Design tips for waterproof and dustproof structures of lithium battery packs

Lithium battery packs often face the risk of water vapor and dust intrusion in outdoor, industrial or consumer electronics scenarios, which may lead to short circuits, corrosion or even thermal runaway. Therefore, the waterproof and dustproof design needs to take into account sealing, heat dissipation and maintainability. The following analysis is carried out from three aspects: design principles, structural schemes and key processes.

First, the core principles of design

The concept of hierarchical protection

The protection level (such as IP67, IP68) should be determined based on the application scenario, rather than blindly pursuing the highest standard. For instance, the battery packs of electric vehicles typically require IP67 (30 minutes in 1 meter of water depth), while the battery cabinets of energy storage power stations may only need IP54 (dust and splash-proof).

Distinguish between enclosure protection (such as enclosure sealing) and internal protection (such as conformal coating on circuit boards) to avoid increased costs due to overdesign.

Dynamic sealing design

Consider the sealing reliability of the battery pack after thermal expansion and contraction, vibration or multiple disassembly and assembly. For instance, a certain drone battery pack failed to reserve space for thermal expansion, causing the sealant to crack and seep water at high temperatures.

Elastic sealing materials (such as silicone rubber, fluororubber) or floating sealing structures are adopted to adapt to deformation without failure.

Balance heat dissipation and protection

High protection levels may impede heat dissipation, and the contradiction needs to be resolved through heat conduction path optimization (such as heat conduction pads + heat dissipation fins) or active heat dissipation (such as fans + waterproof filters). For instance, a certain power tool battery pack adopts graphene thermal conductive film and waterproof breathable valve, which combines IP67 and efficient heat dissipation.

Maintainability and cost

Avoid maintenance difficulties caused by the sealed design. For instance, detachable sealing covers (equipped with sealing rings) or modular designs can be adopted to reduce the impact of single point failures.

Second, key structural design techniques

Shell sealing scheme

Shell material selection: High-strength engineering plastics (such as PC/ABS) or aluminum alloys (surface anodized) are preferred, taking into account both strength and corrosion resistance. For instance, a certain outdoor energy storage cabinet adopts an aluminum alloy frame and plastic panel, which reduces weight while meeting IP65 standards.

Sealing structure form:

Flat sealing: Sealing between shells is achieved through O-rings or sealant, and the compression amount needs to be controlled (usually 10% to 20% of the rubber diameter).

Maze sealing: Extends the entry path of water vapor/dust through complex channels, suitable for high-dust environments (such as mining equipment).

Potting and sealing: The entire internal circuit board is potting with epoxy resin to completely isolate it from the environment, but the maintenance cost is high.

Interface protection design

Connector protection: Select connectors with waterproof functions (such as aviation plugs with rubber sealing plugs), and design anti-misinsertion structures. For instance, the charging interface of a certain electric vehicle adopts a double-layer sealing ring + drainage hole design to prevent water from seeping in.

Wire through-wall treatment: Seal the gap between the wire and the shell through rubber protective coils, gran heads or potting compound to prevent water seepage due to "capillary phenomenon". For instance, a certain industrial battery pack fills silicone at the wire outlet, and after curing, it forms a flexible sealing layer.

Breathability and pressure balance

Waterproof and breathable valve: In scenarios of high altitude or drastic temperature changes, it is necessary to balance the internal and external air pressure of the battery pack (such as the battery pack of an electric bicycle). For instance, a certain battery pack adopts an ePTFE membrane vent valve, which is waterproof (IP67) and allows for gas exchange at the same time.

Breathing valve design: The direction of gas flow is controlled by a check valve to prevent external water vapor from entering. For example, the battery pack of a certain deep-sea exploration equipment adopts a diaphragm type breather valve, which only allows the internal gas to be discharged.

Internal protection strengthening

Circuit board protection: Apply three-proof varnish (such as acrylic, polyurethane) to key circuit boards, or use local potting (such as the BMS chip area). For instance, a certain drone battery pack is equipped with a silicone sheath in the high-risk area of the circuit board to reduce the risk of short circuits.

Battery cell protection: Fill the battery cells with thermal conductive insulating adhesive (such as alumina filled with silicone), which not only fixes the battery cells but also isolates moisture. For instance, a certain energy storage battery module fixes the battery cells with an overall potting compound to enhance its shock resistance and protective performance.

Third, key processes and verification

Sealing process control

The curing conditions of the glue: If sealant is used, the curing temperature, time and pressure must be strictly controlled. For instance, a certain battery pack suffered from seal failure and water seepage due to incomplete curing of the glue.

The machining accuracy of the sealing surface: The roughness of the shell sealing surface should be lower than Ra3.2 to avoid leakage caused by uneven surface. For example, the sealing surface of a certain medical device battery pack is processed by CNC to ensure that the fit clearance is ≤0.1mm.

Test verification method

IP grade testing: IP67/IP68 certification is conducted through a third-party laboratory, including water immersion test (1 meter deep water for 30 minutes) and dust test (8 hours in talcum powder environment).

Thermal cycling test: Simulate 100 cycles at temperatures ranging from -40℃ to 85℃ to verify the reliability of the sealing structure under thermal expansion and contraction. For instance, a certain battery pack was found to have cracked sealant after testing, and the material formula needs to be optimized.

Vibration test: Conduct vibration in accordance with the standards of GB/T 31467.3 or IEC 62660 to check whether the sealing parts are loose. For instance, after vibration of a certain electric vehicle battery pack, it was found that the gran head was loose and a locking nut needs to be added.

Failure analysis and improvement

Water seepage path traceability: If water seepage is found during the test, the leakage point needs to be located by staining seepage method or infrared thermal imaging. For instance, after the water immersion test of a certain battery pack, water seepage was found at the shell joint. Later, laser welding was used instead of glue sealing.

Long-term reliability verification: Expose design flaws in advance through HALT (High Accelerated Life Test) or HASS (High Accelerated Stress Screening) to shorten the development cycle.

Fourth, typical application scenarios and optimization directions

Electric vehicle battery pack

Design challenges: It needs to withstand high-pressure water gun flushing, wading through water driving and thermal management requirements.

Optimization plan: It adopts an aluminum alloy lower box body + plastic upper cover structure. Double-layer sealing rings are used at the interface, and a waterproof and breathable valve is integrated to balance the air pressure.

Outdoor energy storage power station

Design difficulty: It needs to be exposed to sand, dust, rain and ultraviolet rays for a long time.

Optimization plan: The shell is made of stainless steel and coated with powder for anti-corrosion. The wire outlet is sealed with a gran seal, and the internal circuit board is coated with three-proof paint.

Consumer electronic devices

Design difficulty: It needs to be thin and light and support users to replace the battery by themselves.

Optimization scheme: Adopt snap-type sealing cover plate + silicone rubber sealing ring, select Type-C interface with waterproof plug for connector, and protect the circuit through local potting.