KNX EV Charger Integration and Load Management
Integrate an EV charger with KNX using supported OCPP or local interfaces, per-phase load budgets, solar modes, fallback limits, and commissioning tests.
A three-phase charger does not have 11 kW available merely because the building’s total demand is below its headline supply capacity. One heavily loaded phase can be the limiting condition.
That is the central design issue in KNX EV charging integration. KNX can present modes, collect building information, and pass preferences or bounded requests to a charging system. The charger and an appropriately designed local load manager should enforce the actual charging and site constraints. A wall-panel “boost” button must never bypass them.
The practical objective is a system that explains what it is doing: charging, waiting for capacity, waiting for solar, finished, or unavailable. “Enabled” is not the same as “charging,” and “command accepted” is not the same as measured current.
Draw three responsibilities before choosing a gateway
| Layer | Responsibility | What should remain outside its authority |
|---|---|---|
| KNX user/building layer | User modes, schedules, occupancy-related preferences, status display | Overriding electrical protection or charger operating limits |
| Site charging/load manager | Allocate available capacity, handle charger/phase topology, enforce configured limits | Assuming every requested ampere is actually consumed |
| Charger and electrical installation | Vehicle communication, charging sequence, installed protective functions | Relying on a decorative dashboard as the only protective measure |
The electrical designer must establish the supply, branch-circuit, phase, protection, and installation requirements. Software load allocation supplements a suitable electrical design; it does not replace protective devices or commissioning by qualified personnel.
Prefer a local capacity-enforcement path with documented behavior if KNX, the internet, or a cloud service is unavailable. SMA’s dynamic load-management guidance is one product example that describes local management and distinguishes single- and three-phase charging. Verify the equivalent behavior on the selected equipment rather than assuming it from a brochure.
Choose an interface that can do the required job
| Integration route | Potential use | Check before selecting |
|---|---|---|
| Vendor-supported KNX gateway | Direct mapping of selected controls and status | Exact supported charger models, firmware, objects, and limits |
| Documented local API or Modbus interface | Local setpoints and measurements | Write permissions, timeout behavior, supported range, update rate |
| OCPP through a charging management system | Central management and supported charging profiles | Compatible protocol version, smart-charging implementation, ownership |
| Approved contact input | Simple enable or predefined mode request | Input semantics; usually not a full dynamic-control/status interface |
The Open Charge Alliance OCPP overview defines OCPP as communication between charging stations and charging management systems. KNX does not become OCPP simply by using Ethernet; a gateway or service must implement the required translation and control responsibility.
Also, “supports OCPP” is not a complete smart-charging specification. OCA’s resource-constrained device guidance notes that smart charging is not part of the core profile discussed there. Request evidence for the required charging-profile behavior, not only a protocol label.
Avoid a design where a cloud platform, charger app, and KNX logic all write independent current limits without a defined priority. Name one allocator and explain how other requests are bounded or rejected.
A per-phase budget is more useful than a single kW number
For a simple hypothetical site with no PV or battery, assume an approved allocation limit of 40 A per phase and a further operational reserve of 3 A per phase. The reserve is an example design choice, not a prescribed safety factor.
The non-EV loads are 22 A, 12 A, and 18 A on L1, L2, and L3. For a balanced three-phase EV request:
| Phase | Site allocation limit | Non-EV current | Operational reserve | Remaining current |
|---|---|---|---|---|
| L1 | 40 A | 22 A | 3 A | 15 A |
| L2 | 40 A | 12 A | 3 A | 25 A |
| L3 | 40 A | 18 A | 3 A | 19 A |
The candidate limit is the minimum: 15 A per phase, additionally bounded by the charger, vehicle, cable, branch circuit, and any other applicable constraints. At an assumed 400 V line-to-line voltage and near-unity power factor, that corresponds to approximately √3 × 400 × 15 = 10.4 kW.
This is simplified allocation arithmetic, not a protective-device coordination calculation. Real systems need suitable measurement, response time, phase identity, and treatment of generation, power factor, and other connected loads. If the supply is not a 400 V three-phase system, do not copy the power conversion.
A single-phase vehicle changes the allocation problem. It may add demand to the already busy phase rather than sharing it over three phases. Commission the charger’s phase assignment and actual vehicle behavior; a three-phase charging point does not guarantee a three-phase vehicle load.
Do not subtract the EV twice
If the site meter already includes the EV, subtracting total measured site current from the limit gives additional headroom, not necessarily the new total EV allowance.
In the same simplified, import-only example, suppose L1 measures 32 A while the EV is drawing 10 A. The non-EV component is approximately 22 A. The candidate total EV allowance is:
40 A limit - 3 A reserve - 32 A measured site + 10 A measured EV = 15 A
Sending 5 A as the total allowance would confuse incremental headroom with the permitted total. Conversely, adding the EV’s old requested current rather than its measured current can overestimate capacity if the vehicle is drawing less than requested.
Use time-aligned measurements where the design depends on this calculation. In installations with PV, storage, unbalanced loads, or significant power-factor effects, simple subtraction of RMS-current magnitudes may be invalid. Use the load manager’s supported measurement model and the electrical designer’s approved method. The energy-meter integration article covers measurement boundaries and quality.
Minimum charging current makes solar mode discontinuous
A charger cannot necessarily accept every small positive power allowance. Its supported minimum, the vehicle, active phases, and operating mode determine when charging can start or continue.
For an illustrative 6 A operating point, a 230 V single-phase load is about 1.38 kW; a balanced 400 V three-phase load is about 4.16 kW, both assuming power factor near one. These are calculations at a stated current, not a universal promise that every vehicle supports that behavior.
SMA’s solar-mode instructions discuss surplus availability relative to minimum charging current and include a 6 A three-phase example. Check the actual charger and vehicle limits before turning such a figure into a site rule.
If the allowance falls below the supported minimum, the controller needs an approved pause/wait policy rather than an invalid current request. Automatic phase switching, where available, must be a supported charger function with its own safe sequence. Do not improvise it with generic relays.
Give the user modes with honest meanings
| Mode | Intended behavior | Constraint that still applies |
|---|---|---|
| Solar-only | Wait or pause when usable surplus is insufficient | Minimum supported charge level and all site limits |
| Solar-preferred | Favor surplus but permit agreed grid contribution | Configured grid contribution and site capacity |
| Scheduled | Charge within an agreed window | Available capacity may still delay completion |
| Boost | Request higher priority or prompt charging | Never bypass installation or charger limits |
Ask the user for a departure time or completion preference only if the system can use it meaningfully. If battery state of charge is unavailable, say so in the interface. Do not show a precise completion promise calculated from an invented battery capacity or charging efficiency.
For multiple charging points, define allocation fairness and priority explicitly. Starting several vehicles simultaneously can consume all available headroom before the next measurement. Allocation, ramp behavior, and feedback need to be coordinated by the charging manager. See KNX solar PV integration for the wider multi-load control issue.
Commission failure behavior before handing over the boost button
| Test | What must be demonstrated |
|---|---|
| A large non-EV load starts | Charging allowance responds within the approved control design |
| Vehicle draws less than requested | Capacity calculations use the correct actual-state model |
| Meter data becomes stale | Load manager applies a documented conservative limit or pauses as designed |
| KNX connection fails | Core charging limits remain effective; user status shows unavailable data |
| Internet/backend fails | The agreed local operating policy takes effect |
| Charger or controller restarts | Safe configured limits exist before fresh dynamic allocation |
| Two vehicles connect | Shared capacity and priority behave as specified |
Choose fallback limits from the installation’s assessed constraints. A remembered “last good” dynamic limit can be unsafe after other building loads change. Test each communications failure independently; unplugging the internet does not test loss of the site meter.
Keep command, accepted limit, measured phase currents, charger state, and data age in the acceptance record. Those points make a later complaint—“the car did not charge overnight”—diagnosable without guessing whether the issue was authorization, solar availability, capacity, or a vehicle-side condition.
Can a KNX relay simply switch the charger supply?
Do not use routine mains interruption as a substitute for the supported charging-control interface. Electrical isolation and protective switching must follow the charger instructions and the installation design.
Is a total-power meter enough?
Not where per-phase constraints govern the allocation. Specify the measurement inputs required by the approved load manager, including verified phase mapping.
Does boost guarantee full power?
No. It expresses a preference within the available and permitted capacity. A clear “limited by site capacity” indication is better than a button that appears not to work.