EV Charging Protocols and Services

Charging can fail even when the connector fits and both sides can handle the same voltage. A complete session also depends on signalling, digital protocols, credentials, operator systems, roaming links, and payment services.

The communication layers

EV charging uses several interfaces with different jobs:

Vehicle to EVSE: basic signalling establishes that a vehicle is connected and communicates safe current limits. High-level communication can then exchange charging parameters, identification, certificates, schedules, and energy-transfer messages.
EVSE to management system: the charging station reports status, receives configuration, checks authorization, records transactions, and may receive charging limits from a central system.
Operator to mobility provider: roaming connections exchange locations, availability, tariffs, credentials, sessions, and charge records between organizations.
Contract and payment services: accounts, certificates, bank-card processing, invoices, and settlement determine who may charge and who is billed.

These layers are related but not interchangeable. ISO 15118 primarily covers high-level communication between the vehicle communication controller and the supply-equipment communication controller. OCPP connects a charging station with its management system. Roaming protocols connect charging operators, mobility providers, or hubs.

The electrical power path is covered in EV Charging: The Complete System, while connector geometry and regional interfaces are covered in EV Charging Connectors and Inlets.

What happens before and after power flows

A typical conductive session passes through several checks:

The connector and inlet mate, and proximity or latch circuits indicate the connection state.
Basic signalling tells the vehicle how much current the EVSE can make available and confirms that charging may start safely.
Where supported, high-level communication establishes a digital session and exchanges capabilities and charging parameters.
The driver or vehicle presents a credential. A local list or backend decides whether that credential is authorized.
The station and vehicle complete their safety sequence. During DC charging, they also coordinate voltage and current before closing the high-voltage path.
Meter values, status, and transaction events are recorded while energy moves.
When the session stops, the operator produces a charge record and the responsible service bills or settles the transaction.

Not every session uses every network layer. A private wallbox may authorize locally and operate without a live cloud connection. A public roaming session may involve the vehicle, station, charge point operator, mobility provider, roaming platform, and payment processor.

Authentication, authorization, and payment

These three terms describe different decisions:

Authentication checks the identity or validity of a credential, such as an RFID token, app account, bank card, or vehicle certificate.
Authorization decides whether that credential may start a session at that charge point under the applicable contract or ad hoc terms.
Payment and settlement transfer money and allocate the revenue and fees after or during the session.

An app can authenticate a driver yet fail authorization because the station is outside the provider's roaming coverage. A bank card can support ad hoc payment without creating a mobility-provider contract. European rules usefully distinguish ad hoc charging from contract-based payment and define separate operator and mobility-provider roles. EU Regulation 2023/1804 — Alternative fuels infrastructure

Plug & Charge and ISO 15118

Plug & Charge allows a compatible vehicle to present a contract certificate through the charging connection so authentication and authorization can occur without a separate card or app action. The certificate chain and supporting public-key infrastructure help the parties decide whether the contract credential can be trusted. CharIN — Plug & Charge

Plug & Charge is not a synonym for every automatic start method. Some systems identify a vehicle through a network account or another vehicle identifier without using the ISO 15118 certificate model. Those methods can feel similar to the driver while having different security, privacy, and interoperability properties.

ISO 15118 is also broader than Plug & Charge. The multi-part series defines use cases and communication between the EV and EVSE for functions that include charge control, identification, payment-related information, scheduling, and bidirectional energy transfer. ISO 15118-2 and ISO 15118-20 represent different generations of the application protocol, so a claim of “ISO 15118 support” should identify the implemented part and functions. ISO 15118-1:2019 — General information and use-case definition ISO 15118-2:2014 — Network and application protocol requirements ISO 15118-20:2022 — Second-generation network and application requirements

The 2026 amendment to ISO 15118-20 adds AC distributed-energy-resource and Megawatt Charging System services together with a revised security concept. Publication does not make older vehicles or stations compatible with those functions. ISO 15118-20:2022/Amd 1:2026 — AC DER, MCS, and security amendment

OCPP, roaming, and operator roles

The Open Charge Point Protocol (OCPP) is used between a charging station and a charging-station management system. It can carry authorization, transaction, device-management, monitoring, smart-charging, and ISO 15118 support functions. OCPP support does not prove that a car and station can use Plug & Charge because the vehicle interface, certificates, backend integrations, and mobility contract must also work. Open Charge Alliance — Open Charge Point Protocol

A charge point operator (CPO) manages and operates charge points and provides the charging service. An e-mobility service provider (eMSP) sells charging services or contracts to drivers. One company can perform both roles, but roaming often connects a driver's eMSP to a different CPO.

The Open Charge Point Interface (OCPI) is one protocol used for that operator-to-provider exchange. It can carry locations, tariffs, credentials, sessions, charge detail records, and commands. Other bilateral or hub protocols can fill a similar role, so roaming support must be checked between the specific providers rather than inferred from the connector or car. EVRoaming Foundation — Open Charge Point Interface

A reservation adds another chain of dependencies. The app or provider must request it, the operator backend must accept it, and the station must enforce it against other users. A reservation can reserve access to a charge point, but it cannot guarantee that the hardware is healthy, reachable, or able to deliver its advertised power when the vehicle arrives.

Why compatible systems still fail

The failure location is often different from the visible symptom:

A connector or lock fault stops the physical handshake.
A signalling or protocol mismatch prevents the vehicle and EVSE from agreeing on a session.
An expired or untrusted certificate blocks Plug & Charge.
A station-to-backend outage prevents online authorization even though the power hardware works.
A roaming agreement lacks the required credential, tariff, or location data.
The session starts, but payment or charge-record processing fails later.

Drivers should therefore check more than the plug type. Useful questions are whether the vehicle and station support the same ISO 15118 functions, whether the network enables Plug & Charge for that vehicle and contract, whether the driver's provider roams with the operator, which ad hoc payment method is available, and what happens when the station loses connectivity.

Public fast-charging site design, power sharing, reliability, and route-planning consequences are covered in DC Fast Charging.