What are the challenges with high-voltage EV charging?

With existing battery technologies, higher voltages are the key to faster charging and reduced range anxiety. Current fast charging stations can deliver up to 600 Vdc and 400 A for a total power of 240 kW and charge an EV battery pack up to 80% in about 30 minutes. But that’s still too long to ensure widespread adoption of EVs. In part, that’s limited by today’s charger standards, like SAE J1772 in North America and IEC 61851 in Europe.

This FAQ begins with a brief review of the current status of high-voltage (HV) EV charging, looks at how EV battery packs are evolving to support HV and faster charging, looks at some of the challenges related to designing charger connectors that can handle currents of 500 A or more.

HV pack challenges
There are electrical and mechanical challenges to producing higher voltage battery packs. With a given battery chemistry, like today’s lithium-ion cells, a highly serialized cell count is necessary to increase the battery pack voltage. One mechanical challenge is that for a specific vehicle type, the space available for the battery pack is limited. Scaling up to 1.5 kV from today’s 800 V requires increasing the serial cell count and decreasing the parallel cell count to try and maintain the same physical size of the pack. But it’s not that easy. EV battery packs are highly complex and changing one design element can have cascading impacts on other aspects of the design (Figure 1)

Figure 1. EV battery packs are highly complex mechanical and electrical systems (Image: Volkswagen).

When moving from 800 V to 1.5 kV, increased clearance and creepage distances are needed to avoid insulation failure/partial discharge and meet the battery safety requirements of UL 2580. The increased spacing increases the ‘unused’ volume in the pack and can result in fewer cells fitting into the pack, reducing the effective energy storage, and potentially defeating the purpose of HV.

Another consideration is the need for HV contactors. Contactors are used in the inrush circuit to minimize inrush currents from the battery when the EV starts up and to disconnect the pack for safety when the EV is being serviced. Automotive-rated 1.5 kV contactors are larger and heavier than 800 V devices. More weight negatively impacts the EV range, providing another challenge to the development of HV packs.

High-current charging connectors
Designing HV chargers and batteries has its challenges. So does designing the connectors needed to deliver the power from the charger to the battery pack. One possible solution is to use larger conductors in the connectors to carry more current. That’s not necessarily a practical alternative. First, the larger conductors are heavier and less flexible and bring ergonomic challenges.

Figure 2. This liquid-cooled CCS connector can handle 500 A (Image: Phoenix Contact).

Second and more importantly, there’s already a connector standard for HV DC charging called the combined charging system (CCS). The CCS system is designed to enable a single inlet on an EV to handle either AC or DC charging as needed. Using a dedicated inlet for HV DC charging would increase EV weight, complexity, and cost. Instead, liquid-cooled CCS connectors have been developed that can handle 500 A at 1 kV and deliver 500 kW of charging power with a minimal temperature rise. The solution for 1.5 kV packs and charging infrastructure is still under consideration.

Summary
Next-generation HV EV charging is seen as the key to overcoming range anxiety and enabling widespread EV adoption. EV battery packs are highly complex electrical and mechanical systems. There are numerous challenges to overcome related to battery pack design, integration, and charging before 1.5 kV solutions become available.

References