OCPP for EV charging networks

One operator can run chargers from different manufacturers on one platform because both ends of the link speak the Open Charge Point Protocol (OCPP). OCPP 1.6 calls the managing system the Central System; OCPP 2.0.1 calls it the CSMS. The Open Charge Alliance (OCA) publishes OCPP, and OCPP 2.0.1 is also the international standard IEC 63584:2024.

Versions

OCPP 2.0 was published in 2018 and superseded by 2.0.1 in 2020. If a datasheet says "OCPP 2.0", ask the manufacturer which version the firmware implements.

The version is agreed when the connection opens. The charger offers a WebSocket subprotocol (ocpp1.6, ocpp2.0.1 or ocpp2.1) in the handshake, and the CSMS accepts one. OCPP 2.0.1 is not an extension of 1.6, so a 1.6 charger cannot connect to a CSMS that accepts only 2.0.1. Many installed chargers still run 1.6, and most platforms run several versions side by side. OCPP 2.1 is backward compatible with 2.0.1 functionality.

What OCPP carries

The charger opens the WebSocket to the CSMS and exchanges JSON messages.

The CSMS sets the heartbeat interval in its BootNotification response. The meter value interval is a charger setting (MeterValueSampleInterval in 1.6). Both are set per deployment, so read them from the charger instead of assuming a value.

Connector states differ between versions. OCPP 1.6 reports states such as Charging, SuspendedEV and SuspendedEVSE. OCPP 2.0.1 reports only Available, Occupied, Reserved, Unavailable or Faulted, and the charging state moves into TransactionEvent.

Smart charging

The CSMS sends a charging profile to a charger with SetChargingProfile. A profile holds a schedule: a chargingRateUnit of A or W, a list of chargingSchedulePeriod entries, and optional validFrom and validTo times. For AC charging, numberPhases gives the number of phases and defaults to 3. A limit in amps applies to each phase. A tariff shift is a schedule with a low limit in the expensive hours and a higher one after them.

Each profile has a purpose, which sets what it limits:

Within one purpose, the valid profile with the highest stackLevel wins. Across purposes, the charger applies the lowest of the station maximum, the external constraint and the transaction limit. The transaction limit is the TxProfile, or the TxDefaultProfile if there is no TxProfile. Send GetCompositeSchedule after you install profiles: the reply is the limit the charger will actually apply.

In OCPP 2.0.1, a charger can receive an external limit on a local interface. It then reports the limit to the CSMS with NotifyChargingLimit, and its removal with ClearedChargingLimit.

Profiles act on one charger at a time. OCPP has no message that sets a limit for a whole site. The CSMS divides the site limit between chargers and sends each charger its own profile.

On a 1.6 charger, read these keys with GetConfiguration during commissioning:

• ChargeProfileMaxStackLevel: the highest stackLevel the charger accepts.
• ChargingScheduleAllowedChargingRateUnit: Current, Power or both. A charger that accepts only Current needs a kW limit converted to amps per phase. The kW to amps calculator makes the conversion.
• ChargingScheduleMaxPeriods: the maximum number of periods in one schedule.
• MaxChargingProfilesInstalled: the maximum number of profiles the charger stores.

An AC charger passes its limit to the car through the IEC 61851-1 pilot signal. The lowest current that signal can advertise is 6 A. At 230 V, 6 A is 4.14 kW on three phases and 1.38 kW on one phase. Below that, the CSMS must pause sessions instead of reducing them.

The EV charging capacity calculator starts with the same site and chargers, and a sample day that peaks at 420 kW. It gives the headroom and the number of chargers at full power in each interval:

The headroom must be calculated per phase. Single-phase cars on three-phase chargers load one phase only. If most of them draw from L1, that phase reaches its limit while the three-phase total still shows headroom. Use the worst phase.

When the limit stops arriving

Headroom control has three links: the site meter to the headroom calculation, the calculation to the CSMS, and the CSMS to the chargers. OCPP sees only the last link.

If a 1.6 charger loses its CSMS connection, it keeps the TxProfile of a running session until the session ends. New sessions use the stored ChargePointMaxProfile and TxDefaultProfile. With no profile stored, the charger charges at its full rating. In the example, suppose the connection drops at 06:00 with twelve cars at 14.2 kW. The morning load then rises to 420 kW and the site imports 590 kW.

If the headroom feed to the CSMS stops, OCPP reports no fault. The CSMS keeps dividing the last value it received, unless it rejects old values.

Design a fallback at each end:

1. On the charger, store a TxDefaultProfile at a low stackLevel that is safe at the site peak. Send the dynamic limit as a second TxDefaultProfile at a higher stackLevel, with a short validTo. If the updates stop, the dynamic limit expires and the safe profile applies. Test this with the charger offline, because implementations differ.
2. In the CSMS, set a maximum age for the headroom value and a fallback for when it is older. In the example, an even share of the 30 kW peak headroom is 2.5 kW per charger, which is below the 6 A minimum. The fallback must therefore limit the number of sessions, not only the power per charger.

The loop time is the sum of the meter reporting interval, the headroom calculation, the CSMS allocation, the SetChargingProfile round trip and the car's response to the new pilot signal. Measure it during commissioning: step the published limit down and time the fall on the charger meter. The reserve must cover the largest load increase the site can have in that time. For example, if the loop takes 60 s and a 40 kW chiller can start within 60 s, the reserve must be at least 40 kW.

Security

OCPP 2.0.1 defines three security profiles. The OCA's OCPP 1.6 Security Whitepaper brings them to OCPP 1.6-J, where the charger reports its profile in the SecurityProfile configuration key.

Profile 1 sends the password in clear. Use profile 2 or 3 on any network you do not control, including a public mobile network. With profile 3, the charger's certificate expires. The charger requests a new one with SignCertificate and receives it with CertificateSigned. Test renewal before the first certificate expires. A charger that cannot renew goes offline and falls back to its stored profiles. Both versions report security events, such as failed connections and firmware changes, with SecurityEventNotification.

What OCPP does not do

A charger's MeterValues cover that charger only. OCPP has no view of the building's other loads, so the headroom must come from a meter at the site connection.

OCPP does not talk to the vehicle. ISO 15118 does, and OCPP 2.0.1 carries some of its data to the CSMS, such as the energy and departure time that a car reports in NotifyEVChargingNeeds.

OCPP does not define grid services. A flexibility programme or a DSO sends its signals through other protocols, such as OpenADR, or through the platform's own interface. The CSMS converts them into charging profiles.

Each charger and each platform implements a subset of OCPP. Before you rely on a feature, confirm the profile purposes, the rate units, the security profile and any 2.x features at both ends.

Chargers, the site and EpiSensor

An EpiSensor system measures the site that the chargers share. A ZEM electricity monitor on the incomer reports per-phase current and power. A second ZEM on the charger board separates the charging load from the rest of the site. Edge on the Gateway calculates the headroom from the worst phase and publishes it, with its age, over MQTT, HTTP or Modbus TCP.

Edge does not use OCPP and does not control the chargers. The CSMS reads the headroom through its own API and applies it with charging profiles. Before you choose a platform, confirm these points with its vendor:

1. It accepts an external site limit, and states whether in kW or in amps per phase.
2. It states how often it reads the limit and how quickly it sends new profiles.
3. It states the maximum age of a value, and the fallback it applies when the value is older.
4. It states what it does when the headroom is below the 6 A minimum for all waiting cars: pause sessions, rotate them or queue them.

The EV charging load management application describes the meters, the calculation and the fallback in more detail.