What is DC Fast Charging?

When consumers want to buy an electric vehicle, the time it takes to charge it continues to influence their purchasing decisions. Today, the fastest way to charge an electric vehicle is high-power direct current (DC) fast charging. DC fast charging is different from AC charging and requires specific considerations from OEMs when designing key components of the vehicle's electrical architecture, such as the charging inlet.

The slower charging options (AC Level 1 and AC Level 2) use AC power, which is how electricity is typically delivered from the grid and how it is delivered to homes. AC charging rates can range from 12A to 80A according to the SAE J1772 standard in North America, while charging rates elsewhere vary according to regional standards such as IEC 62196 or GB/T 20234. When EV owners charge their vehicles at home, they plug the EV charger into the vehicle's charging port, and the EV's onboard charger converts the AC power to DC to charge the battery.

DC fast charging is the most common charging method used at commercial public charging stations. These charging stations convert AC power from the grid to DC power, so when owners charge their electric vehicles, the DC power flows directly to the battery. According to the J1772 standard, there are two levels of DC fast charging: DC Level 1 has a charging rate of up to 80A, while DC Level 2 (commonly known as Level 3 charging) has a rate of up to 500A.

The difference in time between AC charging and DC fast charging is huge. AC Level 1 charging has a power output of up to 1.4 kW at 12A and 120VAC, while AC Level 2 charging can provide up to 19.2 kW at 80A and 240V. In theory, even using the highest power level of 19.2 kW for AC charging, it takes about five to six hours to fully charge a 100 kWh battery pack. In contrast, using DC fast charging at 500A and 400VDC, the same 100 kWh battery theoretically takes about 30 minutes to reach full capacity. With the advancement of battery technology, the upgrade of 800V architecture, and the improvement of charging infrastructure, charging time will be further reduced.

Considerations

Several factors affect the speed of DC fast charging, including the following:

Battery Pack Limitations:

Each EV battery has a specific acceptance rate, measured in kilowatts, that reflects how quickly it can store electrical energy. For example, if a DC fast charger is rated at 150 kW, but the vehicle's battery can only accept 100 kW, it will definitely take longer to charge. The battery's acceptance rate can vary based on several factors, including battery chemistry, design, size, and age. Many OEMs are looking to upgrade their battery pack architecture from 400V to 800V to maximize the acceptance rate and reduce charging time.

temperature:

Extreme ambient temperatures, both high and low, can reduce the charge rate or even make the battery completely unacceptable for
charging.

Charging status:

Generally speaking, the acceptance rate of electric vehicle batteries is highest when the battery is at a low state of charge (SoC), and decreases as the battery approaches its maximum SoC. Most electric vehicles follow a clear current curve, and the charging rate slows down significantly as the vehicle approaches full charge, especially once it reaches 80%, in order to reduce the risk of overcharging and damaging the battery.

Limitations of charging infrastructure:

Most DC fast-charging stations on the market today have a maximum charging rate of about 200 A to 300 A. While the number of 500-A DC fast-charging stations is growing, they are expensive and complex to install.

fever:

Due to its high current, DC fast charging generates heat throughout the EV charging system - from the EV charger to the inlet, from the high-voltage cables to the battery connections and the battery itself - and this heat must be managed. Standards such as J1772 and IEC 62196 specify a maximum operating temperature limit of 90°C at the interface between the charging gun and the charging socket to ensure touch-safe conditions when consumers operate the equipment. If the temperature approaches this limit, the charge rate will have to be reduced, so managing and monitoring this interface is key to charging performance.

The challenge for OEMs is to provide a charging interface that is flexible enough to accommodate AC charging and DC fast charging options in different regions, capable of detection and heat dissipation, and in a design that is easy to service.