Charging · 9 min
J1772 pilot, proximity, and PWM as a current limit
The control pilot is not a conversation. Duty cycle is the advertised amp limit. Proximity is the latch interlock. Most writeups mash those together.
30 seconds
The EVSE is advertising a number. The vehicle is reporting a state. Neither side is negotiating.
- CP: 1 kHz square wave. Voltage = vehicle state. Duty cycle = how many amps the EVSE will allow.
- PP: handle seated, latch not squeezed. Interlock, not a current command. Sometimes a resistor that encodes cable amps.
- PE: the reference.
PWM duty is a current limit, not a request. The onboard charger may draw less. It must not draw more.
If you remember one correction: do not call this “the charger talking.”
Bench depth
Three pins that actually matter
On a J1772 coupler: AC L1/L2 (or L1/N), PE, CP, PP. Everything else in the “how charging works” explainers is decoration.
- PE is the reference. CP and PP are measured against it.
- CP carries a 1 kHz square wave from the EVSE.
- PP tells the vehicle the handle is seated and the latch is not being squeezed.
Control pilot states are voltages
IEC 61851-1 / SAE J1772. The EVSE sources about +12 V on CP. The vehicle loads CP to PE. The voltage is the state:
| State | CP vs PE | Meaning |
|---|---|---|
| A | ~12 V | No vehicle, or not detected |
| B | ~9 V | Connected, not asking for power |
| C | ~6 V | Ready to charge, ventilation not required |
| D | ~3 V | Ready, ventilation required (rare on a conversion) |
| E / F | 0 V / −12 V | Error, or EVSE not available |
Usual vehicle network: 2.74 kΩ CP to PE for State B. Close S2 and you put 1.3 kΩ in parallel (~882 Ω) for State C. A series diode on the vehicle side lets the EVSE check that the negative half of the PWM still sits at −12 V. Missing or shorted diode: a lot of EVSE will refuse to close the contactor. Real failure on homemade inlet wiring.
The EVSE does not close mains in State A or B. You see State C, then the contactor. If the inlet interface never pulls CP to 6 V, you sit on “connected” with no AC.
Duty cycle is the amp advertisement
SAE J1772:
- 10% ≤ D ≤ 85%:
I = D × 0.6A. 50% = 30 A. 16.7% = 10 A. 83.3% = 50 A. - 85% < D ≤ 96%:
I = (D − 64) × 2.5A. 96% = 80 A. The 0.6 slope runs out at 51 A. - ~5% duty is the digital-comm flag (ISO 15118 / DIN 70121). It is not “3 amps.”
| Duty | Advertised current | Typical use |
|---|---|---|
| 10% | 6 A | Low-power / shared circuit |
| 16.7% | 10 A | Many 120 V portable EVSE |
| 25% | 15 A | Common 120 V ceiling |
| 50% | 30 A | Common 240 V home EVSE |
| 83.3% | 50 A | 40–50 A circuit EVSE |
| 96% | 80 A | High-current AC, if cable and EVSE support it |
A 6 kW OBC on a 30 A advertisement at 240 V is already at the ceiling (30 A × 240 V = 7.2 kW available; the charger still only takes what it is rated for). If the EVSE drops to 16.7%, the charger follows. That is the point of the PWM.
Proximity is not PWM
PP does not carry the 1 kHz wave. Usual inlet: latch switch in series with a resistor to PE. Seat the handle, PP is present. Press the release, the switch opens, the vehicle is supposed to kill the charger before the pins slide. That is how you unplug without arcing the power pins if the wiring is right.
Some inlet/cable designs encode ampacity on PP as a resistor, still not a PWM. Do not scope PP looking for duty cycle.
Typical PP → ampacity when the detachable cable uses IEC 61851 Annex B encoding (Type 2 / socketed EVSE). Confirm on the inlet you have.
| Rc | Advertised cable current | Notes |
|---|---|---|
| 1500 Ω | 13 A | IEC Annex B |
| 680 Ω | 20 A | IEC Annex B |
| 220 Ω | 32 A | IEC Annex B |
| 100 Ω | 63 A three-phase / 70 A single-phase | IEC Annex B |
| 50 Ω | 80 A | Listed on Beyondlogic EVSE-interface notes; not in IEC Annex B |
A US SAE J1772 tethered handle is usually the latch interlock (commonly 150 Ω seated / ~480 Ω when the release is pressed), not the IEC ampacity table. Do not mash those together.
What people get wrong
- Calling CP “CAN” or “the charger protocol.” Voltage divider + 1 kHz analog advertisement. Digital comm is a different mode at ~5% duty. You are probably not using it on a home conversion.
- Wiring an EVSE-side board into the car. The EVSE generates the PWM. The vehicle loads it. See why off-the-shelf J1772 controllers often do not fit.
- Ignoring the diode check. Resistor-only CP can look fine on a DC meter and still fail PWM.
- Treating PP as optional. Skip the latch interlock and you get a handle yanked under load.
PWM limit is a charge-time constraint, not a pack constraint. Pack current and fuse live on the DC side. Next: pack planning and the charge-time toolbox tab.