Heat dissipation structure design of two-wheeler power battery

With the development of society, social activities are becoming more and more abundant, and short – and medium-distance communication is becoming more and more common.

The comfortable and convenient way to travel has been respected by people. Among them, electric bicycles, electric moped, electric scooters and other two-wheeled vehicles are favored by people.

More and more two-wheelers also produce more and more safety problems, and most of the problems are related to the heat dissipation of the battery.

Table of Contents

Heat dissipation design requirements for two-wheeler battery

The heat dissipation design of the two wheeler electric bike battery shall meet the following requirements:

  • The two-wheeled vehicle battery package shall meet the reliability and safety requirements under complex test conditions;
  • Meet the requirements of thermal environment under complex working conditions;
  • Meet the restrictive requirements for battery cooling systems in a limited structural space.

Heat dissipation design of two-wheeler battery

Use Cooling fin to dissipate heat

In the two-wheeler lithium battery industry, the Cooling fin is generally used to heat the electronic components that are easy to heat in the battery pack circuit protection board, and is generally combined with thermal silicone to complete the heat dissipation. Figure 1 shows the design scheme of Cooling fin.

The PCB board is provided with screw mounting holes, and the studs are riveted and pressed on the Cooling fin by the supplier in advance to form a Cooling fin assembly. The Cooling fin assembly is aligned with the mounting holes and locked on the protection plate of the heating component with screws.

It is generally recommended to reserve enough space between the electronic components and the Cooling fin, 0.3 ~ 0.6mm, the reserved space should not be too large, so as to avoid excessive gluing cost waste, the reserved space should not be too small, to avoid interference between the radiator and electronic components, easy to damage electronic components. At this time, the heat transfer system is formed between the electronic components and the Cooling fin by thermal silica gel.

The Cooling fin bears the function of dissipating the heat generated by the heating component to the surrounding air, so that the temperature rise of the heating component will not or will not reach the situation that is out of control due to overheating, and maintain the normal output working state.

The Cooling fin is usually made of aluminum alloy, bronze or brass into sheets, plates and plates, and the heat dissipation capacity of the Cooling fin is strongly related to the material, thickness and area of the Cooling fin, and the designer can make corresponding adjustments according to the actual situation. This structure is easy to process and install, the cost is relatively low, is the most common way of heat dissipation.


Heat dissipation of aluminum substrate

Aluminum substrate is a printed circuit board made of aluminum as a metal layer substrate. The electronic components take the printed circuit board as the carrier, and the connection and conduction between the components are realized through the copper-coated circuit on the printed circuit board. The traditional printed circuit board substrate is FR-4, which is an epoxy resin insulator, and the conduction effect of heat is not very good.

The heat transfer performance of the printed circuit board with FR-4 as the substrate has been unable to meet the needs of some high thermal conductivity products, affecting the use of printed circuit boards in some specific occasions.

The local heating on the printed circuit board can not be effectively evacuated, and the accumulation of heat over time is easy to cause strikes and even failures of electronic components, etc., and the emergence of the aluminum substrate process can solve the major problem of heat dissipation to a greater extent.

Figure 2 shows the heat dissipation structure of the aluminum substrate. Generally, a single panel of aluminum substrate is composed of a three-layer structure. The first layer is the line layer of the surface layer, and the electronic components can be attached to different points of the line layer through SMT to realize the connection and control of the circuit principle.

The second layer is connected to the upper and lower insulation layer, its material is an insulator, the insulation layer needs to have good thermal conductivity, the better the thermal conductivity, the more conducive to heat dissipation, easy to spread heat.

The third layer is a metal base, which continuously dissipates the heat of the above two layers. The aluminum substrate has better heat dissipation performance and lower thermal resistance, so the aluminum PCB has a long life. In the field of two-wheeled vehicle power batteries, it is generally used in electric motorcycle battery pack with large power and energy storage products.


Heat dissipation of thermal silicone sheet

Thermal conductive silicone sheet heat dissipation scheme is also one of the commonly used heat dissipation methods. Because there is no such thing as an absolutely smooth object. Seemingly smooth planes have different degrees of bumps or depressions under the electron microscope, which is called the surface roughness of the object. When two different structural parts are assembled together, countless micro-voids will be formed due to the existence of surface roughness.

The existence of microvoids is not conducive to heat conduction and increases the thermal resistance of heat propagation. At this time, a thermal conductive silicone sheet that can conduct heat can be introduced between the two objects.

The thermal conductive silicone sheet is given a certain pre-pressure, because the silicone material is soft, it can enter the micro-gap to varying degrees, greatly reducing the thermal resistance, improving the heat conduction between the structural parts, and effectively solving the heat conduction between the two structural parts.

The following is a design case of thermal conductive silicone sheet. Figure 3 shows the heat dissipation design scheme of thermal conductive silicone sheet. The battery cell is installed in the mounting holes of the plastic bracket A and bracket B, and the bracket is provided with a screw fastening structure to hold and fix the battery cell tightly. The series and parallel connection between the cell and the cell is completed by convex spot welding technology.

After the connection, the lithium battery power pack is charged, and the battery pack cannot contact the aluminum cylinder directly. In this case, there is a certain gap between the conductive sheet and the inner wall of the aluminum cylinder. Since the thermal conductivity of the air is only 0.0242W (/m·K), the conduction of heat is seriously hindered.

Eventually, the temperature accumulation of the cell and the conductive sheet will cause the battery to start temperature protection and stop the power supply work. In this scheme, a thermal conductive silicone sheet is introduced into the gap between the conductive sheet and the aluminum cylinder on one side of the battery pack. To consider the manufacturability of the battery, the other side of the battery pack is insulated with a PC sheet.

The battery pack is installed in the set position in the aluminum cylinder, and the battery pack is secured by screws from the top of the aluminum cylinder, so that the battery pack is tightly attached to the upper side of the aluminum cylinder. It should be noted here that the thermal conductive silicone sheet requires a certain preload, so the compression amount of the thermal conductive silicone sheet needs to be considered when designing.

In the range of silicone compression, the greater the compression rate, the smaller the thermal resistance, the better the thermal conductivity effect. The thermal conductive silicone sheet can remove the air in the gap and greatly reduce the contact thermal resistance.

In this way, as for the charging and discharging of lithium ion battery, during the discharge process, the heat generated by the battery core and the conductive sheet can be transferred to the aluminum shell through the thermal silicone sheet, and the heat is dissipated through the aluminum shell body to achieve the purpose of normal operation of the battery pack.


Heat conduction potting adhesive heat dissipation

Thermal potting adhesives are often divided into epoxy system and organic silicone rubber system. The hardness of the epoxy system after curing is rigid, and the soft quality of the organic silicone rubber system after curing is elastic. The organic silicone rubber system is further divided into single-component mechanical potting adhesive and two-component mechanical potting adhesive.

The single component potting adhesive has good bonding performance and strong adhesion, and the corresponding fluidity will be poor. The two-component potting adhesive has poor adhesion, but good fluidity and fast curing. Taking into account the characteristics of production efficiency and curing speed, the overall filling scheme of battery pack generally chooses two-component mechanical filling sealant.

The two-component potting glue is stored separately before filling. When used, the two components are fully and evenly mixed in a certain proportion in the special filling equipment, and enter the battery pack through the filling port. After potting, the potting glue mixture can be cured at room temperature.

Figure 4 shows the heat dissipation scheme of heat conduction potting adhesive. Both sides of the aluminum cylinder are provided with a face shell and a bottom shell, the bottom shell and the aluminum cylinder are fastened by screws to form an aluminum cylinder pre-processing, the battery component is assembled to the aluminum cylinder pre-processing, and then the face shell component is locked with the aluminum cylinder to form a semi-finished product. The bottom shell is provided with two holes, one for gluing and the other for venting.

Figure 5 shows the section of the battery pack. The thermal potting glue enters the potting channel through the potting hole, and then diverts from the potting channel to the inside of the battery to fill the inside of the battery. The battery cell and the protection board are wrapped by potting glue.

When the battery is working, the heat generated by the heat source is transferred to the aluminum shell through the thermal potting glue in time, so as to reduce the temperature of the heat source. At the same time, the thermal potting adhesive scheme can also play a waterproof role in the battery, and improve the waterproof and moisture-proof performance of electronic components, batteries and conductive pieces.

Thermal potting adhesive has certain elasticity, which can effectively reduce the damage caused by external impact on the battery in the reliability test of battery pack vibration, drop, impact, etc. However, the thermal potting adhesive heat dissipation scheme also has the disadvantages of relatively large weight, high cost, and not conducive to battery repair, so it is recommended that designers optimize the design according to the actual situation.


Comparison and summary of heat dissipation schemes

  1. The heat sink heat dissipation scheme is generally combined with thermal silicone, which is used to heat the electronic components that are easy to heat in the battery pack circuit protection board. The cost is more economical, and it is the most basic and most common way of cooling the two-wheeled battery pack.
  2. The aluminum substrate heat dissipation scheme is generally used in electric motorcycles with large power and energy storage products. In the field of two-wheeled battery packs, the use scenario is relatively simple. Under the same shape size conditions, the cost is about 30% higher than that of general printed boards.
  3. Thermal conductive silicone sheet heat dissipation scheme, due to the soft texture of the silicone sheet is easy to trim, greatly improve the convenience of the use of thermal conductive silicone sheet. At the same time meet the insulation function, can take into account the thermal conductivity of the structure and the damping of the structure. However, the thermal conductivity is slightly lower than that of the heat sink.
  4. Thermal potting adhesive heat dissipation scheme has good caulking performance and waterproof function. Thermal conductivity of thermal potting adhesive is general, the advantage is that it can fully wrap the heat source, and the contact area with the heat source is the largest.

Therefore, when the battery pack is waterproof and the battery cell has the requirement of rate discharge, it is a good solution. However, after packaging, it is not easy to disassemble, and repair is more laborious. The weight of the battery after potting is large, the cost is relatively high, if there is a special demand for product cost and repair, it is necessary to carefully consider this structure.

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