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1. General heating methods for electric vehicles

One of the commonly used heating methods for electric vehicles is the heating function combined with the cooling system, such as heat pump air conditioning, heat pipes, phase change materials, etc. In this heating system, the heating process occurs in the form of the reverse refrigeration process, and refrigeration and heating are basically carried out within a system. The switching of refrigeration and heating is achieved through the physical, chemical and other properties of the controller and the system during operation, I'm not sure what the professional name of this system is, but let's give it a name for now - Integrated Thermal Management System. Another heating method, specifically designed for electric vehicles working in cold environments, is an independent heating device that is not related to refrigeration. We will temporarily refer to it as an independent heating system.

The current independent heating systems are mainly divided into two categories: resistance heaters and electric film heating.

2. Types and working principles of electric heating films

Traditional heating film, important in the construction industry, is used as a concealed heating system. The electric heating film is pre embedded under walls or floors, and in cold seasons, it heats the space after being powered on. Compared to traditional centralized heating, heating film can heat the space more evenly, bringing a more comfortable experience.

The specific application of electric heating film in electric vehicles has gradually been mentioned in recent years. No relevant standards have been found yet, and the standards that can be referred to are still those of the construction industry and household appliance industry. JGJ319-2013 Technical Specification for Application of Low Temperature Radiation Electric Heating Film System and JGT286-2010 Low Temperature Radiation Electric Heating Film.

The classification of electric heating films is based on different packaging materials, including metal electric heating films, inorganic electric heating films (including carbon fiber electric heating films, ink electric heating films, etc.), and polymer electric heating films.

Metal electric heating film is the first generation of thin film heating products, which uses film-forming techniques such as gas-phase growth to attach conductive metal materials to insulating materials. Then, a layer of insulating material is covered on the surface of the metal layer, tightly wrapping it inside to form a thin conductive film. After being electrified, the internal resistance of the metal heats up, forming an electric heating effect. Common metal electric heating materials include copper and nickel. Different materials have different electrical resistivity, working voltage, and heating power. Different metal materials are combined with different circuit designs to meet different user parameter requirements. The choice of different metal materials will also directly affect the cost of electric heating films.

Inorganic electric heating film refers to conductive materials that are inorganic, such as graphite, SiC, SiO2, conductive ink, carbon fiber, and other conductive silicates. Inorganic electric heating film, which mixes the above-mentioned inorganic conductive substances with flame retardants, film-forming agents and other auxiliary materials, and applies them together to the insulation substrate to form a conductive film. Apply voltage to both ends of the heating film, and the conductive layer is actually a semiconductor that converts electrical energy into thermal energy.

It should be noted that some inorganic conductive materials are brittle substances at room temperature, such as SiO2, which is an important component of glass. Such electric heating films need to be coated on rigid substrates and used as plate materials. And the other part shows flexibility, such as conductive ink and carbon fiber. Inorganic materials have high temperature resistance, long lifespan, and are easy to obtain, making them a type of high-performance heating equipment. However, the inability to bend and the difficulty in deformation are important reasons limiting the use of inorganic electric heating films. Flexible inorganic electric heating films can be applied more widely.

Polymer electric heating film is a process of adding conductive particles to organic materials, processing them into thin film materials, and packaging them, or applying conductive materials to insulating material substrates to make organic conductive films, and then packaging them with polymer insulation materials. The general working temperature is not as high as that of inorganic electric heating films. In the category of organic electric heating films, silicone heating films, polyimide heating films, and PET heating films are all widely used categories.

Application of 3 electric heating films in power lithium battery packs

The application of electric heating film in power lithium battery packs has been a topic of much discussion in the past two years. The high efficiency and good space utilization effect of the heating film give it great advantages in power lithium battery packs with extremely limited space.

Example of electric heating film product parameters for electric vehicles:

However, there is no precedent for the widespread application of electric heating films in road vehicles, and further investigation is needed regarding their safety and reliability. Referring to the construction industry standards and considering the influencing factors of the automotive application environment, the following aspects should be the focus of the safety requirements for electric heating films.

Insulation, leakage current, and electrical strength. The insulation performance of the heating film is the first consideration, whether it is adhered to the battery or to a metal surface with strong heat transfer ability. The general insulation performance is measured by detecting leakage current and dielectric strength.

The power input deviation, as a heating element, directly affects the accuracy of power control and the system's ability to control the ambient temperature. The thin film heating product standard in the home appliance industry requires an input power deviation between -10% and 5%.

Dry burning temperature resistance refers to the ability of a heating film to withstand electricity. After reaching the highest stable heat resistance temperature for a period of time, the appearance and structure of the product will not be damaged, the insulation performance will not change significantly, and the cold resistance value will change within a certain range.

High temperature resistance: After the ambient temperature of the heating film reaches a certain high temperature and is left for a certain length of time, the basic performance of the heating film does not change. When the heating film can be in normal working condition or when it is in a power-off and idle state.

Low temperature resistance, similar to high temperature resistance, means a range of low temperatures.

Cold and hot alternation, also known as temperature shock, refers to the critical performance changes of heating films made of polymer materials or inorganic substances that undergo repeated and rapid cycles of high and low temperatures, which should be within a certain range.

Impact resistance, an important component of road vehicles, must possess impact resistance. Furthermore, considering the heating film, it is best to examine the impact resistance of the installation state during the working period.

Vibration resistance, similar to impact resistance.

Flame retardancy is also a basic requirement for vehicle materials, especially for electric vehicles, which generally require V0 level.

4. Important parameters of electric heating film

In the past, among the electric heating film companies facing industries such as aerospace, there have been pioneers who have listed the category of electric heating films for power lithium batteries in their product categories. The application of electric heating film in power lithium battery packs is not yet mature. Choosing the right electric heating film and how to apply it to the battery pack may require joint participation in structural and electrical design. Choosing an electric heating film is not only about considering the reliability factors mentioned earlier, but also about the actual usability of the product.

What are the most important parameters for heating film applications?

Generally speaking, the type of electric heating film, installation method, operating voltage range, power density, and operating temperature range should be the key factors to consider.

The type of electric heating film is determined after considering factors such as the compatibility between the life cycle of the battery pack and the life cycle of the electric heating film, cost-effectiveness, reliability, electrical and structural requirements.

Installation method, combined with the specific situation of the battery pack, especially the spatial layout, maintenance plan, type of cooling system, etc., determine the installation position and method of the electric heating film.

Working temperature range, identifying the target area of the product, and determining heating requirements;

Power density is the heating capacity of a heating film per unit area. Based on the previous working temperature range and combined with the actual thermal load of the battery pack, calculate the maximum power density required under extreme temperature conditions.

The working voltage is a designed electric heating film, and its heating power is regulated by voltage, with a wide adjustable range. We need to first select an ideal heating duration, and then select the target power and target voltage based on the thermal load of the battery pack. If you want the heating power to be adjustable, you need to configure a voltage regulating power supply.

5. Application forms in electric vehicles

Describe an actual case of using an electric heating film to heat an electric vehicle. Author Pan Chengjiu, in his paper "Research on Insulation and Heating of Electric Vehicle Battery Pack", introduced the method of using heating film on electric vehicles for low-temperature heating of power lithium batteries.

Problem description: A series connected pure electric drive vehicle has two problems with lithium-ion power lithium-ion batteries in low temperature environments (such as -20 ℃). One is that at low temperatures, the battery's discharge capacity is extremely poor, and even when fully charged, the vehicle's driving ability will be greatly reduced; Another, low-temperature charging, has been considered as the fastest way to reduce the lifespan of power lithium batteries. At low temperatures, the conductivity of charged ions inside the battery is poor, and the transfer and embedding ability of charges within the negative electrode material structure is also poor. If charged at low temperature with normal current, a large amount of lithium ions accumulate on the surface of the negative electrode and cannot be embedded. After obtaining electrons through various channels, they deposit on the surface of the electrode, forming a lithium elemental accumulation; Lithium metal is very prone to the phenomenon of uneven crystal growth. When the dendrites grow to a sufficient scale, they may puncture the diaphragm, causing direct connection between the positive and negative electrodes to form an internal short circuit. In short, starting an electric vehicle at low temperatures can have serious consequences, so before starting, heat the power lithium battery first.

The case uses a heating film heating method, and the polyimide heating film is pasted on a square battery cell, as shown in the following figure. The target heating time is 1 hour. Considering that the battery pack will dissipate some heat during the charging process, the heating power of a single chip is calculated to be 14W; The voltage of the electric heating film is 1.8V. The battery pack is designed for 2P90S, with a total of 180 cells and 180 heating elements. The heating plates are connected in series.

Select the lowest temperature point of the entire system. After testing, the article believes that during the heating process, the lowest point in the figure appears on the positive pole of the individual battery cell. Therefore, a temperature detection point is set here.

Used in conjunction with the heating device, it is to design insulation methods for the battery pack. In this paper, silica gas gel is selected as the thermal insulation material, and the material thickness is 10mm.

Place the battery pack in an environment of minus 20 degrees Celsius and let it stand for 16 hours to achieve a balanced temperature inside and outside. After 54 minutes of heating, the maximum temperature of the battery pack reached 11 ℃, and the maximum temperature difference detected by the battery management system was 2 ℃, reaching a normal charging state. After undergoing such treatment, the battery pack can achieve a single driving range of over 90% at room temperature, which is much higher than the mileage of driving directly without heating (usually around 60% of room temperature driving range).

The small space occupied by the heating film is indeed a rare advantage, but in the use of power lithium battery packs, sufficient verification may be required in terms of reliability and safety. There are many problems with the use of lithium-ion batteries at low temperatures, such as low discharge capacity, decreased discharge voltage, inability to charge, poor cycle rate performance, and lithium evolution issues. Research has found that the fundamental reason that restricts the low-temperature performance of lithium-ion batteries can be attributed to the low temperature hindering the effective transfer of Li+and electrons during the charging and discharging process of lithium-ion batteries. Both the charge transfer process at the electrode/electrolyte interface and the transfer process of Li+in the SEI film, electrolyte, and electrode are affected by low temperature, which increases battery polarization and leads to poor battery performance. There are several specific factors []:

(1) At low temperatures, the viscosity of the electrolyte increases and even partially solidifies, resulting in low ion conductivity;

(2) The compatibility between electrolyte, electrode, and separator deteriorates at low temperatures;

(3) At low temperatures, the negative electrode experiences severe lithium precipitation, and the precipitated metallic lithium reacts with the electrolyte, resulting in the precipitation of its products and an increase in the thickness of the solid electrolyte interface (SEI);

(4) At low temperatures, the diffusion coefficient of lithium ions within the active material decreases, and the charge transfer impedance (Rct) significantly increases.

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