Batteries for electric vehicles (EVs) need protection from harsh external conditions. Generally located on the underside of the vehicle, EV battery enclosures protect the cells from exposure to water, dust, debris, and other elements. Like other enclosures, EV battery enclosures also undergo temperature and pressure changes that can lead to problems without proper venting. But EV batteries present unique venting challenges including potentially dangerous thermal runaway conditions. Considering these challenges, integration of design, manufacturing, installation, and testing are crucial to optimal EV battery venting performance.
Numerous components in an EV require venting. Lighting, horns, electronics, powertrain, and other components all experience temperature changes resulting from weather conditions, moisture, or heat from electronic circuits. These temperature fluctuations produce pressure changes inside the components. Changes in altitude can also trigger pressure changes. A trek up Pike’s Peak, for example, could result in pressure changes of up to 140 millibars (2 psi).
Expanded polytetrafluoroethylene (ePTFE) membranes have proven effective in venting EV enclosures and providing pressure equalization. This material allows gases to flow in and out of the enclosure but helps prevent liquid and particle contaminants from entering the enclosure.
Challenges Unique to EV Packs
Along with exposure to the various elements experienced by other automotive components, battery packs present additional challenges due to their location, size, and chemical makeup. Packs located on the undersides of vehicles can be exposed to water fording or submersion as a vehicle traverses standing water. Roadway water may include salt and other corrosive chemicals. Car washes can expose vehicles to high-pressure sprays of varying temperature. Dust, debris, and vibration can also impact the battery pack.
The size of EV battery packs also presents challenges. Weighing up to 2,000 pounds (907 kg), the packs contain hundreds — sometimes thousands — of cells, with large air volumes that can exert forces on the enclosures as pressures increase. This creates significant venting needs to account for differential pressure due to altitude or temperature change.
Perhaps the biggest challenge with EV battery packs is the potential for thermal runaway. If lithium-ion battery cells are damaged by puncturing, overcharging, manufacturing defect, or other causes, they can release gas and heat. This can trigger other cells to decompose, leading to a thermal runaway condition where rapidly increasing temperatures and pressures released by cells exceed the venting capability of ePTFE membrane vent. At this point, the vent is unable to allow gases to escape fast enough and additional pressure relief is needed to avoid rupturing the battery pack enclosure, which is typically made of lightweight materials to conserve weight.
Dual-Stage Venting Mitigates Thermal Runaway
Dual-stage venting provides an effective solution to the unique challenges of EV battery packs. The first stage — passive venting — handles gradual changes in temperature and pressure via the ePTFE membrane. The second stage — active venting — allows the vent to rupture and gases to quickly escape in a thermal runaway situation.
As an example, an enclosure that would rupture at a pressure of 1,500 millibars (21.7 psi) could be equipped with an active venting system that opens at 300 millibars (4.3 psi), well below the rupture pressure. Without active venting, the enclosure would rupture quickly.
With dual-stage venting, a single assembly can help accomplish both passive and active venting functions and assist in controlling the pressure and location at which gas is released. Typically located on the top or side of the enclosure, the vents can be situated to allow for controlled gas release, which helps mitigate risk by allowing the designer to determine the location of the release.
To properly incorporate dual-stage venting, the solution should integrate design, manufacturing, and testing considerations; for example, dimensions of the vent need to be accounted for in the battery pack and vehicle design. While dual-stage vents can be manufactured with a diameter less than 50 mm (1.9") and a height of approximately 10 mm (.39"), space is often limited in areas of battery packs. Serviceability should also be considered to allow for replacement of the vent.
Ease of installation and efficiency in the manufacturing process are key to the overall battery pack solution, particularly if the packs will be manufactured on a high-volume basis. Different fittings are available, depending on enclosure materials. A bayonet quarter-turn fitting is most common, though threaded fittings are feasible with sufficient thickness of enclosure material. Drainage around the vent should also be provided to allow for runoff of water reaching the vent. Weight and cost also need to be considered in the solution. While vents can weigh less than 15 g (.53 ounce), they contribute to the overall weight and cost of the battery pack.
The number of vents required will vary depending on the application but common practice has been to provide at least two vents and up to 12 vents for full EVs and at least one vent for hybrid vehicles. Airflow needs will depend on free air volume and anticipated nominal operating pressures. An experienced venting partner can help analyze the battery pack and provide venting recommendations.
Because thermal runaway events can generate temperatures exceeding the melting temperature of the membrane, a key feature of the active vent requires designing and manufacturing the vent so it bursts before it melts, allowing hot gases to escape before peak temperatures diffuse throughout the pack. The increased pressure of a thermal runaway event will equalize throughout the pack at the speed of sound, while heat takes time to diffuse, so the vent can burst quickly and allow gases to escape.
Testing is Critical
Vents should be thoroughly tested to demonstrate performance, with quality testing included for multiple pressure and temperature cycles, thermal runaway, ingress protection, vibration, thermal shock, and salt spray. Pressure cycle testing should include a sufficient number of cycles to simulate the design life of the product. Ingress protection (IP) should be provided to at least a rating of IP67. Standard tests are available for most of these parameters, though pressure cycle and thermal testing may require custom testing based on the application.
Compatibility with leak testing is another consideration. Ideally, the vent port can also serve as a port for leak testing, service, and inspection of the pack. This can help minimize the number of holes in the pack. If the vent is to remain installed during testing, it is also important that the vent can withstand the vacuum pressures introduced by leak testing.
With venting a necessity in battery pack design, dual-stage venting efficiently provides both passive and active venting in one assembly. Proper venting, however, requires careful planning and integration in the overall process and should not be treated as an afterthought. To achieve successful solutions, vehicle and battery pack manufacturers should engage an experienced venting partner with the expertise and resources needed to analyze issues and develop precise solutions. The partner should have a proven track record, with solutions approved by other OEMs.
With a strong partnership engaging the expertise of the venting and battery pack manufacturers, dual-stage venting can contribute significantly to the successful production and operation of EVs.