Systematic approach for the test data generation and validation of ISC/ ESC detection methods

Various methods published in recent years for reliable detection of battery faults (mainly internal short circuit (ISC)) raise the question of comparability and cross-method evaluation, which cannot yet be answered due to significant differences in training data and boundary conditions. This paper provides a Monte Carlo-like simulation approach to generate a reproducible, comprehensible and large dataset based on an extensive literature background on common assumptions and simulation parameters. In some cases, these assumptions are quite different from field data, as shown by comparison with experimentally determined values. Two relatively simple ISC detection methods are tested on the generated dataset and their performance is evaluated to illustrate the proposed approach. The evaluation of the detection performance by quantitative measures such as the Youden-index shows a high divergence with respect to internal and external parameters such as threshold level and cell-to-cell variations (CtCV), respectively. These results underline the importance of quantitative evaluations based on identical test data. The proposed approach is able to support this task by providing cost-effective test data generation with incorporation of known factors affecting detection quality

Triggering and characterisation of realistic internal short circuits in lithium-ion pouch cells: a new approach using precise needle penetration

The internal short circuit (ISC) in lithium-ion batteries is a serious problem because it is probably the most common cause of a thermal runaway (TR) that still presents many open questions although it has been intensively investigated. Therefore this article focuses on the generation and characterisation of the local single-layer ISC, which is particularly relevant in practice application. A new, very promising method of precise needle penetration made it possible to generate the most safety-critical short-circuit type, the contact between the Al-Collector and the graphite active material of the anode reliable, demonstrated on a 10 Ah Graphite/NMC pouch cell. The special efforts in achieving high reproducibility as well as the detailed analysis of the initiated internal short-circuit conditions led to more reliable and meaningful results. A comprehensive approach to characterisation has been made by detailed measurement of the dynamic short-circuit evolution and a subsequent post-characterisation, which included the application of different electrochemical measurement techniques as well as a post-abuse analysis. It was shown that the cells demonstrated a very individual and difficult-to-predict behaviour, which is a major challenge for early failure detection and risk assessment of cells with an existing or former ISC. On the one hand, it is found that despite high local temperatures of over 1260 ◦C and significant damage to the cell-internal structure, the cell did not develop a TR even with further cycling. On the other hand, it was observed that the TR occurs spontaneously without any previous abnormalities. Based on the overall test results, it was shown that at the high state of charge (SOC = 100 %) even small, dynamically developing voltage drops must be classified as safety-critical for the cell. For reliable and early failure detection, the first voltage drops of the ISC must already be detected

Systematic approach for the test data generation and validation of ISC/ESC detection methods

Various methods published in recent years for reliable detection of battery faults (mainly internal short circuit (ISC)) raise the question of comparability and cross-method evaluation, which cannot yet be answered due to significant differences in training data and boundary conditions. This paper provides a Monte Carlo-like simulation approach to generate a reproducible, comprehensible and large dataset based on an extensive literature search on common assumptions and simulation parameters. In some cases, these assumptions are quite different from field data, as shown by comparison with experimentally determined values. Two relatively simple ISC detection methods are tested on the generated dataset and their performance is evaluated to illustrate the proposed approach. The evaluation of the detection performance by quantitative measures such as the Youden-index shows a high divergence with respect to internal and external parameters such as threshold level and cell-to-cell variations (CtCV), respectively. These results underline the importance of quantitative evaluations based on identical test data. The proposed approach is able to support this task by providing cost-effective test data generation with incorporation of known factors affecting detection quality

Comparison of model-based and sensor-based detection of thermal runaway in Li-ion battery modules for automotive application

In recent years, research on lithium–ion (Li-ion) battery safety and fault detection has become an important topic, providing a broad range of methods for evaluating the cell state based on voltage and temperature measurements. However, other measurement quantities and close-to-application test setups have only been sparsely considered, and there has been no comparison in between methods. In this work, the feasibility of a multi-sensor setup for the detection of Thermal Runaway failure of automotive-size Li-ion battery modules have been investigated in comparison to a model-based approach. For experimental validation, Thermal Runaway tests were conducted in a close-to-application configuration of module and battery case—triggered by external heating with two different heating rates. By two repetitions of each experiment, a high accordance of characteristics and results has been achieved and the signal feasibility for fault detection has been discussed. The model-based method, that had previously been published, recognised the thermal fault in the fastest way—significantly prior to the required 5 min pre-warning time. This requirement was also achieved with smoke and gas sensors in most test runs. Additional criteria for evaluating detection approaches besides detection time have been discussed to provide a good starting point for choosing a suitable approach that is dependent on application defined requirements, e.g., acceptable complexity

Comparison of model-based and sensor-based detection of thermal runaway in Li-ion battery modules for automotive application

In recent years, research on lithium-ion (Li-ion) battery safety and fault detection has become an important topic providing a broad range of methods for evaluating the cell state based on voltage and temperature measurements. However, other measurement quantities and close-to-application test setups were only sparsely considered yet, not has been a comparison in between methods. In this work the feasibility of a multi-sensor setup for detection of Thermal Runaway failure of automotive-size Li-ion battery modules have been investigated in comparison to a model-based approach. For experimental validation Thermal Runaway tests were conducted in a close-to-application configuration of module and battery case – triggered by external heating with two different heating rates. By two repetitions of each experiment high accordance of characteristics and results has been achieved and the signal feasibility for fault detection has been discussed. The before published model-based method recognised the thermal fault in the fastest way – significantly previously the required 5 min pre-warning time. This requirement was also achieved with smoke and gas sensors in most test runs. Additional criteria for evaluating detection approaches besides detection time have been discussed to provide a good starting point for choosing a suitable approach dependent on application defined requirements e.g. acceptable complexity

Electrochemical and thermal modeling of lithium-ion cells for use in HEV or EV application

To build a lithium battery system from single cells for use in an electric drive system the coupled electrochemical and thermal characteristics of these cells have to be well-known. Otherwise it is not possible to find the best cell for a given application considering capacity, power, thermal and electric management, housing, monitoring, etc. In the presented work an easy to parameterize Simulink model regarding the electrochemical and thermal properties was developed to simulate different cell types for an electric drive system. The model is based on an equivalent circuit diagram which is parameterized by means of impedance spectroscopy as well as of current pulse techniques. Differences of these methods are discussed briefly. By comparing simulation results and measurements it is shown that the coupled electrochemical-thermal model can predict the current - voltage behaviour and the temperature development of the different cells very exactly for both constant current profiles and load profiles appropriate for an electric forklift application

Triggering and characterisation of realistic internal short circuits in lithium-ion pouch cells: a new approach using precise needle penetration

The internal short circuit (ISC) in lithium-ion batteries is a serious problem since it is probably the most common cause of a thermal runaway (TR) that still presents many open questions, even though it has been intensively investigated. Therefore, this article focusses on the generation and characterisation of the local single-layer ISC, which is typically caused by cell-internal impurity particles that cannot be completely eliminated in the cell production. A new, very promising method of precise and slow (1 μm s−1) needle penetration made it possible to generate the most safety-critical reliable short-circuit type—the contact between the Al-Collector and the graphite active material of the anode—as demonstrated on a 10 Ah Graphite/NMC pouch cell. The special efforts in achieving high reproducibility as well as the detailed analysis of the initiated internal short-circuit conditions led to more reliable and meaningful results. A comprehensive approach to characterisation has been made by detailed measurement of the dynamic short-circuit evolution and a subsequent post-characterisation, which included the application of different electrochemical measurement techniques as well as a post-abuse analysis. It was shown that the cells demonstrated a very individual and difficult-to-predict behaviour, which is a major challenge for early failure detection and risk assessment of cells with an existing or former ISC. On the one hand, it is found that despite high local temperatures of over 1260 ◦C and significant damage to the cell-internal structure, the cell did not develop a TR even with further cycling. On the other hand, it was observed that the TR occurs spontaneously without any previous abnormalities. Based on the overall test results, it was shown that at the high state of charge (SOC = 100%), even small, dynamically developing voltage drops (<10 mV) must be classified as safety-critical for the cell. For reliable and early failure detection, the first voltage drops of the ISC must already be detected

Particle Contamination in Commercial Lithium-Ion Cells—Risk Assessment with Focus on Internal Short Circuits and Replication by Currently Discussed Trigger Methods

A possible contamination with impurities or material weak points generated in cell production of lithium-ion batteries increases the risk of spontaneous internal short circuits (ISC). An ISC can lead to a sudden thermal runaway (TR) of the cell, thereby making these faults especially dangerous. Evaluation regarding the criticality of an ISC, the development of detection methods for timely fault warning and possible protection concepts require a realistic failure replication for general validation. Various trigger methods are currently discussed to reproduce these ISC failure cases, but without considering a valid basis for the practice-relevant particle properties. In order to provide such a basis for the evaluation and further development of trigger methods, in this paper, the possibilities of detecting impurity particles in production were reviewed and real particles from pouch cells of an established cell manufacturer were analysed. The results indicate that several metallic particles with a significant size up to 1 mm × 1.7 mm could be found between the cell layers. This evidence shows that contamination with impurity particles cannot be completely prevented in cell production, as a result of which particle-induced ISC must be expected and the need for an application-oriented triggering method currently exists. The cause of TR events in the field often cannot be identified. However, it is noticeable that such faults often occur during the charging process. A new interesting hypothesis for this so-far unexplained phenomenon is presented here. Based on all findings, the current trigger methods for replicating an external particle-induced ISC were evaluated in significant detail and specific improvements are identified. Here, it is shown that all current trigger methods for ISC replication exhibit weaknesses regarding reproducibility, which results mainly from the scattering random ISC contact resistance

Thermal fault detection by changes in electrical behaviour in lithium-ion cells

With this paper a method to detect faults of lithium-ion cells during operation is first presented and later validated by experiment. Since every cell fault will increase the cell temperature towards its process until thermal runaway the method uses the temperature-dependent change of the cell impedance as fault feature. Using a 46 Ah pouch cell the model was parameterised by electrochemical impedance spectroscopy and then validated during dynamic load. For this purpose the Worldwide harmonised Light vehicles Test Procedure (WLTP) was chosen. The presence of a fault was simulated by heating the cell once uniformly and once locally and the progression of the chosen fault feature analysed. For both test cases the method proposed was able to detect the present heat source before the thermal runaway was triggered and venting or voltage drop were observed