DC fast charging：What you need to know

As an environmentally friendly and economical mode of transportation, electric vehicles (EVs) are becoming increasingly popular. Charging is a crucial aspect of EV usage. For many owners, the priority is to charge their vehicles quickly and efficiently. DC charging (Direct Current charging) is an ideal solution to this need. This article provides detailed information about DC charging to help EV owners better understand DC battery charger facilities.

Principles and Applications of DC Charging for Electric Vehicles

Principles of DC Battery Charging

Electric vehicle batteries store energy as direct current (DC), while the power grid supplies alternating current (AC). An onboard charger in the vehicle converts AC to DC to charge the battery. DC fast charging bypasses the onboard charger and directly supplies DC to the battery, significantly reducing charging time. The AC to DC conversion occurs within the DC charging station, which then outputs DC to the vehicle. DC fast charging, also known as DCFC (Direct Current Fast Charging) or Level 3 charging, offers a faster charging experience compared to AC charging.

DC Fast Charging Connectors

In the United States, two main types of DC fast charging connectors are used: CCS (Combined Charging System) and CHAdeMO.

• Combined Charging System (CCS): CCS integrates single-phase AC, three-phase AC, and DC high-speed charging capabilities into one system. It includes connectors and inlets and manages communication between the EV and the infrastructure, providing a comprehensive charging solution.
• CHAdeMO: This DC charging standard allows seamless communication between the car and the charger. Established by the CHAdeMO Association, it ensures compatibility through certification. The association, founded in Japan, has members worldwide, including a European branch in Paris.

Advantages and Disadvantages of DC Charging

Advantages:

1. Fast Charging Speed: It is currently the fastest charging method available.
2. Ideal for Long-Distance Travel: Reduces charging time during trips.
3. Perfect for Quick Stops: Suitable for brief but necessary charging sessions.

Disadvantages:

• Vehicle Current Limitations: Not all vehicles can fully utilize the maximum charging speed. The vehicle's acceptance rate, the maximum power it can absorb (measured in kW), varies by model. For example, the Chevrolet Bolt EV has an acceptance rate of 55 kW, meaning it charges at the same rate regardless of whether the DC fast charger is rated at 62.5 kW, 150 kW, or 350 kW. Conversely, a Porsche Taycan, with an acceptance rate of 270 kW, will only charge at 150 kW at a 150 kW station.

DC fast charging rates and EV acceptance rates are intertwined. Here are some data tables that demonstrate the relationship:

• Higher Costs: DC fast charging is more expensive. Pricing varies by location, station, and network provider (e.g., ChargePoint, Electrify America, EVgo). For instance, Electrify America charges 31 or 43 cents per kWh, depending on membership status, while ChargePoint in Tarboro, North Carolina, charges 27 cents per kWh.
• Heat Generation: High charging rates generate heat, which can affect battery health. Although DC fast charging can impact battery lifespan, the effect is minimal and non-damaging. Research by the Idaho National Laboratory indicates that the rate of capacity decline for EV batteries using DC fast charging is comparable to those using Level 2 AC charging. Each EV battery has a Battery Management System (BMS) to prevent damage by controlling the charging rate and monitoring temperature.

Benefits of DC Electric Vehicle Charging Stations

Classification of DC Electric Vehicle Charging Stations

Charging stations are classified by the voltage level of the power supply. Level 1 chargers connect to a 120-volt AC supply, Level 2 to a 240-volt AC supply, and Level 3 to a 480-volt DC supply. Higher voltage levels correspond to greater power output potential.

Charging stations are categorized into different levels depending on the supply voltage:

Working Principles of DC Electric Vehicle Charging Stations

DC fast charging (DCFC) stations directly supply DC to the EV battery, offering much faster charging than traditional AC charging. The basic process includes:

1. Power Supply: Receiving AC power from the grid.
2. AC to DC Conversion: The station’s rectifier converts AC to DC.
3. Communication and Control: Interaction between the EV and the station determines the optimal charging rate.
4. DC Power Supply: Transmitting DC power to the EV battery through the charging connector.

Fast Charging Process

Once connected to a DC charging station, the charging process involves several stages:

1. Initial Stage: The EV communicates with the station to determine the current state of charge (SOC) and acceptance capacity.
2. High Power Stage: At low SOC, the station charges the battery at maximum power.
3. Deceleration Stage: As SOC reaches about 80%, charging power decreases to protect the battery.
4. Completion Stage: Charging speed further reduces until the battery is fully charged.

Factors Affecting DC Fast Charging Speed for EVs

1. Charging Station Rate: The power output of the station affects the charging speed. DC fast chargers range from 15 kW to 350 kW, with megawatt chargers under development. Generally, higher kW means faster charging; however, the EV's acceptance rate can limit the actual charging speed.
2. EV Acceptance Rate: Different models have varying capabilities to accept charging speeds. The acceptance rate of an EV is the maximum power it can intake, also measured in kW. When a DC fast charger is connected, the EV’s battery management system communicates this information to the station. For instance, an EV with an acceptance rate of 50 kW will charge at the same rate on both 50 kW and 350 kW chargers. Conversely, a Porsche Taycan with a 270 kW acceptance rate will only charge at 150 kW if the station's maximum output is 150 kW.
3. DC Fast Charging Curve: The charging rate decreases gradually during the process to protect the battery and extend its lifespan. The DC fast charging curve is another critical variable in determining EV charging time. Each EV model has a unique charging curve dictating how much power it can consume throughout the charging process. Typically, EVs charge at maximum rate for only a portion of the cycle. Once communication with the DCFC station is established, the EV quickly reaches peak charging speed; then, as the battery charge increases, it gradually reduces power intake. When the EV battery is charged to 80% capacity, the power intake drops significantly. Most manufacturers and studies recommend charging to 80% capacity to extend battery life and allow other EV drivers to use the station, as the charging rate from 80% to 100% is considerably slower.

The expansion of DC charging stations is growing to meet market demand, providing more efficient and convenient charging solutions for EV owners. This expansion not only promotes the adoption and development of EVs but also enhances their practicality and accessibility. We believe that with the continued proliferation of DC charging stations and advancements in technology, electric vehicles will become the preferred mode of transportation for an increasing number of people.