Electric Vehicle Charger Specialities
Level 1 and level 2 are very simple: the charging station (not charger, the battery charger is built into the car) supplies a low voltage square wave where the duty cycle indicates the maximum current the car is allowed to draw. The car then connects a resistor between that signal and ground to indicate it is ready to draw current. The charging station then provides AC voltage to the car (at least in the USA, 120 volts for level 1 or 208/240 volts for level 2). That’s pretty much it. There’s no communication of car identity. Any authorization is done outside, using RFID cards or such. This is specifically how stations with the standard J1772 connector work.

In earlier level 3, the battery charger is actually in the station and provides high-voltage (as high as 500 volts) to the car under direction of the battery monitoring hardware in the car. This dynamically varies the voltage as required. I still don’t believe there is any in-band authorization. This is specifically for the Japanese Cademo stations.

Tesla has a separate connector and protocol for their stations, though I believe the voltages are the same. I’m not sure but I assume their Supercharger L3 stations do in-band authorization.

Additionally, there is now a variation on the J1772 for level 3 charging which adds two high-voltage, high-current pins to the connector. I don’t know how much the protocol changes with this.

All of the stations also use built-in GFCI to protect against faults that could hurt or kill the user. They also self-test the GFCI at the start of every session by leaking current to ground internally.