How do status and meter values travel?
Eight minutes into the session. The app shows the energy rising every minute.
Behind it, the charger sends a meter value each minute and reports a change of state whenever one happens.
The questionWhat does the charger send during a session, and how can a driver trust the numbers?
A composed session. The curve flattens as the battery fills and the power falls; each minute, the charger reports the meter's register value, and the energy is the difference between readings.
Two kinds of meter values
sends meter values on a periodic sampling basis, clock-aligned, or both.
- Sampled during a transaction, for example every minute, so the backend and the app can follow the session.
- Clock-aligned, for example every quarter of an hour, independent of sessions, useful for energy management and billing of the site.
Readings a driver can check
Where measuring law requires it, as in Germany, the meter signs its readings. OCPP transports these from the charger to the backend; at minimum, a signed start and a signed end value are needed. The meter's public key belongs to the meter itself and changes only when the meter is replaced, so a driver or operator can check the readings against it.
Status as a timeline
Each connector reports a change of state: available, preparing when a car is plugged in, charging, suspended when the car pauses, finishing, faulted. Strung together, these reports show what happened at the connector minute by minute, and they feed the availability that apps show.
Check your understanding
3 questions. Answer all of them to complete this stop. Each answer explains itself.
How is the energy of a session determined?
The meter's register counts up; the session's energy is end reading minus start reading.
What are clock-aligned meter values for?
They arrive at fixed intervals, for example every quarter hour, whether a car is charging or not.
Why does a meter sign its readings?
The signature and the meter's public key let anyone check that a reading is authentic and unaltered.
- OCPP Open Charge Point Protocol: the open protocol between a charger and its operator's backend. EV charging fundamentals, stop 03 Glossary
- Signed meter value A meter reading signed by the meter itself, so drivers and operators can check that it is authentic and unaltered. Glossary
- Open Charge Alliance: What is new in OCPP 2.0.1? Whitepaper, version 1.0, March 2023. https://openchargealliance.org/wp-content/uploads/2024/01/new_in_ocpp_201-v10.pdf. Checked 09 Oct 2026. Licence: CC BY-ND 4.0.
- Open Charge Alliance: Signed Meter Values (Eichrecht paper), version 1.0, February 2025. https://openchargealliance.org/wp-content/uploads/2025/05/signed_meter_values-v10-1.pdf. Checked 09 Oct 2026. Licence: CC BY-ND 4.0.
- Open Charge Alliance: Signed Meter Values (Eichrecht paper), version 1.0, February 2025. https://openchargealliance.org/wp-content/uploads/2025/05/signed_meter_values-v10-1.pdf. Checked 09 Oct 2026. Licence: CC BY-ND 4.0.