Current control

How does the EVSE limit charging current to the car?

I’m not familiar with the protocols involved here, but the EVSE just has a basic make or break contactor and no apparent way to control the current flow. So is this done by informing the vehicle how much it can have and which responds by only taking that much?

Clearly the EVSE can simply open the contactor if the current exceeds the configured maximum, but that’s something of a sledgehammer and not really ‘current control’.

So how does this all work?

The “Charging Point” is barely more than a power outlet. The charger, where the a.c. is converted to d.c. for the battery and where the voltage and current control takes place is on board the vehicle. The charging point and the vehicle negotiate via control/signalling wires in the lead & plug.

Ref: SAE J1772 - Wikipedia

Yes, exactly that. There’s a PWM signal generated by the EVSE that tells the car what current it can draw. In the simplest case that’s configured inside the EVSE with dipswitches by the installer - typically set to match the upstream wiring and breaker, mindful to local regulations. More advanced EVSEs also use that PWM signal to reduce the current so as to track excess PV in realtime.

In addition, the cable between the EVSE and the car will have its own current limitations. That’s indicated by a resistor between two of the pins. The car will charge at the lower of the two specified currents (PWM and Resistor).

Thanks for confirming what I had surmised.

When first thinking about EVs and EVSEs, I had assumed that the EVSE controlled the current. Then never gave it another thought, even while assembling my own OpenEVSE. Only later did it occur to me that the EVSE had no current control, apart from the contactor. So it had to be the car.

All I need now is to get the Open EVSE actually working. Currently it is dead and doesn’t boot, but that’s a separate issue from this topic.

Thanks again.

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