ISO 15118 is the international standard series that defines high-level digital communication between an electric vehicle and a charging station, covering everything from the physical signal on the charging cable to the messages that negotiate power, authenticate the driver and control bidirectional energy flow. Among charging standards, ISO 15118 is the protocol layer that sits above connector standards such as CCS and the basic signaling of IEC 61851.
The standard is best known for enabling Plug and Charge, but its scope is wider: it lets the car and charger exchange detailed charging parameters, schedules, tariffs and, in its newest edition, instructions for vehicle-to-grid operation. DC fast charging with CCS depends on it or on its predecessor DIN SPEC 70121.
The parts of the ISO 15118 series
ISO 15118 is a multi-part standard, each part covering one layer of communication or its testing. The table summarizes the parts most often referenced by vehicle makers and charger manufacturers.
| Part | Content | Practical significance |
|---|---|---|
| ISO 15118-1 | General information and use cases | Defines roles, scenarios and terminology |
| ISO 15118-2 | Network and application protocol, first generation | Basis of most chargers in service today |
| ISO 15118-3 | Physical and data link layer for wired charging | Power line communication on the cable |
| ISO 15118-4 and -5 | Conformance tests for parts 2 and 3 | Used to certify interoperability |
| ISO 15118-8 | Wireless communication layer | Wireless charging and pantograph systems |
| ISO 15118-20 | Second-generation network and application protocol | Bidirectional charging, stronger security |
How ISO 15118 communication works
The vehicle side is called the EV communication controller (EVCC) and the charger side the supply equipment communication controller (SECC). On wired chargers they talk over HomePlug Green PHY power line communication, a signal modulated onto the control pilot wire that IEC 61851 already uses for basic analog signaling.
Above the physical link, the devices use IPv6, TCP and optionally TLS. Application messages are written in XML schemas but transmitted in the compact EXI binary encoding, which keeps messages small enough for embedded controllers. The vehicle first locates the charger’s address with a discovery protocol and then runs a fixed sequence of request and response pairs.
| Phase | Example messages | Purpose |
|---|---|---|
| Session setup | SessionSetup, ServiceDiscovery | Identify both sides and available services |
| Authorization | PaymentServiceSelection, Authorization | Choose external payment or Plug and Charge |
| Parameter exchange | ChargeParameterDiscovery | Agree on voltage, current and power limits |
| Safety checks | CableCheck, PreCharge | Test insulation and match voltages |
| Energy transfer | PowerDelivery, CurrentDemand | Control current in a loop of short cycles |
| Shutdown | WeldingDetection, SessionStop | Verify contactors opened and end the session |
During energy transfer, the vehicle remains in charge of the battery. It sends a target current and voltage in each CurrentDemand request, typically several times per second, and the charger reports what it actually delivers. If the vehicle stops sending requests or a limit is exceeded, the charger must ramp down and open its contactors within defined time limits, which makes the message loop part of the safety concept rather than a mere data exchange.
ISO 15118 compared with neighboring standards
ISO 15118 is often confused with the connector and back-end standards it works alongside. Each covers a different interface in the charging chain.
| Standard | Interface covered | Relationship to ISO 15118 |
|---|---|---|
| IEC 61851 | Basic charging modes and pilot signaling | Provides the analog layer beneath it |
| IEC 62196 | Plugs, sockets and connectors | Defines the physical hardware it runs over |
| DIN SPEC 70121 | Early DC charging communication | Predecessor, still widely supported |
| OCPP | Charger to operator back end | Carries data from ISO 15118 sessions onward |
| CHAdeMO | Separate DC charging system | Uses CAN bus instead of ISO 15118 |
A useful mental model is a chain: IEC 62196 is the plug, IEC 61851 the basic handshake, ISO 15118 the conversation between car and charger, and OCPP the conversation between charger and operator.
What the second generation, ISO 15118-20, adds
ISO 15118-20, published in 2022, is a substantial redesign rather than a minor revision, and it is not backward compatible at the message level with part 2. Devices that support both editions negotiate the version at the start of each session.
- Bidirectional power transfer, allowing vehicle-to-grid and vehicle-to-home operation under charger control.
- Mandatory TLS 1.3 encryption for all sessions, not only Plug and Charge.
- Multiple contract certificates per vehicle, useful for shared and company cars.
- Dynamic and scheduled control modes, letting the grid side adjust power during a session.
- Support for wireless and automated connection systems, including pantographs for buses.
Regulators increasingly require it. European rules on alternative fuels infrastructure call for ISO 15118-20 support at new public chargers from 2027, which pushes manufacturers to update hardware and firmware.
Implementation challenges
Despite being an international standard, ISO 15118 suffers from interoperability problems in practice, because implementations interpret optional features and timing differently.
- Timing failures, where one side exceeds a message timeout and the session aborts.
- Partial implementations, for example chargers supporting part 2 without Plug and Charge.
- Certificate management effort, as root and charger certificates must be kept current.
- Slow fleet renewal, since many vehicles on the road cannot be updated to part 20.
Industry testing events and conformance testing under parts 4 and 5 help, but field testing across many vehicle and charger combinations remains the decisive step before broad rollout.
For charger operators, a practical rule is to treat firmware and certificate updates as routine maintenance. Logging the message sequence of failed sessions shows at which step a vehicle and charger disagree, which turns vague complaints about a station that will not start into a fault that a manufacturer can reproduce and fix.
A non-technical view
ISO 15118 is the shared language of electric cars and chargers. They use it to talk to each other through the charging cable. Before any power flows, the car says how much it can take. The charger says what it can deliver. Then they agree on the details.
The same language lets the car prove who is paying, with no app or card. In its latest version, the car can even send energy back to a house or the power grid. Every brand follows the same rules. So a car from one maker can charge safely at a station built by another.
A concrete example
A municipal utility runs a pilot project in which 50 electric cars of residents support the local grid during evening demand peaks. The project requires bidirectional DC wallboxes and vehicles that both implement ISO 15118-20 with bidirectional power transfer.
During the day, the cars charge from rooftop solar. Between 6 and 8 p.m., the utility’s system sends power targets through the wallboxes, which translate them into ISO 15118-20 control messages, and each car discharges up to 5 kW while keeping a minimum charge set by its owner. Owners whose cars only support the older part 2 can join for smart charging, but not for discharging, which shows in practice why the edition supported by the vehicle matters.