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Power battery shell waterproof design

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In this Article We are discussing the power battery shell waterproof design, which can be used for reference by other peers.

Introduction

With the development of the global sharing economy, fields such as shared mopeds, shared scooters, shared balance cars, shared motorcycles, and battery swaps have sprung up. At the same time, these products also put forward higher requirements for their power batteries. Shared products work in the wild for a long time, the ambient temperature is changeable, and the working conditions are complex and diverse. If water enters the battery, it may cause the battery to short circuit, catch fire or even explode. Therefore, the waterproofing of the power battery casing has become the focus of structural design, which also requires that the waterproofing level of the product needs to be raised to at least IP67 and above. According to the relevant technical information, combined with many years of work experience, this paper summarizes and introduces the waterproof design of the power battery casing, which can be used for reference by other peers.

1. Waterproof Design Method

In the process of structural design for many years, a variety of waterproof design methods have been summarized to ensure that the power battery casing can reach IP67 and above. The so – called IP67 waterproof means that dust cannot enter the object. Under normal temperature and pressure, the casing will not enter water after being immersed in 1 m deep water for 30 minutes.

1.1 Ultrasonic Welding

In the case of small product size and low level of waterproof requirements, ultrasonic welding is generally used. Figure 1 shows the ultrasonic welding design. Ultrasonic welding has strict requirements on the positioning of the upper and lower plastic casings of the product, the structure of the ultrasonic fuse wire, the material of the plastic, the selection of the wall thickness, and the frequency and power of the ultrasonic equipment. The main principle of ultrasonic welding waterproofing is to use ultrasonic energy to rapidly soften and melt the contact surface of the triangular column, and after the plastic is melted into a liquid state, the upper and lower plastic shells are tightly bonded together. Therefore, the design and positioning of the ultrasonic fuse wire is the key to the success or failure of waterproofing.

1.1.1 Design of Positioning Method

The upper and lower plastic parts need to ensure the accurate positioning of the triangular column during the ultrasonic welding process, and the limit height is generally not less than 1 mm. Cylinder and groove positioning structures can be added to the matching positions of the upper and lower plastic parts to ensure accurate positioning of the entire plastic shell. In the design process, it is also necessary to consider the flatness and deformation of the shape of the plastic shell. Therefore, the design of the plastic shell needs to increase the anti-deformation design and add polishing to the parting line of the mold.

1.1.2 Design of Fuse Wire

There are many types of fuse wire designs. One of them is that the product surface of the upper mold is made into a breakpoint triangular column, and the surface of the lower mold product is made to be a horizontal plane; the other is that the fuse wire is made into a concave-convex groove structure, the surface of the ultrasonic upper mold product is made into a convex structure, and the other half is made of a concave-convex groove structure. A groove structure is formed, and the size of the convex structure is greater than the depth of the concave structure, which is 0.5 to 1 mm.

1.2 Glue Groove Dispensing Waterproof

In the waterproof treatment, the most common method is dispensing waterproofing dispensing glue to the gap, and sealing it after the glue solidifies to achieve the purpose of waterproofing. The dispensing groove structure can be designed as a waterproof rib and glue groove structure similar to the sealing ring, which is used for the dispensing and sealing of the gap between the shell and the material; generally, ribs are arranged on the upper cover, and corresponding grooves are arranged on the lower cover; The depth of the groove is 2 to 4 mm, and the ribs are 1 to 2 mm higher than the bottom of the groove.

The glue used for dispensing and waterproofing is generally RTV waterproof sealing dispensing. RTV glue has good hydrophobicity and bonding properties, and can be turned into an elastomer after curing. Therefore, it can still maintain the original rubber elasticity after reliability tests such as vibration, drop, and mechanical impact, which is helpful for the shockproof and anti-vibration of power batteries. fall.

1.3 Gluing and Waterproofing

Glue-filled waterproofing is currently the highest level of waterproofing, which can reach IP68 and above. However, due to the characteristics of high weight, high cost, and inability to rework, the cost of the entire power battery increases, and many low-cost projects cannot be used. At present, many manufacturers are doing technical research on these shortcomings in order to improve these shortcomings. For example, the use of foam glue can solve the problem of heavy weight, and the use of matching solvents to melt glue can solve the problem that cannot be reworked.

For potting and waterproofing, epoxy resin potting glue or AB glue is commonly used, which can completely pot the inside of the power battery shell, with functions such as waterproof, shockproof, anti- drop, and anti-shock. Epoxy resin is a kind of high molecular polymer. When reacted with the corresponding curing agent, it will form a stable film that isolates water molecules and makes them impenetrable. In the process of glue filling, because the flow of the glue can fill the gap between the cells, it can also form a stable and reliable heat conductor. From the current test results, the overall heat dissipation is better than that of plastic.

1.4 Sealing Ring Waterproof

The waterproof sealing ring is the most common design in existence. It has the characteristics of wide application range, space saving, simple operation, strong stability, good rework ability and good waterproof effect. O -ring seals are most commonly used for sealing ring waterproofing. The O -ring seal can prevent the circulation of water or air, which is embodied in the deformation of the O -ring to become a block able wall. The sealing effect of the O – ring seal depends on whether the size of the O -ring and the groove size are correctly matched, and whether the compression and tension of the sealing ring are reasonable.

1.4.1 Design of O -ring seal

1. Compression Ratio:       

The formula for the stretch amount below is modified in the same way
In the formula:-
W —— Compression ratio %;     
d0 —— The cross-sectional diameter of the O-ring in the free state, mm;
h —— The height of the compressed section of the O-ring, mm
The compression rate of the O -ring is 15% to 30%, and the power battery casing generally uses a static seal, so the selected compression rate cannot be too high; plus, the pressure of the battery casing is fixed: so only increase the width of the overall groove and try to improve the compression rate of the O -ring, in order to improve the overall waterproof level to IP68.
In the formula:-
a —— stretching amount, %;
d —— shaft diameter, mm;
d1 —— inner diameter of O-ring, mm;

1.4.2 Material Selection of O – ring

Most O -rings are made of rubber. During the material selection process, materials need to be selected based on many different application requirements. The main application requirements are operating temperature, UV resistance, flame resistance, chemical resistance, seal thickness and size, overall cost, and more. The comparison of the characteristics of each rubber sealing material is shown in Figure 4. Power batteries have lower requirements for UV resistance, oil resistance and chemical resistance, and higher requirements for working temperature and wear resistance, so EPDM and NBR are generally preferred.

1.4.3. O Ring Seal Groove Design

There are various shapes of grooves, such as rectangular grooves, dovetail grooves, semi-circular grooves, trapezoidal grooves, triangular grooves, etc. The actual test results shall prevail. The design of the sealing ring groove generally follows the following principles:

  1. The groove width is larger than the maximum diameter of the O – ring after deformation.
  2. The expansion coefficient of the sealing ring needs to be considered.
  3. The groove should not be too large to avoid wear. Generally, the groove width is set to 1.5 times the cross-sectional area of the O – ring.
  4. The groove depth design should be reasonable. The depth of the groove plus the clearance must be smaller than the cross-sectional diameter of the O – ring in the free state to ensure the deformation of the O – ring compressed in the groove.
  5. Reasonable size and easy to process.

Selection of Hollow Sealing Ring

The main factors that affect the waterproof of the casing are: the cross-sectional area of the sealing ring; the compression rate; the volume -to- gap ratio; the casing pressure, that is, the friction; the design of the groove; the selection of the sealing material. If sealing ring waterproofing needs to achieve a higher level of waterproofing, there must be an important improvement in one or several of these factors.

Compared with solid O -ring seals, many companies have begun to design and produce hollow seals. Hollow sealing rings have these advantages: higher compression ratio, which can reach 30% to 50%; smaller shell pressure; higher volume -to – gap ratio. Therefore, the hollow sealing ring is more applied to the waterproof casing above the IP68 level. Hollow sealing rings have various shapes, such as hollow O -shape, hollow D -shape, hollow U -shape, hollow P -shape, etc.

1.5 Liquid Silicone Over Molding

Liquid silicone is an inert, doorless, five-toxic, flexible thermoset material with low viscosity, fast curing, shear thinning, and high coefficient of thermal expansion. Over the years, liquid silicone has been widely used in aerospace, automotive, IT and other fields, this method can improve productivity, ensure the consistency of parts, and ensure high-volume molding.

Liquid silicone secondary injection molding is to combine liquid silicone and other elastomers with metal or plastic parts through secondary injection molding. Two parts can be combined into one part by the use of secondary injection molding, which is not easy to fall off in the assembly process and has high reliability. The sealing ring in the power battery casing can also be replaced by liquid silica gel secondary injection molding, which can better ensure the quality and stability of the product; however, because it requires secondary injection molding of the mold, the cost is higher.

Conclusion:

This article introduces a variety of waterproof methods, and can choose different waterproof methods according to the needs of different power batteries during the design process.

  1. Ultrasonic welding is generally used for battery specifications with smaller size and lower waterproof level.
  2. The glue tank dispensing waterproof is mainly used for battery specifications with small wall thickness and waterproof level above IP67 ; but the rework ability is very poor, and the shell cannot be reused.
  3. Glue-filled waterproof is generally used for IP68 and above waterproof level, and the battery requires a battery with anti-prolongation and high temperature balance. The disadvantage is that it cannot be reworked.
  4. The waterproof sealing ring requires a battery with large wall thickness and uniform pressure. The waterproof level can reach IP67 and above, which is easy to rework and the shell can be recycled.
  5. Liquid silicone secondary injection molding is generally used for batteries with small size, but requires IP67 and above waterproof level. Compared with the sealing ring, this method is more stable, but the cost is higher.

More Articles:

BMS Battery Management System,
Battery Electrical Performance Test,
Safety Analysis of Li-Ion Battery,
IEC Battery Safety Standard for Power Batteries,
POWER BATTERY SHELL WATERPROOF DESIGN,
BATTERY SAFETY PERFORMANCE TEST,

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