The control pilot is the single most important circuit in an AC charger. It establishes that a vehicle is connected, confirms the protective earth is intact, communicates how much current the installation can supply, and gives the vehicle a way to say it is ready.
Nothing else in the charger is permitted to energise until the pilot says so, which makes it a safety function rather than a communication convenience.
What the pilot signal carries
The charger drives a square wave onto the pilot conductor. Its duty cycle encodes the maximum current the installation can supply, and the voltage the vehicle pulls that signal down to encodes the vehicle's state.
That is the whole protocol in one sentence, and its elegance is that both directions of information travel on a single conductor with no digital communication at all.
The states, and what each one permits
The transitions between these states are what the firmware state machine implements, and the ordering constraints are not optional. Energising outside the ready state is a safety failure, not a bug.
- No vehicle connected: the charger holds the pilot at its idle level and must not energise.
- Vehicle connected, not ready: the vehicle has pulled the pilot down one step. Energy is still not permitted.
- Vehicle connected and ready: pulled down a further step. The charger may now close the contactor.
- Ventilation required: a further state used where charging produces gases requiring extraction.
- Error or no power: the pilot is outside any valid range and the charger must not energise.
Duty cycle communicates the current limit
The proportion of each cycle the signal spends high tells the vehicle how much current it may draw. This is how a charger applies a limit, and it is also the mechanism load management uses: reduce the duty cycle and a compliant vehicle reduces its draw.
There are reserved duty cycle values with special meanings outside the normal current-encoding range, and an implementation that treats the whole range as linear will misbehave at the extremes.
The proximity circuit is separate and often confused with it
Proximity pilot is a different conductor doing a different job. On a socketed charger with a detachable cable, it tells the charger what current the cable itself is rated to carry, and it also detects that the connector is latched.
A charger that ignores proximity can offer more current than the cable is rated for, which is a fire risk rather than an inconvenience. The current actually offered is the lowest of the installation limit, the cable limit and the charger rating.
Where implementations commonly go wrong
- Treating the pilot as an input to poll rather than as a real-time safety signal, introducing latency between a state change and de-energising.
- Failing to detect a diode fault in the vehicle, which the specification requires and which distinguishes a real vehicle from a fault condition.
- Not handling the transition to a lower state during an active session, which must stop energy flow promptly.
- Applying a duty cycle that does not correspond to the actual protection rating of the circuit.
- Ignoring proximity on a socketed unit, or assuming a fixed cable rating.
Timing matters more than the state diagram suggests
The specification places bounds on how quickly the charger must respond to state changes. Firmware that samples the pilot on a slow loop, or that shares a timer with lower-priority work, can drift outside those bounds under load.
This is a common source of intermittent behaviour that appears only when the processor is busy, which makes it hard to reproduce on a bench and easy to encounter in the field.
Vehicle behaviour is not uniform
Vehicles differ in how quickly they transition states, how they respond to a duty cycle change mid-session, and how they behave when the pilot is interrupted briefly. All within specification, and all different enough to matter.
This is why interoperability testing against real vehicles remains necessary even where the implementation is provably compliant. The specification defines a contract; vehicles occupy different parts of the space it allows.