Liquid-cooled connectors are used for carrying high power levels like those found in extreme fast charging (XFC) EV chargers. Connectors for liquid cooling are more common and used for cooling EV battery packs, cooling XFC EV charging stations, and other thermally demanding applications.

This FAQ reviews the performance and use cases for liquid-cooled connectors and connectors for liquid cooling in EVs and look at efforts to combine liquid and vapor cooling for even higher levels of thermal dissipation.

When it can be used, air cooling is the preferred solution. It combines simplicity with low cost. But it’s limited in its ability to dissipate large amounts of heat. Water-based liquid cooling systems can be up to 10 times more effective at dissipating heat. Using other liquids can further increase thermal efficiency. Liquid cooling systems can be prefabricated, sealed designs with liquid inside, and ready for installation. That can simplify initial system fabrication, maintenance, and upgrades.

Faster charging means more heat
Faster charging times are important in the wider adoption of EVs. Transferring more energy to EV batteries involves using higher voltages and higher currents. Increasing the voltage is important but is also limited. Most of the EVs on the road today have battery pack voltages of about 400 V, with 800 to 900 V battery packs representing the leading edge. The goal of XFC is to deliver up to 500 kW of charging power. Even with a 900 V battery pack, that demands lots of current and dissipates lots of heat.

In the US., the EV industry has mostly standardized on the combined charging system (CCS) connector, also called the SAE J1772 combo connector, that can support AC charging or DC fast charging equipment. Without liquid cooling, CCS connectors can support up to about 200 kW of charging power; with the addition of liquid cooling for the contacts, the power rating can be boosted up to 500 kW (500 A at 1 kV).

Liquid cooling also enables the use of smaller, lighter-weight cabling to handle high power levels. Without active cooling, the cables can become too heavy and unwieldy for users to handle.

Liquid cooling is a necessary but not sufficient condition to support 500 kW EV charging efficiently. Active thermal management, including temperature monitoring, is required in high-current EV chargers. Real-time monitoring is needed to ensure the temperature does not exceed the +50°C specification limit (Figure 2). For example, if an overload occurs or the ambient temperature rises unexpectedly (the sun comes out from behind a cloud), the system needs to be able to respond to ensure safe operation quickly. Depending on the circumstances and system design, the response can be to increase the cooling rate or decrease the charging rate to maintain the connector contact temperature rise below the +50°C limit.

Keeping EV batteries and inverters cool
There are several options for cooling EV batteries and inverters, including air and liquid cooling. Air cooling can be active or passive:

• Passive air cooling circulates air from the cabin or the vehicle’s exterior to maintain the temperature and can dissipate up to a few hundred watts.
• Active cooling can include air from an air conditioner or heater and provide up to about 1 kW of thermal management.

While air cooling is less costly, liquid can carry much higher thermal loads and is currently the dominant form of thermal management for EV inverters and battery packs. Liquid cooling systems are also available in active and passive designs. The high thermal loads of battery packs, battery chargers, and inverters generally require active cooling. Depending on the system, coolants include ethylene glycol, oils or other dielectric fluids, or water with high-performance systems using refrigerants.

EV inverter operating temperatures are higher than the operating temperatures of batteries, and the batteries can be more sensitive to excursions outside of an optimal range. During discharge, most current EV batteries must be kept between -30 and 50°C. When being charged, they need to be kept between 0 and 50°C. Especially when being charged or discharged at high rates, batteries can generate a lot of heat. Depending on the battery chemistry, temperatures above 70 to 100 °C can result in thermal runaway, causing a chain reaction that damages or destroys the battery and can result in fires and explosions.

Batteries can also get too cold and require heating to support high discharge levels. That can be especially important for high-performance EVs that guarantee some minimum rate of acceleration under all environmental conditions.

Integrated liquid cooling
The use of separate thermal management systems for EV battery packs and inverters is being replaced with integrated liquid cooling systems. Designing a single optimized cooling loop can significantly reduce the thermal management function’s size, weight, performance, and cost (SWAP-C). Key elements in integrated liquid cooling systems include:

• Quick-connect liquid connectors, hoses, and other hardware to ensure durability, reliability, and ease of maintenance.
• Cold plates optimized for the specific thermal profile of the battery pack and inverter.
• Thermal interface materials to minimize thermal resistance between the cold plate and the components being cooled.
• Highly efficient radiators or heat exchangers are sometimes supplemented with heat pipes to improve thermal performance.

Reconciling the different thermal management needs of inverters and battery packs is challenging when designing integrated liquid cooling systems. Cold plate optimization for each system is a key consideration and includes the optimization of the quick-connect liquid couplings (Figure 3).

Holistic EV thermal management
Liquid cooling is not limited to the batteries and inverters in EVs; it extends to EV charging infrastructure. As noted above, the connectors that link the EV with an XFC charging station require liquid cooling and quick-connect couplings.

Quick-connect couplings also play an important role in the design of EV charging stations. A slow 22 kW AC charger can take about 2 hours to deliver an additional 200 km range. That can be reduced to only 16 minutes with a 150 kW fast DC charging station. However, the power converter in the 150 kW fast DC charger needs to be packaged in a compact footprint and can experience a temperature rise of 200°C or more during a 10-minute charge if not properly cooled. XFC charging stations have much greater demands on thermal management systems. Liquid cooling is not an option; it’s a requirement (Figure 4).

Combining liquid and vapor cooling
So far, only single-phase cooling systems have been considered. Two-phase systems are under development that combine the sensible heat of the coolant (rise in the coolant’s temperature) and the latent heat (phase change of the coolant from liquid to vapor) to support much higher thermal loads. Various boiling techniques can be used for coolant phase change, including channel flow, pool, jet impingement, and spaying. Many are impractical for implementation in XFC EV charging cables. For the cables, a technique called subcooled flow boiling is a potential alternative.

To achieve 5-minute EV charging for large commercial vehicles is expected to require delivery of 1,400 A. most of today’s EV charger designs are rated for under 150 A, and even XFC chargers can deliver only 500  A. Delivering 5-minute charging will require the cable and connector to handle high currents and be rated for up to 2,500 A.

A prototype 2,500 A EV charging cable system has been developed that includes a pump, a tube the same size, current XFC charging cables, controls, and instrumentation. The prototype uses two-phase subcooled flow boiling to dissipate in liquid and vapor forms. The system has been proven to dissipate up to 10 times the heat compared with a single-phase liquid cooling implementation (Figure 5).

Summary
High currents are a key to faster EV charging, but high currents produce high thermal dissipation. Liquid cooled connectors and connectors for liquid cooling are key components that will enable the delivery of XFC EV charging. In addition, the design of integrated cooling systems for EV batteries and inverters can support EV SWAP-C requirements. Future generation EV chargers may employ advanced cooling technologies like two-phase subcooled flow boiling to support current levels up to 2,500 A and reduce EV charging times to 5 minutes or less.

References

Advancements in Liquid Cold Plates & Liquid  Cooling Systems for eMobility Applications, Boyd
Consolidated theoretical/empirical predictive method for subcooled flow boiling in annuli with reference to thermal management of ultra-fast electric vehicle charging cables, International Journal of Heat and Mass Transfer
Electric vehicles could fully recharge in under 5 minutes with a new charging station cable design, Purdue University
EV Battery Cooling: Challenges and Solutions, Laserax
High Power Charging, Phoenix Contact
Quick Connect Solutions for Electric Vehicles, CEJN Industrial