Ive done Hybrids, they have a standard 12v starting batt as stated above.
But a full electric is different.
The main batt is high voltage and you do not want to tap into it.
They do have a 12v feed for standard car electronics and accessories and you can use this for low power items.
I wanted to use a Chevy Volt as a demo car and be the first to do 150+ in an electric car, but I cannot afford one.
But I will tell you that it can be done, and how I was going to to it was to put in a 2nd battery bank of XS Power batteries and just charge em up as often as I got the chance.
If there was some rotational mechanism in the car like an AC that I could attach to, Id do an alternator, but Ive never seen inside one.