Analysis of crimping performance of high voltage wiring harness terminals for electric vehicles

With the rapid development of the new energy vehicle industry, new energy wiring harnesses have gained development opportunities. The difference between high-voltage wiring harnesses and low-voltage wiring harnesses is that high-voltage terminals carry a large current, which is prone to heat generation, resulting in a decrease in the mechanical strength of the terminals and the insulation performance of the wiring harness, while causing conductor oxidation, further exacerbating the heating problem.

High voltage terminal crimping requires consideration of both crimping reliability and low temperature rise at the crimping point. This article mainly introduces the impact of cold crimping on terminal temperature rise.

Common crimping types for high voltage terminals

Crimping, resistance welding, high frequency welding.

Crimping uses crimping equipment and dies to connect wires and terminals together through a crimping process. High-frequency welding uses a high-frequency welding machine to weld wires and terminals together. Resistance welding uses special resistance welding equipment to connect wires and terminals together.

Advantages of ordinary crimping: simple operation, easy use and maintenance, low manufacturing cost, high operating efficiency, suitable for mass production. Disadvantages: It cannot meet the requirements of wiring harnesses and terminals that require high current flow rate and low resistance after the connector is connected to the wire and terminal.

The common crimping method for large square high voltage terminals is shown in Figure 1, which is a hexagonal closed terminal.

The advantages of ordinary crimping are obvious, but how to make the most of ordinary crimping and avoid its disadvantages and minimize crimping resistance is particularly important. Reducing resistance means reducing heat generation, which can reduce product temperature rise and make product life and quality better.

Hazards of terminal heating

When the terminal is heated, it is easy for itself and the contact surface of the wire conductor to oxidize, forming an oxide film, which increases the contact resistance. The rate of increase increases exponentially with the increase in temperature, further increasing the terminal temperature rise rate, and in severe cases, it may cause a fire. At the same time, it will cause the elastic elements of the contact structure to anneal, reduce the contact pressure, and further increase the contact resistance. In addition, the heat will cause the insulation layer of the wire connecting the terminal to age and become brittle, causing the insulation performance to deteriorate, and there is a risk of fire caused by leakage and overheating.

Three major heat sources of terminals

Wire conductor: The conductor itself has resistance. The smaller the cross-sectional area, the higher the resistance, and resistance will cause heat.

Terminal crimping: Insufficient compression ratio will make the conductor loose, resulting in high resistance and easy heating. Excessive crimping will easily reduce the cross-sectional area and insufficient current carrying capacity, causing heating.

The contact between male and female terminals: The terminals are in poor contact, or the contact surface of the terminals is oxidized, causing severe heat.

Methods to reduce terminal temperature rise

Reduce contact resistance:

Use materials with low resistivity. Commonly used high-voltage terminals are H62, H65 copper or high-conductivity copper. Products greater than 125 A are recommended to use high-conductivity copper with low resistivity. Reduce the contact resistance of the conductor. Compact the terminal and the conductor as much as possible to reduce the crimping resistance. Increase the cross-sectional area of ​​the conductor, increase the cross-sectional area, and reduce the temperature rise of the wire.

Increase the conductor heat dissipation area:

Adopt forced cooling, such as air cooling, water cooling, etc. Arrange conductors reasonably, and arrange wiring harnesses with large current in spaces that are easy to dissipate heat, which is conducive to natural heat dissipation.

Impact of crimping on temperature rise

Crimping, refer to 4.2.6 voltage drop test requirements in QC/T 29106-2014 "Technical Conditions for Automotive Wiring Harnesses" and GB/T 20234.1-2015 "Connection Devices for Conductive Charging of Electric Vehicles Part 1 General Requirements" for verification. The process is shown in Figures 3 and 4, and the data obtained is shown in Table 1.

Compression ratio/compression rate calculation method

(1) Refer to VW60330-2013 standard

(2) Refer to SAE/USCAR21-2014 standard

(3) The difference between conductor compression ratio and terminal compression ratio

According to the VW60330-2013 standard, the calculation of the compression ratio only includes the conductor and does not include the terminal compression. It can more intuitively reflect whether there is a gap in the conductor. When the compression ratio is ≤100%, there should be no gap. We can call it the wire compression ratio.

According to the SAE/USCAR21-2014 standard, the calculation of the compression ratio includes the compression of the conductor and the terminal. Although it cannot directly reflect whether there is a gap in the conductor, it can more directly reflect the actual cross-sectional area of ​​the crimping point. In order to facilitate data comparison, this article defines the terminal compression ratio = 100T/(At+Ac).

Both calculation methods have their own advantages.

Analysis result

01

As shown in Table 1, when the conductor compression ratio of the 3# product reaches 104%, the tension has reached the standard of 50mm² and wire tension ≥ 2 700N specified in QC/T29106-2014. However, the crimping part is not completely compacted at this time, which poses a great safety hazard. Therefore, the tension cannot be used as a quality criterion for high-voltage terminals.

02

The resistance values ​​in the table do not completely correspond to the temperature rise trend. This is probably due to the fluctuations caused by the differences in individual contact resistance of the terminals and the inconsistency caused by oxidation of the terminal plating. However, from the general trend, it basically conforms to the corresponding relationship that the lower the resistance, the lower the temperature.

03

The compression ratio of the 7# terminal is 73%. There will be an oxide layer on the surface of the conductor and the terminal. The oxide layer will be gradually destroyed as the compression ratio decreases. When the terminal compression ratio is 73%, the conductor oxide layer begins to collapse, making the copper wire more tightly fused and the temperature rise slightly reduced, indicating that the terminal compression is more appropriate at this time.

04

Judging from the fluctuation of temperature rise value, the impact of crimping on temperature rise can reach 10°C. For high-voltage terminals, the impact of crimping is relatively large.

05

When the compression ratio of the 10# product terminal is 60%, the theoretical cross-sectional area of ​​the conductor at the terminal crimping is only 30mm². According to the SAE/USCAR21-2014 terminal compression ratio calculation, the cross-sectional area of ​​the crimping should include the cross-sectional area of ​​the terminal in addition to the conductor. The actual cross-sectional area of ​​the conductor and the terminal after crimping is 66.97mm², which is larger than the nominal conductor cross-sectional area of ​​50mm². Therefore, this compression ratio will not cause the compression point to become a bottleneck, which is more in line with the actual situation. It also shows that high-voltage terminals are more suitable for the calculation method of terminal compression ratio.

06

Too low a compression ratio will cause the temperature rise at the crimping point to be too high. According to actual measurements, when the terminal compression ratio reaches below 40%, the terminal crimping resistance will gradually increase. The terminal temperature rise will begin to rise slightly.

07

Analysis of terminal compression ratio applicable to hexagonal crimp terminals.

First of all, no matter what the situation, the cross-sectional area of ​​the crimping point cannot be less than the nominal conductor cross-sectional area;

Secondly, due to the differences in the material thickness selection of each high-voltage terminal, according to the terminal compression ratio = 100T/(At+Ac), some terminals have a lower At cross-sectional area. After compression, the terminal compression ratio that is too low will cause the cross-sectional area of ​​the crimping to be smaller than the nominal conductor cross-sectional area.

Therefore, the cross-sectional area after crimping needs to be as large as possible. In addition, from the analysis of the tensile force data, a too low compression ratio will lead to a decrease in the mechanical tensile force value of the terminal, affecting the reliability of the terminal crimping.

Summarize

Based on the above analysis, considering the reliability of terminal crimping strength and terminal resistance performance, the actual terminal compression ratio is controlled at 65% to 75%, and the conductor compression ratio is 65% to 80%.

From the experimental data, the fluctuation of some resistance and temperature rise data is related to the terminal plating and oxidation conditions as well as the terminal contact structure. Therefore, it is far from enough to reduce the temperature rise only from the perspective of crimping quality.

It is necessary to pay attention to the daily light-proof storage of the terminals, the quality of the terminal coating, the plug-in and plug-out life of the terminals, the plug-in force, the contact area, etc. For products with large temperature rise, it is recommended to use high-frequency welding. High-frequency welding makes copper generate heat through friction at ultra-high frequency, melts the copper and adheres it. This method has lower resistance and therefore has a better effect in controlling temperature rise.