EV charging load management
A meter at the connection and another on the charger board, so the headroom the chargers may use is measured rather than assumed.
- Measure
- Site import at the connection, the charger board separately, and the rest of the site by difference
- Calculate
- Available headroom for EV load balancing, interval by interval, with a stated reserve
- Sensors
- ZEM-65, Class 0.5S metering, per-phase currents kept separately ZEM-63 datasheet (PDF, opens in a new tab)
- On site
- ZGW-20 Gateway, the headroom calculation and the limit it publishes ZGW-20 datasheet (PDF, opens in a new tab)
Who controls the chargers
The charge-point management system knows the sessions, the users and the tariffs, and it is what actually changes a charger's current. This guide measures the electrical constraint they all share and publishes it. Keep the two jobs separate, and agree what happens when the link between them is lost.
Hardware for EV charger energy monitoring
Two measurements are the minimum: the whole site at its connection, and the charging board on its own.
The site connection, and the charger board or group
- Choose the CT rating per point: 120 A split-core, or 300 A, 1 kA and 3 kA Rogowski coils
Class 0.5S to IEC 62053-22 with its CTs, per-phase currents available
Calculates the headroom on site and publishes it to the charging platform
If the charging equipment speaks Modbus RTU
A charger controller or load-management unit that exposes Modbus RTU
What decides how much charging the site can take
Headroom is the site limit, minus a reserve, minus everything that is not charging. Each part of that sentence is a measurement or a decision.
| Position | What it tells you | Reference | Sensor |
|---|---|---|---|
| Site connection | The constraint every load shares, and how close it is | The contracted or physical import limit, written down with its source | ZEM-65 (on this page), connection |
| Charger board | How much of the site's demand is charging, separately from the rest | Phase allocation recorded where the chargers are single phase ZEM-63 datasheet (PDF, opens in a new tab) | ZEM-65 (on this page), chargers |
| Per-phase currents | Whether one phase reaches its limit before the total does | Keep the three currents rather than a balanced total | ZEM-65 (on this page) |
| Charger state | What the charging equipment says it is doing, where it exposes it | Vendor register map, with its version recorded ZMB-3X datasheet (PDF, opens in a new tab) | ZMB-31 (on this page) |
Single-phase chargers unbalance a site. A site with headroom on its total can still be at its limit on one phase. Keep the per-phase currents and calculate on the worst of them.
Charging protocols are the platform's job. EpiSensor does not speak the charging protocols the chargers use. The limit is published for the platform that does, which is why the fallback matters.
Headroom and its fallback in Edge
The Gateway calculates the number and publishes it. What the chargers do with it is the platform's decision, and what happens when it stops arriving is yours.
- Headroom, interval by interval
- A calculated sensor takes the site limit, subtracts the reserve and the non-charging demand, and never returns less than zero.
- The reserve is explicit
- The reserve is a number somebody chose and wrote down, not a rounding in the arithmetic.
- Published, not commanded
- The limit leaves over MQTT, HTTP or Modbus to the charging platform. Edge does not talk to the chargers itself.
- A safe number when data stops
- If the site meter goes stale, the published limit falls back to an agreed safe value rather than the last good one.
Commissioning checks
The failure that matters here is the one where nothing looks wrong, so test the broken paths deliberately.
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Both boundaries are proven
Switch a known load at the charger board and watch both meters.
Pass when the charger meter and the connection meter move together, with the right sign and the right phases.
-
Headroom responds to the site
Step a large non-charging load and watch the calculated headroom.
Pass when the headroom falls by about the same amount, within one interval.
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The platform receives the limit
Change the published limit and watch what the charging platform reports.
Pass when the platform's own view of the limit changes, and its response is visible in the charger meter.
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Every broken path has a behaviour
Break the site meter, the charger link and the WAN in turn.
Pass when each failure produces the agreed fallback, and every one of them is written down.
-
Local override works
Ask the site to override the limit by its agreed method.
Pass when the override takes effect, is visible in the record, and expires or is cleared deliberately.
What monitoring does not replace
This measurement produces a number. Somebody else's system acts on it.
- EpiSensor does not replace charger protection, the charging platform or the distribution design.
- Charging protocols and their smart-charging profiles have to be verified at both ends: support is per charger and per platform.
- Vehicle energy needs, departure times and tariff optimisation belong to the charging platform, not to this measurement.
- A headroom figure is not a network study: diversity, protection and phase balance stay with the designer.
Sources and related guides
Sources
- Open Charge Point Protocol (opens in a new tab) Open Charge Alliance. The versions and smart-charging capabilities a charging platform may support. Implementation still has to be verified per product.
- ZEM-63 Wireless 3-Phase Electricity Monitor datasheet (opens in a new tab) EpiSensor. Specifications, ranges and ordering codes.
Related
- Battery demand response Application guide Measure the connection and the battery separately, read state of charge and limits from the BMS, and record events at the resolution the programme wants.
- Solar PV monitoring Application guide Independent AC metering at the inverter and the grid connection, inverter registers for context, and an energy balance that adds up.
- EV charging site-headroom worksheet Engineering tool See how many EV chargers a site can run at once, interval by interval, within its import limit.
- Interval demand calculator Engineering tool Turn the energy recorded in one demand interval into average demand in kW.
Reviewed by EpiSensor Engineering on . Revision 4.
Measure the headroom your chargers are allowed
We will place the meters, agree the reserve and the fallback, and define how the limit is published.