How Does Electric Vehicle Supply Equipment (EVSE) Work?

EVSE infrastructure encompasses everything from home EV chargers to heavy-duty public and fleet EV charging stations. Let's look at some of the key components and considerations of EVSE.

Charging Methods

Two types of charging power are prevalent:

• Alternating current (AC): EVSE supplies AC power at the connector. AC is then converted to DC inside the vehicle to charge the batteries.
• Direct current (DC): EVSE supplies DC power directly to an EV's batteries, bypassing its onboard charger.

Complementary to that is the nature of the conduit used for the power transfer:

• Conductive/wired charging: This is the most common charging option, where a charging cable connects the EVSE to the EV.
• Inductive/wireless charging: Instead of a plug-in cable, it uses inductive or resonant technology to deliver energy electromagnetically to the batteries.
• Pantograph charging: Pantographs connect a vehicle to an overhead charging line. It's used for electric heavy-duty vehicles in fleet and urban settings where quick, automated charging without manual plugging is desirable.

Charging Standards

The charging standards are comprehensive frameworks that cover several aspects of charging between an EVSE and an EV. There's no single globally accepted standard. Instead, a variety of global and regional standards exist:

• Combined Charging System (CCS): The CCS standard specifies aspects such as negotiation of charging parameters, authentication, payment, and advanced features like Plug and Charge (PnC). CCS connectors support both AC and DC charging. The CCS type 1 connector is popular in North America and type 2 in Europe and the rest of the world.
• North American Charging Standard (NACS): The NACS utilizes a single, compact connector for both AC and DC charging.
• CHArge de MOve (CHAdeMO): The CHAdeMO standard is popular in Japan and uses two separate ports for AC and DC charging.
• Guobiao Tuijian (GB/T): These standards are prevalent in China. Previously, they used separate ports for AC and DC; however, the new ChaoJi version will utilize a common port.
• Megawatt Charging System (MCS): The MCS standard enables electric heavy-duty vehicles, such as trucks and buses, to be charged quickly using high-power DC.

These standards cover the following aspects:

• Electrical parameters, including charging levels, supported voltages, currents, and power limits for different speeds of charging
• Connectors, including the shape, pinout, and mechanical locking mechanisms of the physical interfaces
• Communication protocols, including the control signals between EVSEs and EVs and between EVSEs and grids
• Safety mechanisms, such as proper grounding and insulation

Charging Levels

Charging speed is determined by the charging levels as explained below.

AC charging

The AC charging levels from the Society of Automotive Engineers (SAE) J1772 standard are integrated into CCS:

• Level 1 charging: It relies on a standard 120 volts (V) AC household outlet that can deliver around 1-2 kilowatts (kW) of power at 10-16 amps (A). It's suitable for small electric bikes, not electric cars.
• Level 2 charging: Most home chargers and public EV charging solutions use level 2 chargers. They operate at 208-240 V AC. Power ranges from 7-19 kW and current from 32-80 A.

DC charging

CCS specifies these five DC power classes:

• Low-power charging: Charge power remains below 8 kW.
• DC charging: These EVSE can deliver above 8 kW up to 50 kW.
• Fast charging (FC): FC EVSE can supply 50-100 kW for at least 30 minutes.
• Ultra-fast charging (UFC): They can deliver 100-150 kW for at least 20 minutes.
• High-power charging (HPC): HPC EVSE can supply more than 150 kW for at least 10 minutes.

These classes and power levels are also used for bidirectional charging.

Megawatt charging system (MCS)

Megawatt charging is intended for heavy-duty electric buses, semis, earth-moving machinery, aviation equipment, and even maritime vessels. It involves DC voltages of 1,000+ V and currents from several hundreds to 4,000 A. Existing MCS systems can supply 0.4-6 megawatts (MW), and future systems are being designed for 12-24 MW.

MCS currently specifies five power levels:

• MCS level 1: Level 1 EVSE uses non-cooled charging cables to deliver up to 350 A at 1,250 V.
• MCS level 2: They can supply up to 1,500 A at 1,250 V using liquid-cooled charging cables.
• MCS level 3: Level 3 EVSE uses liquid cooling for both charging cables and EV inlets to supply up to 3,000 A at 1,250 V.
• Ruggedized MCS (R-MCS): Ruggedized MCS can deliver up to six MW with currents reaching 4,000 A at 1,500 V. It's designed for harsh environments like mining, aviation, and maritime.
• X-MCS: This upcoming standard will enhance R-MCS to deliver 12-24 MW.

Communication Between EVSE and EV

The EVSE and EV communicate with each other using protocols like the International Organization for Standardization (ISO) 15118 specifications that define how to:

• negotiate charging levels
• control the charging sequence involving handshakes and authentication
• facilitate smart charging (where charging times and rates are based on real-time grid load, energy prices, and renewable energy availability)
• implement PnC with automated authentication and billing
• ensure safety through state-of-charge reporting and error handling
• communicate securely

Safety Mechanisms

Charging standards specify safety mechanisms like:

• grounding and insulation to prevent electric shocks
• ensuring that power is delivered only when a secure connection is established
• mechanisms to detect overcurrent, overvoltage, short circuits, ground faults, and abnormal temperatures, and to shut down power safely
• temperature monitoring, especially for high-power DC charging

Grid Integration

EVSEs are increasingly integrated into smart grids to implement:

• V2G (vehicle-to-grid) for bidirectional energy flow
• load balancing
• demand response
• smart charging based on grid load and renewable energy availability

Some EVSEs can be powered by solar energy and other distributed energy resources.

For safe and reliable EV charging, thorough EVSE testing is essential. EVSE testing must address the aspects outlined below:

• Interoperability: EVSE must be able to charge every EV of any manufacturer or model successfully to maintain consumer confidence in e-mobility.
• Protocol conformance: Many inoperability issues stem from communication errors. With numerous EVSE standards and advanced features, thorough protocol conformance testing is crucial for ensuring interoperability.
• Safety: EVSE testing validates safety-critical events, such as emergency shutdowns and the discharge of internal high-voltage sources. Insulation resistance testing is also a key safety check.
• Performance: EVSE testing must validate performance under fault conditions, extreme temperatures, and dynamic grid events.
• Simulation: Realistic simulations of different EVs, batteries, positive scenarios, and error conditions enable pushing systems to their limits from the early stages at low cost without risking EVSE installations or EVs.
• Automation: Test automation enables repeatable evaluation through hundreds of tests, continuously verifying interoperability and conformance.
• Electromagnetic compatibility (EMC): EVSE emissions can interfere with other electronic devices and wireless communications. EMC testing ensures that EVSEs always operate in their intended electromagnetic environment.
• Protocol security: Secure authentication and data exchange are verified by testing transport layer security, public key infrastructure for PnC, and XML security for application layer integrity and non-repudiation.
• Debugging: For debugging and root cause analysis, protocol trace viewers and man-in-the-middle analyzers enable observation, capture, and decoding of communication and power signals between an EV and EVSE.

Interoperability testing uses specialized test systems and methods to ensure seamless communication and power transfer between different grids, EVSE, and EVs. Let's look at these techniques below.

• Conformance testing: Rigorous protocol conformance verification using official test case specifications of various charging standards ensures strict adherence to the rules that govern charging negotiations. Testing negotiation and fallback behaviors ensures compatibility with both new and legacy EV models. EVSEs can be designed to support evolving standards.
• EVSE emulation: The test system emulates a configurable AC or DC EVSE to test any EV.
• EV emulation: The test system acts as a universal configurable EV that enables functional, safety, and performance testing of EVSE. It uses an electronic load and a regenerative power supply to simulate EV behavior.
• Battery emulation: The test system can emulate any EV battery capacity, technology, state of charge, or internal resistance.
• Man-in-the-middle testing: A test system is placed between a real EV and a real EVSE to passively monitor, capture, and decode the communication and power signals. This enables real-time analysis of errors and their causes during an actual charging session.
• Automated testing: Extensive libraries of automated test cases based on industry specifications can be run systematically and continuously validate behaviors, inject modified parameters, and set pass/fail conditions.
• Component-level testing: The EV communication controller (EVCC) in the vehicle and the supply equipment communication controller (SECC) in the charger are tested individually using a communication interface tester that emulates the counterpart controller and executes pre-programmed test cases. 

What’s the Difference Between EVSE Conformance Testing and Functional Testing?

EVSE conformance testing rigorously checks adherence to official charging standards and communication protocols, including its behaviors under normal, extreme, and error conditions. It's a prerequisite for type approval and certification.

Functional testing goes beyond conformance testing, verifying that an EVSE is behaving and performing as intended and expected by users.