Author: Zhu Yulong

With the launch of the XPeng G9, the Chinese electric vehicle market will kick off a round of ultra-fast charging competition. The main heating mode of electric vehicles during charging is direct current charging. In the charging scene, a large amount of heat is generated in the entire high-voltage circuit, and the greater the power, the greater the heat loss. According to the law of energy conservation, heat needs to find a way to be discharged, including from the charging facility, the charging stand to the battery charging interface, and from the high-voltage contactor to the inside and surface of the battery cells.

At present, for high-voltage connectors and charging connectors under a 400V system, the charging current is controlled within 250A, and a charging cooling system is not required. The charging power is about 70 to 90KW. Therefore, liquid cooling for charging interfaces has become a standard configuration for the next-generation charging piles.

Why use charging liquid cooling technology

Compared with traditional DC charging guns and cables, charging guns and cables with liquid cooling add cooling pipelines to the charging gun, cable, and charging pile circuit, and add cooling liquid pipelines inside the cable. This can be compatible with two different requirements of 400V and 800V to ensure that the temperature rise of the charging interface does not exceed the standard.

Cooling the terminals with cooling liquid can reduce the cross-sectional area requirements of the cable, reduce the weight of the actual plug and cable, and facilitate operation.

Liquid-cooled fast charging only reduces the charging temperature. About 50% of this charging circuit from the charging pile to the charging gun relies on this system, while the other half from the vehicle-mounted charging socket to the battery fast charging port needs to withstand high currents. Therefore, we can introduce the derating curve, which is the relationship between the highest safe continuous current that the connector can bear and the external environment temperature within a certain range that matches a specific wire connector.

Here we need to consider that at higher ambient temperatures, the highest safe continuous current that the connector can bear is gradually reduced. The hottest point of the connector is the mating position of the terminals, and the source of heat includes the working environment input and connector current carrying. Assuming that the circuit resistance, energization time, and application environment are constant, the greater the current, the greater the heat generated.Therefore, when the operating temperature of the connector application environment increases, under the restriction of the cutoff temperature, the remaining temperature rise allowance that can be generated by the current carrying capacity of the connector will be lessened. With the circuit resistance and electric time unchanged, the only way to limit temperature rise is to restrict current.

Thus, we can see that we need to consider that both the charging socket and battery charging connector need to withstand relatively high currents.

Here, the electrical equivalent circuit is composed of thermal resistance and thermal capacity. The method of controlling temperature rise is to take away heat by increasing the cooling pipeline.

Within the socket, the method of reducing self-generated heat is by reducing contact resistance through changing connection methods and reliability, such as changing the terminal plating, wire and terminal crimping method (ultrasonic welding), increasing the crimping area, etc.

Difficulty

Currently, it appears that the biggest difficulty in the future lies in the connector terminals of the battery pack, because the current it carries is actually the same as the socket. Tesla has hard hit this aspect and used a custom metal-faced contact connector for the fast charging connector on the Model Y, ensuring the structural strength of this high current connection through the aluminum tube positioning, non-shielded wires, and metal material.

Currently, we have not seen car companies designing in this direction. However, with the increase of fast charging current and consideration of integrating cooling BDU, it is not impossible that there might be further changes in the latest design.

Summary: The thermal management of fast charging might see significant changes in the future – we not only need to use high voltage of 800V, but also continue to increase the current from 250A to about 500A in order to consider how to enter the 400kW stage.

This article is a translation by ChatGPT of a Chinese report from 42HOW. If you have any questions about it, please email bd@42how.com.