During the V2GTP (Vehicle-to-Grid Transport Protocol) session setup over PLC (power line communication) or HLC (high-level communication), the EV initiates a SupportedAppProtocolReq message listing the ISO 15118-20 namespace as a prioritized entry (e.g., 'urn:iso:std:iso:15118:-20:AC' for AC bidirectional or 'urn:iso:std:iso:15118:-20:DC' for DC); the EVSE responds with SupportedAppProtocolRes selecting a matching protocol.
In the SessionSetupReq / SessionSetupRes exchange, the EV and EVSE establish a session ID; proceed to AuthorizationSetupReq to determine whether Plug & Charge (PnC) or External Identification Means (EIM) authorization will be used.
After authorization, the EV sends a ServiceDiscoveryReq; the EVSE includes BPT services (service ID for AC_BPT or DC_BPT) in its ServiceDiscoveryRes — if the EVSE does not offer a BPT service, fall back to unidirectional charging.
The EV selects the BPT service in ServiceSelectionReq, then exchanges ChargeParameterDiscoveryReq and ChargeParameterDiscoveryRes to share EVSEMaximumDischargePower, EVSEMinimumDischargePower, EVSEMaximumDischargeEnergy, and corresponding EV constraints.
Depending on negotiated control mode (Scheduled or Dynamic), proceed to ScheduleExchangeReq/Res (Scheduled) or invoke DynamicChargeLoopReq/Res (Dynamic) for ongoing power setpoint negotiation during the session.
On session end, send PowerDeliveryReq with chargeProgress 'Stop' before transitioning to SessionStopReq/Res to cleanly close the session.
Known gotchas
ISO 15118-20 defines separate physical-layer bindings for AC and DC BPT; AC bidirectional typically targets V2H/V2B applications while DC is used for V2G grid export — confirm the EVSE hardware supports the correct binding before attempting protocol negotiation.
The SupportedAppProtocolReq priority field is an integer: lower values mean higher priority; if an EV lists both 15118-2 and 15118-20, the EVSE will select the highest-priority match it supports, which may fall back to 15118-2 (unidirectional only).
ISO 15118-20 is a paid ISO standard; implementers must purchase it through their national standards body — publicly available drafts may differ from the published text, and implementations should reference the normative published edition.
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp