Your AC Charger Does Not Charge: The OBC Sets the Power

Plug a car into a 22 kW wallbox and the dashboard may report 11 kW, or 7 kW, or 3.6 kW. The instinct is to blame the charger. Most of the time the charger is delivering exactly what it was asked to deliver, and the conversion into direct current is happening somewhere else entirely — inside the vehicle.

That distinction sounds academic until it lands on an installation bill, a metering dispute or a warranty claim. An AC charging point is not a small DC charger. It is a protected, metered power path plus a control box that negotiates. Knowing which part does what changes what you can promise a site, and what you can reasonably complain about.

What an AC Charger Actually Contains

Open one and the parts list is short. There is a contactor that connects and disconnects mains power. There is residual current protection, either built in or required upstream. There is a meter, a controller running the communication stack, a user interface of some kind, and the pilot signal circuitry that talks to the vehicle. Then there is the enclosure, the cable or socket, and the mounting.

What is not in the list matters just as much. There is no rectifier. There is no large transformer stepping voltage up to pack level. There is no high-power switching stage. Those components are large, they are hot, and they sit in the vehicle. A wallbox is closer in spirit to a smart switchboard with a payment terminal attached than to a power converter.

This is also why AC units are cheap relative to their rating. The expensive conversion hardware was already paid for by whoever bought the car.

Where the Rectifier Sits

The onboard charger, usually abbreviated OBC, converts the AC supply into the DC voltage the pack needs. Its rating is a property of the vehicle, fixed at the factory, and it varies far more between models than buyers expect. A compact hatchback may accept a single phase. A long-range saloon may accept three. A van may accept three phases at a modest current because its pack voltage is lower.

Three consequences follow, and they explain most of the confusion around AC charging.

First, the nameplate on the charger is a ceiling, not a delivery figure. A 22 kW unit connected to a vehicle with an 11 kW onboard charger settles at 11 kW and stays there for the whole session, with nothing wrong at either end.

Second, the same charger produces different results on different cars. That is normal behaviour, not evidence of a fault, and it is the single most common source of unnecessary service calls on AC installations.

Third, the phase count is decided by the vehicle, not by the installer. A three-phase supply at the charger is wasted on a car that can only rectify one phase.

CP, PP and the PWM Signal

Two low-voltage wires carry the entire negotiation, and neither of them carries power.

The control pilot, defined in IEC 61851-1, is the main conversation. The charger holds the line at a defined voltage and applies a pulse-width modulated square wave. The duty cycle of that wave encodes the maximum current the vehicle is permitted to draw. The vehicle responds by changing the voltage on the same line to signal its state: connected, ready, charging, requesting ventilation, or faulted. The charger watches that voltage to know when it is safe to close the contactor.

The proximity pilot is the second wire, and it exists on socketed units. It tells the vehicle what the cable plugged into it can carry. A cable rated for a lower current presents a different resistance, and the vehicle reduces its draw accordingly without the charger being involved at all.

The practical point is that both signals communicate permission. Neither one instructs the vehicle to take a specific amount. When a driver reports a lower figure than the label promises, the useful question is which permission was the binding one — the duty cycle, the cable, or the vehicle's own converter.

Case A, B and C: Three Ways to Connect

The standards describe three connection arrangements, and the difference is not cosmetic. It decides who owns the cable and therefore who owns the problem when the cable is the limit.

In a tethered arrangement the cable is permanently fixed to the charging point. Its rating is known, it is part of the product, and responsibility for its condition sits with whoever supplied the unit. In a socketed arrangement the charging point presents an industrial socket and the driver brings a cable. That cable may be rated below the charger's output, may be damaged, may be an adapter, and may have been bought at a price that reflects none of this.

This is why both forms exist in the same product family rather than one being strictly better. Public and semi-public sites often prefer the socketed form because it works with cables drivers already carry and reduces strain on a fixed assembly that sees heavy use. Fleet depots and workplaces often prefer the tethered form because the cable can be specified, inspected and replaced as a controlled item. Outputs of 7, 11 and 22 kW appear in both forms, so the choice is about operating model rather than about power.

Why the Same Charger Gives Different Power on Different Cars

Four variables account for nearly all of the variation seen on site.

The onboard charger rating is the first and largest. It is a hard ceiling that no downstream setting can move. The phase count is the second: a vehicle that rectifies one phase draws from one phase regardless of what is available on the other two. The pack voltage is the third, because a converter designed for a lower pack voltage reaches its current limit sooner. The fourth is thermal and state of charge, which is the same behaviour described in the charging curve: a cold pack or a nearly full one accepts less, and the AC supply dutifully follows.

A quick diagnostic separates them. Compare the same vehicle on a different charging point of known output. If it hits the same figure elsewhere, the vehicle is the ceiling. If it reaches a higher figure elsewhere, look at the cable and the pilot signal at the first point. If it varies session to session at the same point, the cause is thermal or state of charge, and nothing needs replacing.

Full technical parameters for the AC range, including the socketed and tethered variants, are listed on the AC wallbox product page.

What This Means for Metering

The meter sits on the supply side of the charging point. It measures what leaves the charger, not what enters the battery. Between those two points sit cable losses, the conversion losses inside the onboard charger, and whatever the vehicle consumes for thermal management while charging.

For most commercial billing this is the correct place to measure, because it is where the operator's cost is incurred and where the supply is contractually delivered. It does mean the figure on the invoice exceeds the figure the vehicle reports, and that a driver comparing the two finds a gap that has nothing to do with accuracy.

Where the energy figure is used for reimbursement, sub-billing or statutory reconciliation, the meter itself becomes a regulated item rather than a component. Units in this range can be supplied with a MID-approved meter, which addresses the instrument but not the losses downstream of it. The distinction between a compliant meter and a compliant measurement chain matters, and it is the subject of separate treatment in a separate note on metering and documentation.

This changes how an AC project should be specified. The useful question is not how much power the charging point can deliver, but which vehicles actually park there and what their onboard chargers accept. A site full of three-phase long-range cars justifies three-phase points. A site serving a mixed fleet, including plug-in hybrids and small urban vehicles, gains less from the extra phases than from more points at a lower rating, because the vehicles cannot use what the higher rating offers.

The same logic applies to the cable. Specifying a tethered unit removes one variable from every future fault report. Specifying a socketed unit removes a wear item but hands the cable rating to the driver, and with it a recurring question about why the session was slower than the label.

Neither answer is wrong. What is wrong is specifying an AC installation without knowing which vehicles use it, then treating the resulting figures as a performance problem.