Googling finds evidence that Tesla Roadster electric car can detect cases when it is plugged in via extension cord.
Specifically this page lists error codes 937 and 9 meaning "extension cord detected". And this post says that when a cable with 2.5 square millimeters cross-section in each wire is used the charging goes just fine but when a cable with 1.5 square millimeters cross-section in each wire is used the car displays "extension cord detected" message.
Now the latter post says it's because of notable voltage drop on a thinner cable. Looks reasonable but the lower voltage on the car end of the cable can be because the mains voltage gets lower than the nominal, not because of voltage drop in the cable.
How can such check be implemented so that it only detects thin wires and ignores cases when mains voltage is low?
Best Answer
The car can draw a variable amount of current. By measuring how much the voltage changes at two different current levels, it can distinguish between low source voltage (low mains voltage) and high source impedance (small-gauge extension cord).