A conventional EV charging session involves no communication more complex than a pilot signal indicating that the vehicle is connected and ready to accept power. The EVSE supplies AC or DC power and the vehicle's onboard charger manages the rest. This model is simple, robust, and entirely inadequate for the requirements of modern smart charging infrastructure.
Grid operators need to dynamically adjust charging rates based on grid load. Fleet operators need to authenticate vehicles and bill automatically without manual interaction. Vehicle owners need to schedule charging for off-peak tariffs and return power to the grid during peak demand. ISO 15118, the international standard for communication between electric vehicles and charging infrastructure, provides the protocol architecture that makes all of this possible.
What ISO 15118 Defines
ISO 15118 is organised as a multi-part standard covering the complete communication stack between an EV and an EVSE. The key parts relevant to embedded implementation are Part 1 covering general information and use case definition, Part 2 covering the network and application protocol over wired Power Line Communication, Part 20 covering the next-generation protocol with enhanced V2G and wireless support, and Part 3 covering the physical and data link layer for wired PLC communication.
The Plug and Charge Mechanism
Plug and Charge (PnC) is ISO 15118's most commercially significant feature. When a PnC-capable EV connects to a PnC-enabled EVSE, the following occurs automatically without any driver interaction: the physical connection triggers a Power Line Communication link establishment; the EV and EVSE perform a V2G communication session setup including TLS handshake and protocol version negotiation; the EV presents its contract certificate to the EVSE; the EVSE validates the certificate against the V2G Root CA and authorises the charging session; and at session end, billing is captured against the authenticated contract without the driver presenting a card or app.
ISO 15118 Communication Stack Architecture
| Layer | Technology | Role |
|---|---|---|
| Physical / Data Link | HomePlug GreenPHY (PLC) or Wi-Fi | Physical communication over charge cable or wireless |
| Network / Transport | IPv6, TCP | IP connectivity between EV and EVSE |
| V2G Transport | V2G TP (Part 2) | Session management and message framing |
| Application | EXI-encoded XML messages | Charging service exchange, authentication, scheduling |
The application layer uses Efficient XML Interchange (EXI) encoding, a binary XML format that reduces message size significantly compared to text XML, important for the constrained bandwidth of Power Line Communication channels. Embedded teams implementing the application layer must handle EXI encoding and decoding correctly for all defined message types, which number several dozen across the full ISO 15118-2 and 15118-20 message catalogues.
V2G and Bidirectional Charging Under ISO 15118-20
ISO 15118-20, published in 2022, extends the protocol to support bidirectional energy transfer, enabling EVs to return energy to the building (V2H) or grid (V2G) during peak demand periods. From an embedded software perspective, bidirectional charging adds scheduling complexity to the charging session. ISO 15118-20 defines Dynamic Charging Mode, in which the EVSE updates the power schedule throughout the session in response to changing grid conditions, and Scheduled Charging Mode, in which a fixed schedule is agreed at session start.
The embedded BMS and charging ECU must expose the battery's current state of charge, maximum charge and discharge rates, and departure time parameters accurately to the ISO 15118 application layer, creating a tight integration requirement between the charging protocol stack and the vehicle's battery management software.
Security Architecture: TLS, PKI, and Contract Certificates
ISO 15118's security model is based on TLS mutual authentication using X.509 certificates. Both the EV and EVSE must hold valid certificates and must validate each other's certificates against the V2G Root CA before the charging session proceeds. This certificate chain involves the V2G Root CA at the top, OEM Provisioning Service Certificates for vehicle OEMs, Mobility Operator Certificates for charging service providers, and Contract Certificates issued to individual vehicles representing specific charging contracts.
For embedded EVSE developers, the security implementation requirements include TLS 1.2 or 1.3 with the cipher suites specified in ISO 15118, certificate storage in a secure element or HSM, online certificate status checking or cached CRL validation, and certificate update mechanisms for maintaining a current contract certificate throughout the vehicle's life.
Implementing ISO 15118-Ready Embedded Software with RAPIDSEA
RAPIDSEA supports ISO 15118-enabled EVSE and EV charging ECU development through its bootloader and OTA update infrastructure, protocol communication foundations, and hardware-abstracted embedded platform support. The bootloader suite enables the secure firmware update path that EVSE controllers require throughout their operational life — including certificate pool updates, software maintenance releases, and security patches. A/B redundancy and anti-rollback protection ensure that EVSE units in the field can be updated without bricking the charging station during a firmware delivery failure.
RAPIDSEA's hardware abstraction layer and MISRA-C compliant firmware foundations support Renesas, NXP, Infineon, and STMicroelectronics MCU families commonly used in EVSE controller designs, providing a portable embedded software base on which ISO 15118 application stacks can be integrated.
Conclusion
ISO 15118 is the communication backbone of smart EV charging infrastructure, enabling Plug and Charge automation, dynamic grid-responsive charging schedules, and bidirectional V2G energy services. For embedded developers building EVSE controllers and vehicle-side charging ECUs, correct implementation of ISO 15118's layered protocol stack, EXI message encoding, and TLS-based security model is the technical foundation upon which all of these capabilities rest.
RAPIDSEA provides the embedded firmware infrastructure — secure bootloader, OTA update delivery, and hardware-portable HAL — that EVSE controller and EV charging ECU projects require alongside their ISO 15118 application stack.
Ready to discuss embedded software for your EVSE or EV charging ECU project? Contact our team to request an evaluation build or book a technical demo.