Further guidance
The term Electrical Vehicle Supply Equipment (EVSE) is used to cover all EV charging equipment that sits between the fixed wiring installation within a building (or similar) and the vehicle itself (i.e. not a charging station connected directly to the main distribution grid).
The Control Pilot (CP) line is used by the vehicle's On-Board Charger (OBC) and the EVSE to communicate the charging system state, the EVSE's maximum charging current and any errors.
With the vehicle and EVSE disconnected from each other, the vehicle side of the CP line, between the OBC and the charging port, has a 0 V potential and the EVSE side has a 12 V potential.
On initial connection of the vehicle and EVSE, the CP line becomes a potential divider circuit, formed between the 12 V source (in the EVSE) and a ground (in the OBC), consisting of two resistors (one in the EVSE and the other in the OBC). The divider circuit causes the CP line voltage to drop to 9 V.
The change of CP line voltage indicates to the EVSE that there is a good connection between the EVSE and the vehicle's OBC. When confirmed, the EVSE changes the CP line circuit source from a constant 12 V supply to a PWM generating source which produces a square wave switching at 1,000 kHz (1,000 cycles per second) and between -12 V and 12 V.
When the switching source is at 12 V there is a complete circuit between it and the ground connection in the OBC, meaning there is a potential difference across the resistor (in the EVSE) which creates a 9 V potential on the CP line. However, when the switching source is at -12 V the diode in the OBC has the effect of opening the circuit, meaning that there is no potential difference across the resistor (in the EVSE) and we see the full -12 V potential on the CP line.
The EVSE sets the PWM duty depending on the maximum charging current available. The table below gives the approximate relationship between the two:
| Current (A) | Duty cycle (%) |
|---|
| 6 | 10 |
| 12 | 20 |
| 18 | 30 |
| 24 | 40 |
| 30 | 50 |
| 40 | 66 |
| 48 | 80 |
| 65 | 90 |
| 75 | 94 |
| 80 | 96 |
With advancements in Vehicle-To-Grid (V2G) communication and EVSE technology, the duty cycle can be varied by the EVSE throughout charging.
Once the available maximum current has been communicated to the OBC, it can initiate the charging process. To do so, the OBC controller switches in another path to ground on the CP line circuit through another (lower resistance) resistor. This has the effect of dropping the CP line potential to 6V, which is the signal to the EVSE that it can deliver a charge. In the example above, this process takes around six seconds but the time will vary with the vehicle manufacturer.
When the OBC initiates charging, you may notice vehicle battery cooling fans starting up or light indicators informing the user that charging has commenced. You may also see additional, related, noise on the CP line itself.
In total, the peak CP line voltage (as measured on the vehicle) indicates one of six charging states (not all states are available on every vehicle), as described in the following table:
| State | Peak voltage (+/- 1) V | Vehicle connected | Status | Charging possible | Notes |
|---|
| A | 0 | No | Standby | No | EVSE not connected to the vehicle |
| B | 9 | Yes | Vehicle connected | No | |
| C | 6 | Yes | Charging allowed | Yes | |
| D | 3 | Yes | Ventilation | Yes | |
| E | 0 | Yes | EVSE shutdown | No | EVSE problem or pilot short to earth |
| F | -12 | Yes | Error | No | EVSE not available |
EVSE and vehicle handshake sequences are not always the same. However, there is always a change of state between state B, where the PWM signal peak is at 9 V, and state C, when it drops to 6 V and the vehicle initiates charging.
Problems
What happens if charging doesn't start?
- Check the SOC of the HV battery. It may be fully charged and unable to accept any more current.
- If the SOC is not at 100% and charging still fails to commence, then safely carry out the Proximity Pilot (PP) line resistance check (Type 1) to determine the condition of its circuit.
- Safely check the condition of the cables and any temperature related switches inside the connector. Please make sure that the EVSE cable is disconnected from any voltage source prior to an inspection.
We should reiterate that the CP line duty cycle indicates the maximum charging current available from the EVSE, not the charging current being consumed by the vehicle. You can verify the latter by safely connecting a current clamp around the HV charging input to the OBC.
If all of the above checks prove okay and no charging current is being delivered, then suspect the EVSE.
