Efficient thermal management is needed for drive batteries, electric motors and power electronics in electric vehicles. Due to their wide range of consistencies and their robustness, silicone-based thermal interface materials prove indispensable in this field.

Most experts agree: tomorrow’s cars will be electric. By 2025, roughly 25 percent of world light vehicle production will have an electric engine with a battery, as found in hybrids, plugin hybrids, battery electric or fuel cell vehicles.

The automotive industry has recognized the signs of the times and is now working flat out on the development of electromobility. One challenge is how to effectively dissipate the heat generated in the various components while the battery is being charged and when the vehicle is on the road.

Such thermal management is especially critical for the battery serving as the power source. Lithium-ion batteries only deliver their best performance at temperatures between 20 and 35 °C. Consequently, to ensure acceptable performance and life span, they need to be prevented from overheating. Heat is also generated by the electric motor and the power electronics. Again, to avoid heat-related damage or failure, this thermal energy also needs to be dissipated quickly and effectively.

Thermal interface materials (TIMs) play a key role here. They fill the gap between the assembly which needs to be temperature-controlled and the heat exchanger or heat sink, and thus lower the thermal transfer resistance. With this, they enhance thermal coupling between the components. Thermal interface materials are therefore becoming increasingly attractive to car makers as they develop electric vehicles for mass production.

The heat-related issues of the battery are the key factors that determine its performance, safety, life and cost. First of all, the temperature level of lithium-ion batteries directly affects their energy and power performance in use. When the temperature is low, the available capacity of the battery will rapidly decay. Charging the battery at a too low temperature (such as below 0°C) may cause an instantaneous voltage overcharge phenomenon, which will cause internal lithium deposition and cause a short circuit. Secondly, the heat-related issues of lithium-ion batteries directly affect the safety of the batteries. Defects in the manufacturing process or improper operation during use may cause partial overheating of the battery, which will cause a chain exothermic reaction, and eventually cause serious thermal runaway events such as smoke, fire or even explosion, which threatens the lives of vehicle drivers and passengers Safety.

In addition, the operating or storage temperature of lithium-ion batteries affects their service life. The suitable temperature of the battery is about 10~30°C, too high or too low temperature will cause the battery life to decay quickly. The large-scale power battery makes the ratio of its surface area to volume relatively reduced, the internal heat of the battery is not easy to dissipate, and it is more likely to have problems such as uneven internal temperature and excessive local temperature rise, which further accelerates battery degradation, shortens battery life, and increases users’total cost.

The importance of thermal management system

The battery thermal management system is one of the key technologies to deal with battery heat-related issues and ensure the performance, safety and life of power batteries. The main functions of the thermal management system include:

• Effective heat dissipation when the battery temperature is high, to prevent thermal runaway accidents;
• Warm-up when the battery temperature is low, increase the battery temperature, and ensure the charging and discharging performance at low temperatures And safety;
• Reduce the temperature difference in the battery pack, inhibit the formation of local hot zones, prevent the battery from decaying too quickly at high temperature locations, and reduce the overall life of the battery pack.

battery pack under selected fast-charging conditions.

The thermal management system of tesla poadster battery

Tesla Motors' Roadster pure electric vehicle uses a liquid-cooled battery thermal management system. The vehicle-mounted battery pack is composed of 6831 18650-type lithium-ion batteries, of which 69 are connected in parallel to form a set (brick), 9 sets are connected in series to form a sheet, and finally 11 layers are stacked in series. The cooling fluid of the battery thermal management system is a mixture of 50% water and 50% ethylene glycol.

Figure 1. (a) is the thermal management system inside the sheet. The cooling pipe is arranged in a zigzag between the batteries, and the coolant flows inside the pipe to take away the heat generated by the battery. Figure 1. (b) is a schematic diagram of the structure of the cooling pipe. The inside of the cooling pipe is divided into four channels, as shown in Figure 1.(c). In order to prevent the gradual increase in the temperature of the coolant during the flow of the coolant, the heat management system adopts a two-way flow field design, and the two ends of the cooling pipe are both the liquid inlet and the liquid outlet, as shown in the figure. As shown in 1(d). Fill between batteries and between batteries and pipes with materials with electrical insulation but good thermal conductivity . The functions are: 1) Change the contact form between the battery and the heat dissipation pipe from line contact to surface contact; 2) Yes It is beneficial to improve the temperature uniformity between the single cells; 3) It is beneficial to increase the overall heat capacity of the battery pack, thereby reducing the overall average temperature.

Through the above thermal management system, the temperature difference of the individual cells in the Roadster battery pack is controlled within ±2°C. A report in June 2013 showed that after driving 100,000 miles, the capacity of the Roadster battery pack can still be maintained at 80~85% of the initial capacity, and the capacity degradation is only obviously related to the mileage, but is related to the ambient temperature. The relationship between vehicle age is not obvious. The achievement of the above results depends on the strong support of the battery thermal management system.

Heat dissipation principle of Tesla battery thermal management system

Heat dissipation principle: When the battery is working, a lot of heat is generated. The heat is transferred to the water-cooled tube by the thermally conductive silicone pad, and then the water-cooled tube is transferred to the coolant. The liquid in the water-cooled tube flows in the battery pack to remove the heat.

Heat dissipation principle of battery thermal management system of a well-known domestic automobile manufacturer

The principle of Thermal dissipation: the use of a fan to actively dissipate heat, and the fan supplies the wind, and the wind blows toward the battery flow channel to heat the inside of the battery pack.

Heat dissipation principle: Because the temperature difference inside the battery pack is not controlled within 5°C, a thermally conductive silicone sheet is attached to the upper and lower parts of the battery pack, and the silicone sheet guides the temperature to the outer aluminum shell. The temperature difference of the entire battery module is controlled within 5°C. It meets the requirements of the battery pack design, which makes the battery pack longer life and more stable performance during driving.

Heat dissipation principle: The battery pack adopts passive heat dissipation. A thermally conductive silicone sheet is pasted between the battery pack and the aluminum heat sink. The silicone sheet transfers the temperature to the aluminum plate, and the aluminum plate exchanges heat with the air.

AOK technologies for battery packs

By applying advanced thermal interface material technologies, car manufacturers will be able to reduce investment and operating cost, improve battery life and long-term performance, protect critical electronics and enable lightweighting, thereby meeting consumer demands. Indeed, thermal interface material solutions help overcome key challenges through bonding, sealing, coating and thermal management at the individual cell level all the way through the battery pack, continuing to the power conversion systems and control units. For battery system modules in specific, thermal interface material play an integral role in the construction and ensuring durability. This is because they provide significantly higher resistance to fatigue than riveted or welded assemblies while at the same time enabling the use of down-gauged and lightweight substrates.

AOK offers a variety of thermal interface material that provide greater and more consistent connectivity throughout the battery ecosystem while protecting components from the harsh conditions in which EV batteries operate, including noise and vibration, battery technology and manufacturing is a major cost driver, optimizing the production processes with high performance materials can drive down overall electric vehicle prices.

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