All the car needs to store is some private key corresponding to yourself, to prove it's that car. On modern hardware you can make these really hard to steal, short of taking the entire chip with you.

Worse case, if someone steals that key (or the hardware that stores it), they can steal electricity with it. Which also means they'll be caught approximately tomorrow.

If you have that level of access/capability, there's better things to do than using it to get 50 dollars worth of electricity and probably being picked up by police as you siphon it.

Besides, there's easier and stealthier ways to get free electricity.