Unless the grid is paying a significant premium for the energy why would you want to offer up your car battery? With charging inefficiency and losses converting AC to DC and back to AC for the grid would be just donating your money to the grid. Or am I missing something here?,(reply>), If your car knows to charge itself during periods of low demand, and sell that energy back to the grid during periods of high demand, then you will be buying low and selling high which will make a profit.