The problems with NiMH batteries for cars are technical, not legal. They do not really work well when its 120 degrees out or work at all at 40 below. They must be charged every couple weeks (think of coming back to the dark, remote airport long term parking lot after a month of travel to . . . a dead battery), and they have a limited life when used for sudden heavy discharges (like starting an engine) followed by fast recharging (like the 100+ amp alternators on modern cars). Li ion batteries can be made to be ideal for this work and are used for it in airplanes, but are kinda pricey for cars.