Turning Two into One
Innovative Pressure Compensation for the Batteries of Electric Vehicles
Christian Kleinke
The transition towards e-mobility brings new challenges to vehicle design in respect to pressure equalization. Hermetically sealed housing of lithium-ion batteries for vehicles must be equipped with special pressure equalization devices. Traction batteries for electric vehicles, which run on voltages of several hundred volts, must be well protected. Electrical components are packed into a thin-walled yet stable metal housing so that water and dirt/dust particles cannot damage them. But the housing cannot be fully hermetically sealed. An electric car should be able to cross the Rocky Mountains and be used in mid-summer during hot temperatures, and the housing would noticeably deform in those situations due to fluctuations in the external air pressure or the temperature. That is why housings of this kind are equipped with at least one pressure compensation valve – mostly in the form of microporous film. A second valve is a safety measure: It only opens if damage to one or several battery cells results from a malfunction, and the buildup of gases has to be released. Freudenberg Sealing Technologies has developed an entire family of smart pressure compensation elements called “DIAvent,” which combines both functions into a single component, making electric vehicles more reliable and economical. The solutions range from combined pressure equalization and emergency degassing during a worst-case scenario – the thermal runaway of a cell – to the two-way ventilation of a greasy environment
The combination of pressure compensation during normal operation and pressure reduction with an emergency gas release is technically demanding. The reason: Under the normal pressure-compensation conditions, just a few litres of air are exchanged bi-directionally per minute. But in an emergency, it is imperative to drain off all the gas emitted by a damaged cell in just a few seconds. That keeps the housing from bursting in response to the strong internal pressure. Pressure compensation elements that self-destruct on a controlled basis are used here. The trend toward battery systems with ever-higher energy densities and accordingly low dead volume in the battery housing also makes the merger of the two functions more difficult. That trend is increasing both the amount of gas that must be drained off quickly and the speed of the pressure compensation needed during normal operation.
The DIAvent resolves the apparent contradiction between the need for high air permeability related to surface area and, simultaneously, a remarkably high degree of watertightness. The key is to replace the usual microporous film with a combination of two nonwovens with differing characteristics. During a slight pressure difference, the water-repellent nonwoven element on the outer side enables an air exchange of about 8 litres per minute and is water-tight up to a water column of 100 mm. If the water pressure also rises, the outer layer is pressed onto an internal layer that is completely impermeable to media, preventing the water from penetrating the housing. In an emergency, degassing is enabled by an umbrella valve arranged in a ring around the nonwoven. It reliably opens as soon as the pressure in the housing exceeds more than 50 millibar atmospheric pressure and can then drain off 40 litres of gas per second. One major advantage of this solution is that the umbrella valve then closes. In practical terms, this facilitates the safe removal of a damaged battery. Since it opens and closes completely reversibly, it can also support pressure compensation in normal operation, for instance, when batteries with a high energy density are exposed to rapid temperature fluctuations.
How DIAvent Highflow is Changing the Industry – Four times faster in an emergency
In early 2020, first series production of “DIAvent” was launched, to offer a ventilation valve that combines regular housing venting and rapid emergency degassing in a single component. One year later, Freudenberg Sealing Technology presented a new generation of this valve, which makes the emergency degassing four times faster. Even with increased pressure of 300 millibars inside the housing, “DIAvent Highflow” enables a flow rate of 92 litres per second. The basic design of the valve stays the same: A centrally positioned; water-repellent nonwoven element ensures effective air exchange during normal operation. If water hits the valve at high pressure, the outer layer is temporarily pressed onto a completely media-tight interior layer, preventing any water from penetrating the housing.
The DIAvent Highflow pressure equalization element has been developed specifically for battery systems with higher energy densities. The higher energy density also results in larger gas volumes that need to be dissipated quickly in case of an emergency. DIAvent Highflow is primarily designed for use in lithium-ion batteries in larger vehicles with very high energy densities and has four times the mass flow rate of the standard DIAvent in case of an emergency. To be able to achieve this in an emergency, the opening created by the sealing lip has not only been enlarged, but additional exhaust ducts have also been integrated.
Emergency degassing is enabled by a ring-shaped umbrella valve surrounding the nonwoven membrane. It automatically opens as soon as the pressure inside the housing exceeds the atmospheric air pressure by more than 40 millibar. After the pressure is equalized, the reversible umbrella membrane closes again and restores the water tightness. The significantly enhanced performance of the valve can be attributed to the optimized gas flow inside the component. Engineers at Freudenberg Sealing Technologies have calculated the flow-through of the valve in detail for this purpose. Minor geometric changes, such as additional openings on the sides of the umbrella, prevent air from accumulating under the screen and enable a significantly increased flow rate.
The Daily Challenge: Pressure Fluctuations During Operation
Air pressure changes within the battery housing of a vehicle occur during normal driving operation. For example, when the vehicle drives over a mountain pass or the temperature in the battery housing changes in the period between a cold start and high speed on the highway. These pressure fluctuations are compensated for by an air exchange of several litres of air per minute in both directions. In the past, porous PTFE foils were used as a pressure equalization element for this purpose. The disadvantages of these solutions are the high sensitivity for mechanical influences as well as contamination by particles and liquid media, which ultimately reduce the air permeability. Here, the DIAvent family offers a practical alternative.
The Challenge in Case of an Emergency: Equalizing Pressure in Seconds
Foreign bodies in the cell, internal and external short circuits, defective overcharging or the destruction of the battery cell in an accident result in the release of considerable amounts of energy and gas volumes. The resulting gas then flows directly into the battery housing.
To reduce the resulting pressure as quickly as possible, battery housings usually have built-in rupture discs as predetermined breaking points to release the gas quickly in a controlled manner. Although these prevent the housing from being damaged, the destroyed disc leaves an opening to the outside after emergency degassing. This has significant disadvantages. Ambient air can be drawn into the battery after pressure equalization to create a combustible gas-air mixture that can cause further exothermic reactions inside the hot battery housing.
DIAvent – the Safe and Reliable Solution
The DIAvent pressure equalization elements are not only extremely robust against mechanical stress, but also immediately closes the diaphragm that is opened in the event of emergency degassing. One the one hand, environmental pollution is prevented. On the other hand, it increases safety for anyone in the immediate vicinity of the vehicle – especially during the transportation of the damaged and fire dormant battery system.
Summary:
- DIAvent Highflow builds on Freudenberg’s patented DIAvent pressure compensation technology and design and provides precise pressure equalization, faster emergency degassing, long-term reliability, and cost and complexity savings over the standard, two-component solution. It is specifically engineered to manage the pressure and safety requirements of larger batteries and is unique in the industry.
- DIAvent Highflow requires 50 to 70 percent less space than competitive pressure regulation and degassing solutions for the same airflow. Its compact size and lower part content increase the ease and efficiency of installing the component in the battery system. The component design can also be adapted to fit any battery space required.
- DIAvent Highflow offers customers weight saving through its innovative use of a UL94-V0-compliant plastic, rather than aluminum, housing. It is less costly than two separate components, requires less installation effort, addresses supply chain complexity, and supports the industry’s use of higher density batteries to address range, power, and safety requirements in BEVs. Several major automotive OEMs committed to DIAvent Highflow at SOP Dec. 2022
Christian Kleinke Manager Research & Development Diaphragms at FST.
