. (8)–(12):
where
2.4 Identification procedure
The variations of
The main decision process is described in
Figure 2
. First, the terminal voltage and the operation current of the
The forgetting factor
In the previous method [12],
where
After that, the threshold is set to ±3. We can consider the significance calculation in this way: the extent of the extreme value deviating from the average value
3. Results and discussion
The nickelcobaltmanganese (NCM) ternary lithiumion battery is used for this experiment. The battery module is a parallel connection of three batteries and then connected in series. The layout of the battery cells on the test rig is shown in Figure 3(a) . The cell has a capacity of 50 Ah. The identification algorithm treats these three parallel groups as one series group with each cell having a capacity of 150 Ah. The equivalent resistance replacement method is employed to simulate the electrical characteristics of the internal short circuit battery. The dynamic stress test (DST) cycle is used as an input. The DST working condition is shown in Figure 3(b) , which can represent typical state changes of the battery under the practical operation conditions of electric vehicles. A data collector is used to measure the current and voltage of each cell. The accuracies for the current and voltage measurement are 0.1%, and the sampling frequency is set to 1 Hz. The maximum charge/discharge rate of the DST cycle is set to 2C. When the minimum cell voltage decreases to 2.75 V, the DST test is terminated. After 10 min rest, the battery pack is charged with a constant current of 75 A (0.5C) until the voltage of any cell reaches 4.2 V. After resting for another 10 min, the next cycle is proceeded again.
An equivalent resistor with a precision of 0.1% is used in the experiment. The internal short circuit can be triggered or cancelled via a switch. In this experiment, three different values for the internal short circuit resistance are used including 1, 10, and 100 Ω, respectively. The results of the three resistances are used to validate the identification algorithm and compared with those of the previous method [12]. The terminal voltage and the current of each cell are measured and inputted to the internal short circuit identification algorithm programed in MATLAB.
3.1 Experimental results
The results of the modified identification method are given in
Figures 4
–
6
for an external resistor of 1, 10, and 100 Ω, respectively. First, the measure values of the terminal voltage are displayed. Then,
For each case, the internal short circuit cell (i.e. Cell 1) already reaches the lowest voltage after the first DST cycle, leading to a much lower SOC. When the SOC drops to a very low value, the OCV is more sensitive. Therefore, a sudden drop of
The change rate of
The results for the detection time are listed in
Table 1
and compared with those in [12]. It can be seen that the detection time of the modified identification method is much shorter than the previous one. The reason is mainly attributed to the judge criteria being relaxed. To prevent the occurrence of false alarm, both the conditions for
Parameter  Criteria  100 Ω  10 Ω  1 Ω 


( 
42 min  27 min  6 min40 s 


11 h 19 min  1 h 18 min  24 min 


4 h 43 min  1 h 17 min  22 min 
Improvement (%) c  85.2%  64.9%  69.7% 
3.2 Analysis of significance test
The results of the significance calculation for an external resistor of 1 Ω are shown in the right column of
Figure 4
. The significance value for
3.3 Results for early stage identification
If an internal short circuit can be detected at the beginning, protection measures can be adopted in time, and the system safety can be improved significantly. However, when a selfinduced internal short circuit emerges at the early stage, the ISR resistance is very large. Accordingly, the variations of the OCV and the ISR current are very small. The possibility of this method for the earlystage internal short circuit identification is studied in this section [21]. An external resistance of 1000 Ω with a 0.1% precision is used in this equivalent experiment. The results are shown in
Figure 7
. Because the variation magnitudes of
3.4 Discussion
The algorithm is based on the meandifference model. There is a certain deviation in this model. In addition, the internal short circuit itself also generates noises for the identification algorithm. These may cause the internal resistance of the battery fluctuating more than other normal batteries. The true internal short circuit should be produced by the internal electrochemical reaction and the growth and change of the internal dendrites. At the same time, the temperature change caused by the internal short circuit will affect the electrochemical structure inside the battery, so the resistance fluctuation of the actual internal short circuit battery is larger than that of the normal battery. The external resistance used in the simulation of the internal short circuit may be different from the actual internal short circuit, but the equivalent experiment method still can be used to simulate the electrical characteristics of the internal short circuit battery. The battery resistance fluctuation of the equivalent experiment is smaller than the resistance fluctuation of the actual internal short circuit battery. As the internal short circuit battery ages, the lithiumion and active materials in the battery decrease. The internal short circuit resistance of the battery fluctuates more, and the open circuit voltage will also drop faster.
The problem of internal short circuit detection caused by inconsistency is tricky, especially for the characteristic parameter of
In the case of extreme SOC, the algorithm is limited by the inherent characteristics of the battery itself, and the internal shortcircuited battery may be affected by the inconsistency, the polarization, and the solidphase diffusion in the battery electrode particles at the extreme low SOC, causing the algorithm to produce a possible misjudgment. However, in electric vehicles, the working range of the battery generally would not reach the extreme SOC, so the algorithm can be used for internal short circuit identification in the normal driving range.
For the identification of early internal short circuit, it is difficult to separate the difference between internal short circuit and selfdischarge. The voltage characteristics of selfdischarge and internal short circuit are consistent. The threshold cannot be set to completely distinguish between selfdischarge and internal short circuit. Therefore, early internal short circuit identification is easy to be misreported.
4. Conclusions
In this chapter, an identification method is presented for the internal short circuit of lithiumion battery based on the meandifference model and the recursive least square algorithm with a forgetting factor. This method is a modification of our previous investigation. The working principle and test procedure are described at first. Then, its performance is compared with that of the previous method. Subsequently, the influence of the parameters of the significance test on the detection time is analyzed. Finally, the possibility of this method for the earlystage internal short circuit identification is estimated.
The criteria of the significance test for the internal short circuit detection are adjusted, and two characteristic parameters (
Because the selfinduced internal short circuit resistance is very large at the early stage, this method cannot detect the internal short circuit event when the SOC is in the range of normal operation conditions. However, if the selfinduced internal short circuit resistance reduces close to 100 Ω, the internal short circuit event can be identified successfully by this method, which can be integrated into the battery management system to monitor the operation of lithiumion battery within the normal working range. A warning signal can be specified if an internal short circuit is detected so that the security of electric vehicles can be improved.
Acknowledgments
The authors would like to thank for the support of the National Natural Science Foundation of China (Grant No. U1564205 and 51876009), the Beijing Natural Science Foundation (Grant No. 3184052), and the Cooperation Project of Chongqing Changan New Energy Vehicle Technology Co., Ltd (No. 1811420009).
Abbreviations
DST  dynamic stress test 
ISC  internal short circuit 
NCM  nickelcobaltmanganese 
OCV  open circuit voltage 
RLS  recursive least square 
SOC  state of charge 
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