Application of electric heating film in power battery pack under low temperature environment

As the core energy storage unit of electric vehicles, the performance and life of power battery packs have an important impact on the driving range, charging speed and safety of electric vehicles. The reduction of battery performance and shortened life in low temperature environments have become a major problem restricting the development of electric vehicles. Taking new energy electric vehicles as an example, the driving range and charging and discharging performance of pure electric vehicles such as Tesla Models, Nissan Leaf, Chevrolet Volt, BAIC New Energy EV series, and JAC New Energy IEV series are all severely challenged by low temperature environments.

锂离子动力电池的特性受环境温度的影响比较显著，尤其是在低温环境中，其可用能量和功率衰减比较严重，且长期低温环境使用会加速动力电池的老化，缩短使用寿命。在低温条件下，随着放电倍率的增加，动力电池的功率特性和容量特性均存在衰减的问题。常用的电动汽车锂离子动力电池在-10℃时，容量和工作电压会明显降低，在-20℃时性能更加恶化，表现为其可用放电容量骤降，仅能保持常温时比容量的百分之三十左右，影响的不仅仅是续航里程，甚至车辆的动力性，能量回收等。

In addition, it is also difficult to charge lithium-ion batteries in low-temperature environments. The smaller diffusion coefficient will hinder the diffusion process of lithium ions in the negative electrode graphite, which will easily produce "lithium deposition" on the surface of the negative electrode particles, causing damage to the battery. The growth of "lithium dendrites" will pierce the battery separator and cause an internal short circuit in the battery, which will not only damage the battery, but also induce thermal runaway of the battery, greatly reducing its safety. The safety issue of low-temperature charging of electric vehicle battery systems has greatly restricted the promotion of electric vehicles in cold areas.

Low temperature characteristics of electric vehicle power battery pack

Low temperature preheating technology for electric vehicle power battery pack

In order to overcome the problems caused by low temperature environment, research and development of low temperature preheating system has become a key research direction to improve the working performance and reliability of battery packs under low temperature conditions. Battery preheating technology is an important part of battery thermal management. By preheating the battery pack, the battery temperature can be quickly raised to the optimal operating temperature, promoting the electrochemical reaction rate and ion migration rate, thereby improving the performance and reliability of the battery. The following are several common low temperature preheating technologies:

Internal heating: Internal heating is to install the heating element directly inside the battery pack, and increase the temperature of the battery pack through the heating effect of the heating element. Internal heating is usually achieved using thermistors, heaters or PTC thermistors inside the battery pack. PTC is widely used due to its safety, high heat conversion efficiency, rapid heating, no open flames, and automatic constant temperature. Its low cost is a favorable factor for the current high-priced power batteries, but the PTC heating element is large in size and will occupy a large space inside the battery system.

External heating: Insulated flexible electric heating film is another type of heater. It can be bent according to any shape of the workpiece to ensure close contact with the workpiece. It can be easily pasted on the surface of the battery PACK. In applications that require light weight, small space, exposure to vacuum, chemical products or oils, PI heating film is an ideal choice. External heating is one of the most commonly used low-temperature preheating technologies. By transferring heat to the battery pack under the action of an external heating source of the battery pack, the temperature of the battery pack is increased. The thin and light PI heating film can only heat the position that needs to be heated, and the part that does not need to be heated can avoid heating. It can provide fast and efficient heat transfer, and the heating surface is uniform with minimal temperature difference. It can meet different heating power requirements and heating temperature requirements in different areas, realize temperature distribution on the battery heating surface, reduce battery aging, shorten service life, and improve battery safety.

Application of PI heating film in battery modules

Silicone rubber heating film is mainly composed of nickel-ming alloy heating wire and silicone rubber high-temperature insulating cloth. It is a soft electric heating film element composed of high-temperature resistant, high thermal conductivity, good insulation performance, good strength silicone, high-temperature resistant fiber-reinforced material and metal heating wire circuit. It heats up quickly, has uniform temperature, high thermal efficiency, high strength, is easy to use, has high safety, long life, and is not easy to age. Due to its thickness and high temperature resistance, silicone rubber heating film is more suitable for use in large areas and exposed environments, such as outdoor use. The waterproof level of silicone rubber electric heating sheet can reach IPX6, and the fire protection level can reach UL94V0. In some special high-temperature use places, it is more reliable to choose silicone rubber material.

Application of silicone rubber heating sheet between battery cells (side view)

In addition to the above-mentioned method of using thermistors and electric heating films to preheat the power battery pack, the mainstream preheating methods for electric vehicle power battery packs on the market currently include liquid cooling temperature control and phase change material heating. However, these new technologies have not been verified on a large scale at this stage. In the future, the comprehensive application of different low-temperature heating technologies can further improve the low-temperature heating effect and uniformity of the battery. Tightly integrating components such as heating devices, sensors, and control systems will achieve more efficient energy utilization and overall performance improvement.