What is the difference between AC and DC EV charging?

EV charging is highly standardized, and the standards are primarily focused on safety and connectivity. The standards between regions. One common thread is the availability of separate standards for AC and DC charging. This FAQ reviews the basic electrical architectures of AC and DC charging, looks at different charging modes and levels, and closes by presenting the EV charging connectors found in each regional market.

Of course, it takes DC power to charge batteries. The difference between AC and DC charging is the source of the DC power. With AC charging, AC power from the grid flows directly into the EV. With DC charging, an external battery charger feeds DC power to the vehicle. AC chargers can include safety devices but no power conversion electronics. There’s an onboard charger (OBC) that converts the AC to DC for battery charging. With DC charging, the DC bypasses the OBC and is fed directly to the battery pack (Figure 1).

Figure 1. AC charging uses an OBC that’s bypassed by DC charging (Image: EV Charge).

Modes and levels
Charging modes and levels are used to categorize EV chargers. Modes describe the method of power delivery, protection, communication, and control of the charger. Levels describe the rated power in terms of voltages and currents. The most common standards including SAE J1772 in North America and IEC 61851 in Europe use different definitions of modes and levels.

SAE J1772 recognized three modes or levels:

Level 1 is basic AC charging. It uses single-phase 120 Vac to deliver up to 1.9 kW in residential and similar settings.
Level 2 uses single phase 208/240 Vac to deliver up to about 19 kW. It’s sometimes referred to as ‘fast AC charging.’
Level 3 defines DC charging with the initial specification calling for 600 Vdc at up to 400 A for a charging power of 240 kW. More recent designs can deliver up to 1 kV and 500 A for a charging power of 500 kW.

IEC 61851-1 adds a third AC charging mode for a total of four modes. The first three are for AC charging:

Modes 1 and 2 are low-power AC charging. In mode 1, the AC power is fed directly to the EV. Mode 2 adds an in-cable control and protection device and supports higher levels of AC power, up to 15 kW.
Mode 3 is called ‘fast AC charging’ and uses an external charging station to deliver up to 120 kW to the EV. A high-level communications (HLC) connection between the external charging station and OBC for charging control is an optional feature in mode 3.
Mode 4 is fast DC charging. It requires an HLC connection to provide feedback to the external charger. Depending on the output voltage and current of the external charger, Mode 4 systems can deliver several hundred kW to the EV.

Connector standards
EV charging connectors provide connectivity for power and HCL communication. North America and Europe both use variations on the combined charging system (CCS) standard while Japan and China have developed different standards. In North America, Tesla uses a proprietary connector while in Europe, the company is adopting the CCS standard. It’s called the CCS system since a single inlet on an EV can be used to connect to AC or DC charging. With the other standards, separate inlets are needed for AC and DC charging (Figure 2).

Figure 2. Each region has its unique EV charging connector standard, and Tesla has a proprietary one (Image: Enel X).

Summary
AC charging feeds an OBC that’s bypassed with DC charging. In addition, AC charging is more restricted in the amount of power that can be delivered and therefore limited to slower charging. There are a range of different charger modes, levels, and connector standards that vary by region.

References