1.Test content
With the rapid popularity of electric vehicles (EVs), fire accidents caused by battery thermal runaway have become frequent, with over 90% of EV fires related to this. Lithium ion batteries have become the core components of electric vehicles and energy storage systems (ESS) due to their high energy density and long cycle life, but their safety hazards have always constrained the development of the industry. Although the mechanism of thermal runaway and gas release at the battery cell level have been studied, there is still a significant lack of systematic research on commonly used soft pack battery modules for EVs, especially experimental data for EV level soft pack modules with 12 or more cells is scarce.
Soft pack batteries are widely used in mid to high end EVs due to their high volume energy density and strong adaptability, but the propagation law of module level thermal runaway is still unclear. How does State of Charge (SOC) affect the intensity of thermal runaway? Does the triggering position (edge or center) change the propagation characteristics? These issues directly affect module safety design and fire prevention and control strategies. This study focuses on the NMC 622 EV grade soft pack module with 12 battery cells, and systematically explores the propagation characteristics of thermal runaway by regulating SOC and trigger position.
2. Materials and Experimental Methods
2.1 Experimental Module Parameters
This study selected EV grade soft pack lithium-ion battery modules for experiments, using a 12 cell configuration of 2 parallel 6 series (2p6s). The rated capacity of a single battery cell is 55.6Ah, with a thickness of about 9mm; the total energy of the module is 2.4kWh, and the size is 139.7 × 431.8 × 114.3mm, which is consistent with the specifications of mass-produced electric vehicle battery modules, ensuring that the experimental data has engineering application value.
2.2 Experimental Variables and Design
The experiment revolves around the two core variables of State of Charge (SOC) and thermal runaway trigger location. Six control experiments are designed, each using the same specification module to ensure variable singularity:
SOC variable: Set three gradients of 50%, 75%, and 100%, charge to the target value using a high-precision CT-4008-5V12A-DB battery tester, and let it stand for 2 hours to eliminate polarization effects;
Trigger position variable: divided into two methods: edge trigger (heating film attached to the surface of negative side cell 1) and center trigger (heating film placed between cells 7-8), ensuring close contact between the heat source and the cell.
2.3 Monitoring device and triggering system
(1) Parameter monitoring system
Building a multidimensional monitoring system to achieve data collection throughout the entire process of thermal runaway:
Temperature monitoring: Using 7 K-type thermocouples (T1-T7), T1 corresponds to cell 1, T7 corresponds to cell 12, and the rest are evenly distributed in the gaps between cells, fixed and sealed with high-temperature tape, with a measurement accuracy of ± 0.1 ℃;
Voltage monitoring: Using CA-4008-1U-VT data acquisition to record the total voltage of the battery module in real time, using voltage drops greater than 2V/min as the basis for thermal runaway judgment, with an accuracy of ± 0.001V;
Monitoring of heat release rate (HRR): Install a 5MW cone calorimeter 1.2m directly above the module, record HRR and total heat release rate (THR) based on ISO 24473 oxygen consumption method, with a measurement range of 0-5000kW and an accuracy of ± 5%.
(2) Thermal runaway triggering system
Thermal Runaway Test Machine
Implement the “step heating” strategy: gradually increase the power from 63W to 141W at 5-minute intervals, simulating a slow short-circuit scenario for electric vehicles. Real time monitoring of exhaust, voltage, and temperature signals during the experiment, and immediate termination of heating in case of thermal runaway.
3. Experimental results and analysis
3.1 Visual characteristics of thermal runaway process
Using a high-speed camera to capture the entire process of thermal runaway, it was found that the state of charge (SOC) and trigger position have a significant impact on flame morphology and ignition timing
The impact of SOC: The 100% SOC module ignites within 1-2 seconds after exhaust, with a high-intensity jet like flame (height>30cm) accompanied by a large amount of black smoke; A 50% SOC module takes 15-20 seconds to ignite, and the flame is in the form of a weak fireball (height<10cm) with less smoke. Under high SOC, the positive electrode material has a high lithium extraction amount, poor thermal stability, high concentration of combustible gas, and is more likely to meet ignition conditions.
The influence of triggering position: The edge triggering module only sprays flames towards one side away from the edge, presenting a “unidirectional combustion”; The central triggering module sprays simultaneously to both sides, forming a “bidirectional combustion” that directly determines the direction of thermal runaway propagation.
3.2 Temperature and voltage laws of thermal runaway propagation
3.2.1 Temperature changes and propagation time sequence
The temperature time curve reveals two major laws:
The impact of SOC on propagation rate: Under the same triggering position, the higher the SOC, the earlier the thermal runaway triggering time and the shorter the propagation interval. For example, when triggered at the edge, the first triggering of a 50% SOC module takes 1370s, and the cell propagation interval is 68s; the first triggering of a 100% SOC module takes only 952s, and the interval is shortened to 42s.
The influence of triggering position on propagation direction: The temperature of the edge triggering module rises sharply in a “linear time series”, corresponding to unidirectional propagation; The central triggering module exhibits a “symmetrical timing”, corresponding to bidirectional propagation, and the total propagation time is only 50% of the edge triggering.
3.2.2 Voltage variation and propagation characteristics
Further verification of propagation differences through voltage time curves:
Edge triggering: The voltage drops sharply in a “step like” manner, with each step corresponding to a group of battery cells experiencing thermal runaway, with intervals consistent with temperature propagation;
Central trigger: voltage drops sharply in a cliff like manner due to multiple groups of battery cells losing control simultaneously;
Common feature: All modules experience a “brief voltage rebound” during the initial stage of thermal runaway, which is caused by the redistribution of short circuit energy inside the battery cell.
3.3 Heat Release Rate (HRR) and Fire Intensity
The HRR curve and key parameters indicate that:
The influence of SOC on heat release intensity: The peak heat release rate (PHRR) and total heat release (THR) significantly increase with SOC, with the PHRR of 100% SOC module being 2-3 times that of 50% SOC, and THR reaching 5.7 times.
The influence of trigger position on heat release concentration: Under the same SOC, the PHRR of the central trigger module is higher but the difference in THR is small. At 100% SOC, the PHRR of the central trigger module is 2.5 times that of the edge trigger, but the THR of the edge trigger is slightly higher due to longer combustion time.
4. Conclusion
This study conducted a thermal runaway experiment on a 12 cell EV grade soft pack lithium battery module to investigate the influence of state of charge (SOC) and trigger position on the propagation characteristics of thermal runaway. The following key conclusions were drawn:
SOC dominant thermal runaway intensity: 100% SOC modules trigger thermal runaway early and propagate quickly (with an interval of 42 seconds), presenting intense jet flames with a peak heat release rate (PHRR) of 236~590kW; 50% SOC modules trigger later and propagate slowly (with an interval of 68 seconds), with flames in the form of weak fireballs and a PHRR of only 105~176kW. It is recommended to avoid long-term full charging during daily charging (>90% SOC).
The triggering position affects the propagation path: edge triggering triggers unidirectional propagation (total duration of 212-338 seconds), while center triggering triggers bidirectional propagation (total duration of 107-139 seconds), and the PHRR of center triggering is higher (1.7-2.5 times that of edge triggering), indicating the need to set up thermal barriers in the central area during design.
The heat release characteristics need to be comprehensively considered: the total heat release (THR) is significantly affected by SOC, and the THR of high SOC modules reaches 5.7 times that of low SOC when triggered at the edge; PHRR is closely related to the triggering location, with the central trigger being 2.5 times that of the edge. This study provides important experimental evidence for the safety design and fire prevention of EV battery modules.