StartTransaction

Tells the CSMS a charging session has begun, and gets back the transactionId used for the rest of it.

Version: OCPP 1.6
Direction: Charge Point → CSMS
Feature profile: Core

StartTransaction opens a charging session. The charge point reports which connector, which token, the meter reading at the start and the time; the CSMS assigns a transactionId that identifies the session in every message that follows.

The meter value is in watt-hours, and it is the charger's lifetime register reading, not a per-session counter starting at zero.

Request

Field	Type	Required	Description
`connectorId`	integer	Required	The connector in use. Must be greater than 0 — a transaction cannot run on connector 0.
`idTag`	IdToken	Required	The identifier that authorized the session.
`meterStart`	integer	Required	Meter reading in Wh at the start of the session.
`timestamp`	dateTime	Required	When the session started.
`reservationId`	integer	Optional	The reservation this session fulfils, if any.

json

[2, "19223201", "StartTransaction", {
  "connectorId": 1,
  "idTag": "04A1B2C3D4E580",
  "meterStart": 1204500,
  "timestamp": "2026-09-09T10:22:00.000Z"
}]

Response

Field	Type	Required	Description
`transactionId`	integer	Required	Identifies this transaction in MeterValues and StopTransaction.
`idTagInfo`	IdTagInfo	Required	The authorization decision, re-evaluated at session start.

json

[3, "19223201", {
  "transactionId": 8814,
  "idTagInfo": { "status": "Accepted" }
}]

Implementation notes

A rejected StartTransaction still gets a transactionId. If idTagInfo.status is anything but Accepted, the charger must stop the session — but the CSMS has already assigned an id, and the charger must use it in the StopTransaction that follows. Implementations that skip the id on rejection break reconciliation.

meterStart is a lifetime register. Energy delivered is meterStop - meterStart, and a charger that resets the register mid-session produces a negative total.

Offline sessions. A charger that starts a transaction while disconnected queues this message and sends it on reconnect, with the original timestamp. The CSMS must accept a start time in the past.