WebSocket

A connection that the charger opens to the backend and keeps open, so messages can travel in both directions at any time.

Explained in

OCPP in depth · Stop 02How does a charger connect to its backend?WebSocket, boot announcement, heartbeat and status: the first seconds after a charger starts, and the connection it keeps open all day.

In OCPP over JSON, the charger opens the connection: an HTTP request that is upgraded to a WebSocket, to an address that contains the charger's own identity. That suits the real world, where chargers sit behind mobile networks and routers that would not let the backend call in. Once open, the connection carries messages in both directions, so the backend can still send commands at any time.

From “Why the charger calls”, stop 02

Also in

  1. OCPP in depth · Stop 09One day of messages, fully understoodThe day in eight moments
  2. OCPP in depth · Deep diveOne session as raw OCPP messagesWhat you are looking at
  1. OCPPOpen Charge Point Protocol: the open protocol between a charger and its operator's backend.
  1. Open Charge Alliance: OCPP-J 1.6 specification (October 2015): connection URL, CALL, CALLRESULT and CALLERROR, synchronicity and time-outs; free download. https://openchargealliance.org/download-ocpp/. Checked 13 Oct 2026.