EVS Battery Pack Sealing: Techniques for Optimal Performance and Safety
Electric vehicles, and electric boats (EVS) are leading the way in automotive and marine innovation, but how do they ensure their battery packs are fully protected against the elements? Bonnen’s senior engineer has created a guide to showcase their cutting-edge method for ensuring battery pack waterproofing and sealing performance. By designing a durable battery box seal that meets the highest safety standards for dustproofing and waterproofing, Bonnen is helping to make EVS one of the safest and most exciting developments in the automotive industry today. In this blog post, we will take a closer look at how Bonnen’s design helps to keep EVS safe and protected.
1. EVS Battery Pack Sealing Structure Analysis
2. How To Design The Sealed Structure For EVS Battery Pack?
3. How To Test The Battery Pack Sealing?
1. EVS Battery Pack Sealing Structure Analysis
As the output voltage of a pure EVS power battery pack can reach 200V or more, it is essential to ensure that the battery box is properly sealed and waterproof to prevent water ingress and subsequent short circuits. To meet this requirement, the battery box must comply with IP67 standards. For battery packs that rely on natural air cooling, a check valve is added to the upper cover to prevent explosions, while battery packs that rely on forced air cooling cannot ventilate with the atmosphere except for necessary ventilation holes.
The design of the sealed box focuses on the flow of battery cooling airflow, and any leakage must be avoided to ensure consistent performance. To achieve this, the upper cover and the lower bottom of the battery box must be free from any perforations or gaps, and a gasket should be added between them during assembly. Additionally, all plug connectors, inlet and outlet air ducts should be installed with gaskets or made waterproof to further prevent leakage.
2. How To Design The Sealed Structure For EVS Battery Pack?
2.1 Design of the battery box sealing surface
The design of the battery pack sealing surface also plays a crucial role in sealing performance. Its design needs to be needs to be aligned with the box structure and sealing ring of the battery pack.
Machining the upper cover and lower bottom of the battery box requires precision control. To ensure maximum accuracy, it’s strongly recommended to spot-weld more than two plates together when welding these parts – particularly due to their relatively thin construction. Prior to spot welding, apply a 30mm wide adhesive along the inner edges of two overlapping steel plates. Carefully control the current during this process in order to avoid damaging thinner metals with too much heat. If extra flatness is desired for butt welds, brazing can be utilized as an alternative method that reduces both deformation and heat output – afterward these areas should then receive polishing until smooth surfaces are attained. Once upper coverings and lower bottoms have been processed via electrophoresis, sealant must be applied onto the jointed metal pieces before they enter into a drying room where other components await their completion.
A good seal between the upper cover and lower bottom is very important. The upper cover and lower bottom are leaned on two surfaces, and sealing material is added. Generally, there are three types of sealing structures between the upper cover and the lower bottom.
(1) As shown in Figure 1, the upper and lower covers are held firmly with screws, ensuring a tight seal between the two placed flanges. To achieve this strong bond, an additional gasket is added which must be pre-tightened to provide sufficient force – carefully considering each plane’s flatness requirements for optimal results.
(2) Figure 2 demonstrates a construction with screws fastening an upper cover to the lower bottom, then sealed by vertical surfaces, and a gasket is added between the upper cover and the vertical surface of the lower bottom. This type of structure requires relatively higher preload than other structures while maintaining low flatness demands.
(3) As shown in Figure 3, the left and right surfaces of the battery pack sealing surface are inclined surfaces. The front and rear surfaces are flat planes with flanging. Although the design of the slope in the direction increases the difficulty of design and production, it plays a significant role in the sealing effect of the battery pack. In addition to the above, on the other hand, when a small amount of liquid falls on the sealing slope of the battery pack outside the car when going up, the design of the inclined surface can prevent the liquid from falling on the inclined surface from flowing into the battery pack and prevent the liquid from damaging the battery system.
2.2 Compression sealing ring design
The so-called compression seal is to form an appropriate pre-compression amount of the sealing ring, and use the rebound force of the material to compress the sealing surface to perform a sealing function. During the design and processing of the sealing ring and sealing surface of the battery pack, if the compression of the sealing ring is too small, leakage will occur; if the compression is too large, the rubber stress of the sealing ring will relax and cause leakage, and it is also prone to permanent deformation. Therefore, the design of the sealing ring should consider the compression ratio, and the compression ratio W is usually expressed by the following formula:
W=(h0-h1)/h0 × 100%
Where, h0 – sectional height of sealing ring in free state (mm)
H1 – The height of the sealing plane of the upper and lower covers of the battery pack after the sealing ring is compressed (mm).
When selecting the compression ratio of the sealing ring, the following two aspects should be considered:
(1) There should be sufficient sealing contact area.
(2) Try to avoid permanent deformation.
From the above factors, when selecting the sealing ring compression ratio, it is essential to consider several countervailing factors. Over compression can cause permanent deformation and reduce service life while under-compression may result in leakage due to height discrepancies between the seal surface and battery pack. Therefore, when selecting the sealing ring compression ratio, it is necessary to weigh various factors.
The usual sealing gasket is designed as a single-stage seal with a flat ribbon shape, which is simple to manufacture and low in cost. However, the sealing effect is general, prone to permanent deformation, and cannot withstand repeated disassembly and assembly. It can also be designed as a two-stage sealing structure with a metal guide sleeve in the middle. The upper and lower covers are fixed together by bolts through this metal guide sleeve. For example, the sealing ring height is set to 10mm, and the metal guide sleeve height is set to 8mm. Therefore, the maximum compression ratio of the sealing ring is W=(10-8)/10 × 100%=20%. Its compression rate meets the standard for planar seal compression rate (15% – 30%). In addition, the metal guide sleeve can serve as a fixed seal ring and limit the compression rate of the seal ring. This is so that it does not generate an excessive compression rate, causing permanent deformation. There is a groove in the middle of the sealing ring, forming the sealing ring in the shape of No. 2 in Figure 3. This allows the sealing ring to form two sealing layers, the inner sealing layer being the main sealing layer. The outer sealing layer is the auxiliary sealing layer. When inner sealing fails, outer sealing can also play a certain sealing role, and the double-layer sealing structure better enhances the sealing effect.
Therefore, the material to be selected as a compression gasket should consider the working state, working medium, working pressure, working temperature, and cost sources. It should conform to GB8410-200 flame retardant materials characteristics. Testing has shown that EPDM rubber or EPDM foam rubber is a better material for compression gaskets.
2.3 Sealing design of the installation interface between high/low-voltage connectors and the battery box
Most high/low-voltage connectors and mounting nuts use ordinary spot welding, resulting in high sealing failure rates at the mounting interface. Therefore, the sealing of the fixed parts of the high/low-voltage connectors and the battery box needs to be carefully considered, with strict requirements for the flatness of the battery box wall.
In order to ensure the tightness of the fixing point between the high/low-voltage connectors and the battery box, the nuts at the fixing point can be blind-hole butt welded nuts, and the flange surface can be directly butt welded to the battery box wall. This can ensure that the connector butt welded nuts and the lower box wall hole of the battery box are fully integrated into a whole, as shown in Figure 4, thereby effectively improving the sealing level here and improving the qualification rate of the fixed nuts at the connector and the lower box production.
2.4 Sealing design of the mounting surface between the air pressure balancing component and the battery box
During the long-term use of the electric vehicle battery pack, due to changes in temperature, altitude, and other factors, there will be a difference in internal and external pressure, and the pressure that the sealing surface can withstand is certain. Once the internal and external pressure difference exceeds the limit value, the sealing interface will fail, resulting in IP67 failure. To solve this problem, air pressure balancing components, such as air valves or explosion-proof valves are usually installed on the battery pack. If the sealing design between the air pressure balancing component and the battery case is not effective, it is easy to become the weak location where the entire battery pack seal fails. The air valve fixing nut of the upper battery box adopts a spot welding nut with a platform. After the air valve fixing nut is welded to the upper battery box, the nut’s welding surface is flush with the upper battery box. This effectively ensures the coupling between the air valve mounting screw and the upper box wall, ensuring sealing.
3 How To Test The Battery Pack Sealing?
3.1 Air tightness test
The main method for airtightness testing for EVS batteries is to use a gas pressurization system, connect the product to the airtightness tester by using a quick connector, and then charge the gas into the battery box to be tested. After the air pressure stabilizes, observe the change in internal pressure over time. Pressure drop Δ P is at time Δ Measured on the basis of t. If the pressure in the system decreases rapidly, it indicates that there is a large source of leakage. If the system pressure decreases, there are small leaks. If the pressure remains constant, there is no leakage. The leakage rate Q can be easily calculated considering the volume V of the component, which is: Q=(∆ p * V)/∆ t
3.2 Water immersion test
Close the top cover of the battery box, add a gasket in the middle, and plug all the connector holes with the baffle clip gasket. Completely immerse the entire battery box in a container of water, and lift it from the top of the battery box with a bracket. The entire box is immersed in water, and the upper surface is kept 500mm underwater for 10 minutes. This is shown in Figure 4. After the time is up, take out the box, open the top cover, and check whether the box is soaked in water. If the inside of the box is completely dry, it means that the battery box’s airtightness has reached at least IP66 or higher. If water enters the inside of the box, the seal is not enough. It is necessary to continue to find the reason and optimize the design.
The sealing of the EVS battery pack is very critical to the battery pack’s safety in the box. New sealing structures and sealing materials are constantly emerging. Battery pack sealing is constantly being explored, evolved, and improved. A well-designed sealing scheme will definitely improve the technical content of the battery pack and drive the electric vehicle industry development.
Now that we’ve delved into the intricacies of EVS battery pack sealing, it’s clear that precision and attention to detail are key factors in ensuring the safety and reliability of these systems. Whether you’re in the market for lithium batteries for your project or simply want to learn more about EVS battery sealing standards, the experts at Bonnen Battery are here to help. With 10 years of experience and a commitment to quality, we can provide the product consulting, R&D, production services you need to make informed decisions about your battery needs.
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