car batteries will not work. one can charge the battery in a car but you need that charge to move the ev car. if you increase the storage capacity of the battery that is an extra cost that the car owner may not wish to pay for. if you charge you car over night you should have enough energy to get to work to plug it in. But little left to power the grid and you will need stored energy to get back home. This will not happen if you feed power back to the grid while you are at work. The Car Battery would need to be at least 2 times the size just to hold or store grid power. You did not state Hydrogen production to store energy and then reuse via fuel cells. Hydrogen is a better storage meadium,(reply>), You don't need to use the full capacity of the battery for grid-balancing. Just 10% would be plenty. A few kilowatt-hours here and there would probably be enough if spread across several million vehicles. Most people wouldn't worry at all if their car was only 90% full when they came to use it.