HV Batteries: Encapsulation and Thermal Management
Thermally conductive pastes protect against overheating and vibrations
Thermal influences have an enormous impact on the service life, capacity and above all the operational safety of Li-ion traction batteries. In order to prevent possible defects and damage caused by overheating, large quantities of heat-conducting potting materials are used.
Electric mobility is gaining momentum: Vehicles with alternative drive systems are regarded as a central component in the fight against noise pollution and steadily rising emissions. Several European countries are already planning to part with the combustion engine. In China – currently the world's largest market for electric vehicles – binding quota regulations for electric and hybrid cars are scheduled for application as early as 2019.
The heart of these vehicles is the built-in high-voltage (HV) battery. It serves as an electrochemical energy storage device that supplies the electric motor with electricity. As it currently has the best performance of all available battery technologies, lithium-ion technology has emerged as the leading technology. In addition to their high specific power and energy, Li-ion traction batteries are also characterized by their high efficiency when charging and discharging as well as low self-discharge. The disadvantages are their high weight, unsatisfactory loading capacity and comparatively high costs.
Basically, a HV battery consists of hundreds to thousands of individual cells of various shapes and performance classes. These cells are joined into modules, which together form the final traction battery. The design of the battery cells varies depending on the manufacturer or vehicle builder. Key decision criteria here are factors such as energy density, cooling properties or heat dissipation, production costs, weight, scalability or mechanical stability. Cylindrical and prismatic shapes are currently in use, as well as pouch cells, which are also known as "coffee-bag" cells because of their similarity to shrink-wrapped coffee powder.
HV batteries: Thermal management is of key importance
Connected to the HV battery are various mechanical and electronic components that ensure efficient, long-term and above all safe operation. One of the most important components is the housing, which protects the modules of the traction battery from damaging environmental influences such as dust, dirt or moisture.
Thermal influences also play a central role here: for normal operation of an electric vehicle, outside temperatures of 10 to 25°C and operating temperatures of approx. 20 to 40°C are optimal. If the temperatures exceed or fall below this ideal range it can have negative effects on the service life, capacity and especially the operational safety of the HV battery. Higher temperatures, for example, cause faster degradation of the materials used in the battery cells and thus faster aging. Excessive thermal stress can also cause internal short-circuits and, as a result, serious to irreparable damage to the traction battery and vehicle.
In order to prevent this, each electric vehicle has a battery management system (BMS), which, in addition to controlling and monitoring the current state of charge (SoC) of the HV battery, is also responsible for thermal management. In order to ensure safe operation and optimum performance and service life, the traction battery is cooled at high temperatures by means of a built-in cooling system.
Reliable heat transfer thanks to heat-conducting pastes
Moreover, large quantities of thermally conductive paste or gap filler are used. Approximately 5 to 10 liters of these highly viscous materials, which contain heat-conducting fillers, are dispensed per vehicle in between the modules and the battery housing - and the trend is rising. Supporting cooling by means of water or coolant, they ensure a reliable heat transfer between the HV battery and the housing, thus helping to prevent potential defects caused by overheating. Due to their soft, flexible consistency, they also protect the partly sensitive components of the traction battery from shocks and vibrations during driving.
Due to the large volumes that have to be dispensed within the required cycle time and due to their high viscosity and abrasive fillers, the application of heat-conducting media places the highest demands on the applied dispensing technology. Scheugenpflug offers a new system solution, consisting of a dispenser and material feeding system, especially for the encapsulation of HV batteries. This permits high flow rates and thus increased part throughput - even with highly abrasive potting materials.
Up to 80 ml/sec with highly paste-like media
Scheugenpflug has developed the new volumetric high performance dispenser Dos HP based on the proven piston dispenser. With this dispenser, dispensing rates of up to 80 ml/s (max. volume per shot: 316 ml) can be achieved - with consistently high dispensing accuracy. To ensure a sufficiently wide travel range for the media application, the Dos HP is mounted to a robot arm.
The PailFeed200 Abrasive barrel follower plate system for 200 liter drums is used to feed the potting material to the dispenser quickly and reliably. The output of the integrated double piston pump is 480 cm3/stroke, the possible delivery pressure reaches up to 65 bar.