The power battery system is typically composed of a battery module, a Battery Management System (BMS), a thermal management system, and some electrical and mechanical components. Several factors influence the large-scale adoption of new energy vehicles, including battery system cost, driving range, and safety. As new energy vehicle technology advances, safety has become a major concern. Power lithium-ion batteries are prone to thermal runaway due to exothermic reactions under conditions like overcharging, puncture, or collision, which can lead to smoke, fire, or even explosions. Additionally, the performance of power batteries—such as energy density and lifespan—is affected by temperature fluctuations, highlighting the importance of effective thermal management.
First, the significance of thermal management
Under different driving conditions, each individual battery cell generates heat due to internal resistance when delivering electric energy, causing its temperature to rise. If the temperature exceeds the normal operating range, it can negatively impact the battery's performance and lifespan. The power battery system on an electric vehicle consists of multiple cells, and during operation, a significant amount of heat accumulates within the compact battery box. If this heat is not dissipated quickly, high temperatures can reduce battery life or even trigger thermal runaway, leading to fires or explosions.
Currently, domestic thermal management research focuses heavily on heat dissipation, particularly at the system and module levels, such as liquid cooling systems. However, there is less emphasis on insulation measures at the cell level. It is evident from the design of power battery systems that the structure of the battery cells and modules must be considered in the thermal management design. Therefore, when designing the battery module layout, if the cells are densely packed without proper heat dissipation or insulation, the temperature inside the battery pack can rise rapidly, posing serious safety risks.
Thus, it is essential to study battery thermal management technology to enhance both heating and cooling capabilities, ensuring the battery operates within an optimal temperature range and maintains a balanced temperature distribution inside the battery box. Research should gradually expand from understanding the thermal runaway mechanisms at the cell level to analyzing how thermal runaway propagates through the entire battery system.
Second, the difference between insulation and no insulation
Previous studies have shown that placing a thermal insulation layer between battery cells can prevent heat transfer from a failing cell to adjacent ones. At the same time, the insulation layer is not completely sealed, allowing for convection channels between cells, which helps dissipate heat generated during thermal runaway across the entire battery pack and prevents localized overheating. In the "Integrated Study on Thermal Protection and Heat Dissipation of Vehicle Power Batteries," four schemes were analyzed for their thermal performance during thermal runaway: Scheme 1 represents no insulation or heat dissipation; Scheme 2 involves placing an insulating board between cells; Scheme 3 uses heat pipes between cells; and Scheme 4 combines both an insulating board and heat pipes.
The thermal dissipation and insulation performance of the battery pack under normal and thermal runaway conditions were analyzed, verifying the effectiveness of the integrated system and investigating how the thickness of the insulation board affects thermal runaway propagation:
(1) Comparing the four schemes, Scheme 2 shows excellent thermal resistance, effectively delaying thermal runaway propagation, but with poor heat dissipation. Insulation boards alone cannot meet the thermal management requirements of the battery pack. Scheme 3 has good heat dissipation performance, but as the discharge rate increases, the maximum temperature difference rises sharply. After thermal runaway, the thermal resistance of Scheme 3 is much lower than that of Schemes 2 and 4. Scheme 4 not only significantly improves heat dissipation and temperature uniformity across the battery pack but also provides high thermal insulation, effectively preventing thermal runaway propagation.
(2) Increasing the thickness of the insulation board and enhancing the heat dissipation capability of the battery pack can effectively block thermal runaway. When the insulation board thickness increases from 1mm to 2mm, thermal runaway can be prevented before the heat pipe fails, provided the heat pipe functions normally.
(3) Combining appropriate insulation measures with cooling methods can not only improve the stability of the battery pack’s operating temperature range but also effectively prevent thermal runaway.
A classic example is General Motors' Volt, which uses a liquid cooling system for battery thermal management. A metal heat sink (1 mm thick) is placed between the cells, with capillary structures allowing coolant to flow and remove heat. The insulation method involves using foam between the cells.
Third, the application of foam
Battery systems and modules are generally designed based on the shape of the battery cells, which are mainly categorized into cylindrical, square, and soft-pack types. Soft-pack batteries offer higher energy density, making them more widely used under energy subsidy policies. Their advantages include minimal external structural interference, excellent cell performance, and lightweight packaging materials. However, they also have drawbacks, such as increased difficulty in sealing large-capacity batteries and relatively lower reliability. Additionally, the aluminum-plastic composite film used has low mechanical strength, and its limited lifespan restricts the battery's overall service life.
Therefore, it is important to consider the bulging of soft-pack batteries during charging and discharging. If the soft packs rub against each other for a long time, the aluminum-plastic film may be damaged, leading to battery failure or even uncontrolled situations. Foam application between soft-pack batteries is crucial, offering four key benefits:
1. Foam has low hardness and high resilience, helping absorb bulging stress and act as a buffer.
2. During uncontrolled heat release, foam can function as a thermal insulator, slowing heat diffusion and delaying accidents.
3. When a battery catches fire, flame-retardant foam can delay fire spread, increasing escape time.
4. Foam has excellent resilience and a wide compression ratio, making it suitable for positioning.
There are various types of foam, such as PU, CR, EVA, and PE. In practical battery applications, it was found that only foams capable of maintaining sufficient elastic recovery under long-term compression, like PU, are suitable for use between soft-pack batteries. Other foams, such as CR, may lose their recovery ability after prolonged compression, leading to structural instability in the module.
In addition, during the disassembly analysis of the VOLT module, the applied foam did not self-extinguish when exposed to fire, meaning it did not meet the national standard's V0 requirement. This is an interesting point. Perhaps it has excellent insulation properties that complement the liquid cooling system, helping avoid thermal runaway. Alternatively, if thermal runaway occurs, the foam may not play a significant role in flame retardancy. In any case, it's better to focus on improving insulation rather than relying solely on flame retardancy.
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