Method for analyzing causes of thermal runaway accidents of power battery faulty cells

Abstract

A kind of power battery failure cell thermal runaway accident cause research method, by comparing the characteristics of voltage drop curve and temperature change curve when accident occurs, and combining with the appearance observation and analysis of failure cell after fire accident, the effective and rapid judgment of the cause of thermal runaway accident of failure cell in new energy vehicle fire accident investigation is realized.In the present application, the power battery failure cell thermal runaway accident cause research method starts from the voltage and temperature curve characteristics of cell level, and combines with the state of cell sample after accident, which is suitable for distinguishing and judging the cause of the accident.The method is easy to operate, and is convenient for relevant technical researchers to carry out new energy vehicle fire accident investigation and rapid judgment of the cause of the accident.

Description

Technical Field

[0001] This invention relates to a method for determining the cause of thermal runaway accidents in power battery cells. Background Technology

[0002] In recent years, the production and sales of new energy vehicles have experienced explosive growth, but safety accidents have also occurred frequently. Known accidents involving new energy vehicles have revealed that fires can occur during various stages of use, including when the vehicle is stationary, charging, driving, and during collisions, raising concerns about the safety of these vehicles. Only by tracing the root causes can the problem of new energy vehicle fires be prevented.

[0003] Currently, the trigger for fires in new energy vehicles is concentrated on their power source—the power battery. In the power batteries used in new energy vehicles, multiple cells (also called "single cells") form a module. The module, along with the thermal management system, battery management system (BMS), electrical system, and structural components, assembles to form the entire power battery pack.

[0004] The battery cell is the source of energy for the entire power battery, and its thermal runaway is also the key to causing vehicle fire accidents.

[0005] Common accident-causing scenarios include thermal and mechanical factors.

[0006] There are two types of thermal factors. The first is that the battery cell is directly ignited by an open flame. The second is that the battery cell does not come into contact with an open flame but is instead subjected to high-temperature heat conducted through conduction, which causes abnormal heat generation and leads to thermal runaway. Mechanical factors are caused by foreign objects squeezing or piercing the battery cell, resulting in an internal short circuit and thermal runaway.

[0007] Currently, the main method for investigating fire accidents involving new energy vehicles is on-site visual observation. For example, after disassembling the battery module, we observe the top cover and side panels of the individual cells, whether there is mutual compression and deformation between cells, bulging of the electrode sheets, and traces of arcing and melting through the cell shell to infer the cause of the accident. However, we lack the technology to determine the cause of the accident of the faulty battery cell.

[0008] Therefore, developing a method for determining the causes of thermal runaway in power battery cells, combined with existing visual observation and analysis of faulty cells, provides important technical support for the investigation and analysis of new energy vehicle accidents. Revealing the causes of accidents at the cell level is of great significance for improving the effectiveness of investigating the causes of new energy vehicle disasters and for studying the patterns of new energy vehicle fires. Summary of the Invention

[0009] The purpose of this invention is to provide a method for determining the cause of thermal runaway accidents in power battery cells. By comparing the characteristics of the cell voltage drop curve and temperature change curve at the time of the accident, and combining the visual observation and analysis of the faulty cell after the fire accident, the method can effectively and quickly determine the cause of thermal runaway accidents in new energy vehicle fire accidents.

[0010] The objective of this invention is achieved through the following technical solution:

[0011] The method for determining the cause of thermal runaway accidents in power battery cells includes the following steps:

[0012] (1) Observe the characteristics of the temperature curve before the cell temperature rises from normal temperature to a temperature surge (temperature rise rate exceeds 2℃ / s); if the temperature curve gradually rises (i.e. there is a temperature rise period), it is determined that the cause of the cell thermal runaway is "heat factor"; if the temperature remains unchanged (i.e. there is no temperature rise period), it is determined that the cause of the cell thermal runaway is "mechanical factor".

[0013] This is mainly because when thermal runaway is primarily caused by heat, there is a heat conduction process, resulting in a gradual temperature rise phase observed on the temperature curve of the faulty cell. When thermal runaway is primarily caused by mechanical action, the temperature curve of the faulty cell does not show a gradual temperature rise phase before the accident occurs; only after abnormal mechanical action occurs will a sudden temperature spike appear on the cell's temperature curve.

[0014] (2) If the cause of thermal runaway of the battery cell is determined to be "heat-induced" according to step (1), then compare the order of time when the voltage drops to 0V and the temperature surge on the voltage curve; if the voltage drops to 0V before the temperature surge, and the battery cell casing bulges after the accident but is relatively intact, then it is determined that the cause of thermal runaway of the battery cell is thermal runaway caused by the high temperature heat conducted through it.

[0015] This is mainly because the high temperature conduction triggers thermal runaway in the battery cell. The separator inside the cell first melts, causing an internal short circuit. During overcharging at high temperatures, the electrolyte gradually decomposes, generating pressure from the inside out, causing the faulty cell's casing to bulge. However, because the heat conduction process is slower than ignition by an open flame, and the electrolyte decomposition and gas production are also relatively slow, when high temperature conduction causes thermal runaway in the battery cell, the casing bulges but remains relatively intact.

[0016] (3) If the cause of thermal runaway of the battery cell is determined to be "heat-induced" according to step (1), then compare the time when the voltage drops to 0V and the time when the temperature spikes. If the voltage drops to 0V and the temperature spikes at the same time, and the battery cell casing is severely cracked after the accident, and the burn marks on the outer side of the casing are significantly more severe than those on the inner side, then it is determined that the cause of thermal runaway of the battery cell is thermal runaway caused by open flame combustion.

[0017] This is mainly because when an open flame ignites and causes thermal runaway, the battery temperature rises rapidly, accelerating the reaction between the internal positive and negative electrode materials and the electrolyte. This results in a sudden surge of high heat at the onset of thermal runaway, occurring almost simultaneously with the voltage drop. Furthermore, the electrolyte generates a large amount of gas in a short time, instantly rupturing the outer casing and causing severe damage to the casing of the faulty cell. Since open flames are often externally ignited, such as fires involving automotive electrical components, wiring harnesses, or interior trim, the external fire source first burns the outer casing. If the fire continues after the casing ruptures, the inner casing will only begin to be burned. Therefore, in such cases, the burn marks on the outer casing are significantly more severe than those on the inner casing.

[0018] (4) If the cause of thermal runaway of the battery cell is determined to be "mechanical factors" according to step (1), then compare the temperature change curves of the positive electrode, negative electrode and the center of the large surface of the battery cell; if the three temperature curves have good overlap, then it is determined that the cause of thermal runaway of the battery cell is mechanical factors acting in parallel on the internal electrode of the battery cell.

[0019] This is mainly because when external mechanical stress is applied parallel to the direction of the cell electrode, the number of short-circuited electrodes inside the cell caused by the external mechanical action is small, and the thermal runaway propagation speed is relatively slow. As a result, the thermal trend at each location of the cell is not significantly different, and the temperature curves collected at the positive electrode, negative electrode, and center of the large surface of the faulty cell have a high degree of overlap.

[0020] (5) If the cause of thermal runaway of the battery cell is determined to be "mechanical factors" according to step (1), then compare the temperature change curves of the positive electrode, negative electrode and the center of the large surface of the battery cell; if the three temperature curves are significantly different and have low overlap, then the cause of thermal runaway of the battery cell is determined to be mechanical factors acting perpendicularly to the electrode plates inside the battery cell.

[0021] This is mainly because mechanical action perpendicular to the direction of the cell electrode will cause a large number of electrode short circuits inside the cell, generating a concentrated large discharge current at the stress point, instantly generating high heat, and the temperature rises very quickly. The heat is transferred to different parts of the cell with great differences. Therefore, the temperature curves collected at the positive electrode, negative electrode and center of the large surface of the faulty cell have great differences.

[0022] The battery cell mentioned in step (1) is a battery cell for power batteries, mainly a lithium battery;

[0023] The cell temperature curve mentioned in step (1) is the temperature change curve of various parts of the cell over time collected before and after the story occurs, such as the temperature of the positive electrode, negative electrode, and a certain part of the outer casing of the cell.

[0024] The “heat-induced factors” mentioned in step (1) include open flame ignition and heat-induced factors generated by high-temperature heat sources;

[0025] The sources of the open flame include open flames intentionally or unintentionally ignited by the perpetrator of the accident, fires in automotive wiring harnesses, electrical appliances, or interior components, etc.

[0026] The high-temperature heat sources include automobiles operating at abnormally high temperatures, battery cells that first experience thermal runaway, and high-temperature heat sources generated by high-voltage arcing.

[0027] The “mechanical factors” mentioned in step (1) include the factors caused by abnormal mechanical factors (squeezing and puncturing); specifically, these include the squeezing effect caused by a car collision, foreign objects piercing the battery cell from the bottom, and damage to the battery cell caused by scraping.

[0028] The voltage curve mentioned in step (2) is the voltage change curve of the positive and negative terminals of the battery cell before and after the accident, which is collected over time.

[0029] The present invention has the following advantages and effects compared with the prior art:

[0030] (1) In this invention, the method for judging the cause of thermal runaway accident of power battery cell failure starts from the characteristics of cell-level voltage and temperature curves and combines the state of cell sample after the accident. It is suitable for distinguishing and judging the cause of the accident.

[0031] (2) The method of the present invention is easy to operate and facilitates relevant technical R&D personnel to conduct investigations on fire accidents of new energy vehicles and to quickly determine the cause of the accident. Attached Figure Description

[0032] Figure 1 This is a graph showing the voltage and temperature changes of a faulty battery cell in Embodiment 1 of the present invention.

[0033] Figure 2 This is a photograph of the battery cell after thermal runaway in Embodiment 1 of the present invention.

[0034] Figure 3 This is a graph showing the voltage and temperature changes of a faulty battery cell in Embodiment 2 of the present invention.

[0035] Figure 4 This is a photograph of the battery cell after thermal runaway in Embodiment 2 of the present invention.

[0036] Figure 5 This is a graph showing the voltage and temperature changes of a faulty battery cell in Embodiment 3 of the present invention.

[0037] Figure 6This is a photograph of the battery cell after thermal runaway in Embodiment 3 of the present invention.

[0038] Figure 7 This is a graph showing the voltage and temperature changes of a faulty battery cell in Embodiment 4 of the present invention.

[0039] Figure 8 This is a photograph of the battery cell after thermal runaway in Embodiment 4 of the present invention.

[0040] Figure 9 This is a graph showing the voltage and temperature changes of a faulty battery cell in Embodiment 5 of the present invention.

[0041] Figure 10 This is a photograph of the battery cell after thermal runaway in Embodiment 5 of the present invention. Detailed Implementation

[0042] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0043] Example 1

[0044] Taking a certain power battery fire accident as an example, the cause analysis of the thermal runaway accident of the faulty cell is as follows:

[0045] (1) As Figure 1 As shown, when observing the temperature changes at the positive electrode, negative electrode, and center of the large surface of the battery cell from the normal temperature to the temperature surge, the temperature curves all show a slow temperature rise period. The maximum temperature rise rate in the slow temperature rise region is 0.4℃ / s. Therefore, it is determined that the cause of the thermal runaway of the battery cell is "heat-induced".

[0046] (2) Based on the judgment in (1), the cause of the cell thermal runaway is "heat-induced". Then compare the results. Figure 1 When the voltage drops to 0V on the medium voltage curve, it occurs simultaneously with a temperature surge.

[0047] (3) According to Figure 2 The photos of the battery cell after the accident show that the outer casing of the cell is severely cracked, and the burn marks on the outer side of the casing are significantly more severe than those on the inner side. Therefore, it is determined that the thermal runaway of the battery cell was caused by thermal runaway due to open flame combustion.

[0048] Example 2

[0049] Taking a certain power battery fire accident as an example, the cause analysis of the thermal runaway accident of the faulty cell is as follows:

[0050] (1) As Figure 3 As shown, when observing the temperature changes at the positive electrode, negative electrode, and center of the large surface of the battery cell from the normal temperature to the temperature surge, the temperature curves all show a slow temperature rise period. The maximum temperature rise rate in the slow temperature rise region is 0.2℃ / s. Therefore, it is determined that the cause of the thermal runaway of the battery cell is "heat-induced".

[0051] (2) Based on the judgment in (1), the cause of the cell thermal runaway is "heat-induced". Then compare the results. Figure 3 The voltage drops to 0V on the medium voltage curve before the temperature spikes.

[0052] (3) According to Figure 4 The photos of the battery cell after the accident show that the casing of the faulty cell is bulging but the cell is relatively intact. Therefore, it is determined that the thermal runaway of the battery cell was caused by the high temperature heat conducted through it.

[0053] Example 3

[0054] Taking a certain power battery fire accident as an example, the cause analysis of the thermal runaway accident of the faulty cell is as follows:

[0055] (1) As Figure 5 As shown, when observing the temperature changes at the positive electrode, negative electrode, and center of the large surface of the battery cell from the normal temperature to the temperature spike, the temperature curves did not show a gradual temperature rise period. Therefore, it is determined that the cause of the thermal runaway of the battery cell is "mechanical factors".

[0056] (2) Based on the judgment in (1), the cause of the cell thermal runaway is "mechanical factors". Figure 5 The three temperature curves show good overlap, indicating that the cause of thermal runaway in the battery cell is mechanical factors acting in parallel with the internal electrodes.

[0057] (3) According to Figure 6 Photos of the battery cell after the accident show a dent at the bottom of the faulty cell's casing, extending from the outside in. A foreign object pierced the cell from the direction parallel to the cell's electrodes, causing thermal runaway.

[0058] Example 4

[0059] Taking a certain power battery fire accident as an example, the cause analysis of the thermal runaway accident of the faulty cell is as follows:

[0060] (1) As Figure 7 As shown, when observing the temperature changes at the positive electrode, negative electrode, and center of the large surface of the battery cell from the normal temperature to the temperature spike, the temperature curves did not show a gradual temperature rise period. Therefore, it is determined that the cause of the thermal runaway of the battery cell is "mechanical factors".

[0061] (2) Based on the judgment in (1), the cause of the cell thermal runaway is "mechanical factors". Figure 7 The three temperature curves show good overlap, indicating that the cause of thermal runaway in the battery cell is mechanical factors acting in parallel with the internal electrodes.

[0062] (3) According to Figure 8Photos of the battery cell after the accident show a dent on the side of the faulty cell's casing that extends from the outside in. A foreign object pierced the cell from the direction parallel to the cell's electrodes, causing thermal runaway.

[0063] Example 5

[0064] Taking a certain power battery fire accident as an example, the cause analysis of the thermal runaway accident of the faulty cell is as follows:

[0065] (1) As Figure 9 As shown, when observing the temperature changes at the positive electrode, negative electrode, and center of the large surface of the battery cell from the normal temperature to the temperature spike, the temperature curves did not show a gradual temperature rise period. Therefore, it is determined that the cause of the thermal runaway of the battery cell is "mechanical factors".

[0066] (2) Based on the judgment in (1), the cause of the cell thermal runaway is "mechanical factors". Figure 9 It is evident that the three temperature curves cannot overlap, indicating that the cause of thermal runaway in this battery cell is a mechanical factor acting perpendicularly on the internal electrode of the cell.

[0067] (3) According to Figure 10 Photos of the battery cell after the accident show a dent on the front of the faulty cell casing that extends from the outside in. A foreign object pierced the cell from a direction perpendicular to the cell electrode, causing thermal runaway.

[0068] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for determining the cause of thermal runaway accidents in power battery cells, characterized in that... Includes the following steps: (1) Observe the characteristics of the temperature curve change from the normal temperature to the temperature spike before the temperature rises. If the temperature curve gradually rises, it is determined that the cause of the thermal runaway of the battery cell is "heat-induced". If the temperature remains unchanged, it is determined that the cause of the thermal runaway of the battery cell is "mechanical-induced". The temperature surge refers to a temperature rise rate exceeding 2°C / s; (2) If the cause of thermal runaway of the battery cell is determined to be "heat-induced" according to step (1), then compare the time when the voltage drops to 0V and the time when the temperature spikes on the voltage curve. If the voltage drops to 0V before the temperature spikes, and the battery cell casing bulges after the accident but is relatively intact, then it is determined that the cause of thermal runaway of the battery cell is thermal runaway caused by the high temperature heat conducted through it. (3) If the cause of thermal runaway of the battery cell is determined to be "heat-induced" according to step (1), then compare the time when the voltage drops to 0V and the time when the temperature spikes. If the voltage drops to 0V and the temperature spikes at the same time, and the battery cell casing is severely cracked after the accident, and the burn marks on the outer side of the casing are significantly more severe than those on the inner side, then it is determined that the cause of thermal runaway of the battery cell is thermal runaway caused by open flame combustion. (4) If the cause of thermal runaway of the battery cell is determined to be "mechanical factors" according to step (1), then compare the temperature change curves of the positive electrode, negative electrode and the center of the large surface of the battery cell; if the three temperature curves have good overlap, then it is determined that the cause of thermal runaway of the battery cell is mechanical factors acting in parallel on the internal electrode of the battery cell. (5) If the cause of thermal runaway of the battery cell is determined to be "mechanical factors" according to step (1), then compare the temperature change curves of the positive electrode, negative electrode and the center of the large surface of the battery cell; if the three temperature curves are significantly different and have low overlap, then the cause of thermal runaway of the battery cell is determined to be mechanical factors acting perpendicularly to the electrode plates inside the battery cell.

2. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The cell temperature mentioned in step (1) refers to the temperature at a certain point on the cell.

3. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The cell temperature mentioned in step (1) refers to the temperature of the positive electrode, negative electrode, or a certain point on the outer casing of the cell.

4. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The high-temperature heat mentioned in step (2) is caused by a heat source; the heat source includes a car operating at an abnormally high temperature, a battery cell that first experiences thermal runaway, or a high-temperature heat source generated by high-voltage arcing.

5. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The mechanical factors mentioned include compression and puncture.

6. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The mechanical factors mentioned include the squeezing effect caused by a car collision, foreign objects piercing the battery cell after bottoming out, or damage to the battery cell caused by scraping.

7. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The voltage curve mentioned in step (2) is the voltage change curve of the positive and negative terminals of the battery cell before and after the accident, which is collected over time.

8. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The battery cell mentioned in step (1) is a battery cell for power batteries.

9. The method for determining the cause of thermal runaway accidents in power battery cells according to claim 1, characterized in that: The battery cell mentioned in step (1) is a lithium battery.

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