Many EV industry professionals and electric vehicle owners know how to plug in and start charging, but few understand the sophisticated communication process that takes place between an EV and a DC fast charger.
ISO 15118 is an internationally recognized communication standard for interactions between electric vehicles and charging stations. In this article, we break down the complete ISO 15118 communication process step by step, using a detailed sequence diagram to explain how EVs and DC fast chargers communicate throughout the charging session.
1. Basic Concepts: EVCC and SECC
Two key communication components are involved in the DC fast charging process.
EVCC (Electric Vehicle Communication Controller)
Installed inside the electric vehicle, the EVCC manages communication between the vehicle and the charging station.
SECC (Supply Equipment Communication Controller)
Installed inside the charging station, the SECC manages high-level communication with the vehicle.
The EVCC and SECC establish an IP communication channel using Power Line Communication (PLC). Through this channel, they exchange messages to complete protocol negotiation, authentication, charging control, power delivery, and safe shutdown.
2. Basic Hardware States: State A, B, and C
- State A: The charging connector is not connected to the vehicle.
- State B: The charging connector is detected, and the physical connection is established. The high-voltage contactors remain open.
- State C: The vehicle indicates that it is ready for charging. Actual power delivery begins only after the required safety checks and control procedures.
3. Phase 1: Plug-In Detection and PLC Communication Establishment
When the charging connector is inserted into the vehicle, the charging system detects the physical connection and transitions from State A to State B.
The EVCC and SECC then establish their communication network through PLC, including SLAC association and IP communication setup.
The following communication procedures take place:
supportedAppProtocolReq/Res
The EVCC and SECC negotiate a mutually supported ISO 15118 protocol version.
SessionSetupReq/Res
A dedicated communication session is established for the charging operation.
4. Phase 2: Authentication, Service Selection, and Payment Authorization
Once the communication session has been established, the system proceeds to service discovery, payment selection, and authentication.
Step 1: ServiceDiscoveryReq/Res
The EVCC retrieves information about the charging services and additional services supported by the charging station.
Step 2: ServiceDetailReq/Res
The EVCC requests detailed information about a selected service when required.
Step 3: PaymentServiceSelectionReq/Res
The EVCC selects a supported payment option, such as external payment or certificate-based Plug & Charge (PnC).
Optional Certificate Management
Depending on the vehicle’s certificate status, additional procedures may be required.
- CertificateInstallationReq/Res: Installs a contract certificate when necessary.
- CertificateUpdateReq/Res: Updates an existing contract certificate.
Step 4: PaymentDetailsReq/Res
For certificate-based payment, the vehicle provides the required contract certificate information.
Step 5: AuthorizationReq/Res
The charging station processes the authorization request.
Once authorization is successful, the charging session can proceed to charging parameter negotiation.
Note: The specific authentication procedure depends on the selected payment method. Not every charging session requires certificate installation, certificate updates, or PaymentDetails messages.
5. Phase 3: Charging Parameter Negotiation and High-Voltage Safety Checks
Step 1: ChargeParameterDiscoveryReq/Res
The vehicle communicates its charging requirements and battery limitations to the charging station.
These parameters may include:
- Maximum battery voltage
- Maximum charging current
- Battery operating limits
- Other applicable charging constraints
The charging station responds with its supported charging capabilities.
Step 2: Transition to State C
The vehicle indicates that it is ready for charging.
Before DC power delivery begins, the charging system performs the required safety procedures.
Step 3: CableCheckReq/Res
The cable check procedure verifies the insulation condition of the high-voltage charging system.
The EVCC and SECC may repeat this request-response sequence until the required conditions are satisfied.
Step 4: PreChargeReq/Res
During pre-charging, the charging station adjusts its output voltage to match the vehicle battery voltage.
This process minimizes inrush current and helps protect the vehicle’s high-voltage components.
Step 5: PowerDeliveryReq/Res
Following successful safety checks and pre-charging, the vehicle requests the start of power delivery.
The charging station coordinates the required switching operations. High-voltage charging begins once all necessary safety conditions have been satisfied.
6. Phase 4: Continuous Charging and Dynamic Power Regulation
During the charging process, the EVCC and SECC continuously exchange messages to regulate charging performance.
Step 1: CurrentDemandReq/Res
The vehicle periodically communicates its current charging requirements to the charging station.
The charging station dynamically adjusts its output voltage and current according to the vehicle’s requests and its own operating limits.
This allows the charging system to respond to changing battery conditions, including:
- Battery temperature changes
- Increasing State of Charge (SOC)
- Battery voltage variations
- Charging power reduction
Step 2: Optional MeteringReceiptReq/Res
When applicable, the EVCC and SECC exchange metering receipt information.
This procedure supports charging energy measurement verification and transaction records.
7. Phase 5: Charging Termination and Safe Power Shutdown
Step 1: PowerDeliveryReq/Res
When the vehicle reaches its target charging level or the user manually stops charging, the EVCC requests the termination of power delivery.
The charging system initiates the shutdown sequence.
Step 2: High-Voltage Disconnection
The charging station stops power delivery and coordinates the required switching operations.
The high-voltage output is disconnected, and the vehicle returns to the appropriate non-charging control pilot state.
Step 3: WeldingDetectionReq/Res
The welding detection procedure checks whether the relevant high-voltage contactors have opened correctly.
It helps identify potential contactor welding faults before the charging connector is released.
Step 4: SessionStopReq/Res
The EVCC and SECC exchange session termination messages, formally ending the charging session.
8. Complete ISO 15118 DC Charging Communication Sequence
The complete communication process can be summarized as follows:
- Plug-In Detection
- PLC Communication Establishment
- Protocol Negotiation
- Communication Session Setup
- Service Discovery
- Payment Selection and Authentication
- Charging Parameter Negotiation
- Cable Check
- Pre-Charging
- Power Delivery
- Dynamic Charging Control
- Safe Power Shutdown
- Contactor Welding Detection
- Session Termination
Conclusion
ISO 15118 provides the communication framework that enables electric vehicles and charging stations to exchange information throughout the charging process.
From initial connection and authentication to dynamic power regulation and safe shutdown, each communication procedure serves a specific operational or safety function.
The standard also provides a foundation for advanced charging technologies, including Plug & Charge (PnC), automated charging systems, and Vehicle-to-Grid (V2G) bidirectional charging.
Technical Note: This article primarily describes the ISO 15118-2 DC charging sequence using the Extended Message Set. Certain procedures are optional or conditional. ISO 15118-20 introduces a different message structure and additional functionality.
