Backing vehicle out of driveway and went to engage brakes and they did not engage. began rolling backwards down street with steep grade. Kept stepping on brakes with no response. Had to turn into a parked vehicle to keep from rolling down steep grade and potentially off a substantial cliff. Happened today and dealership said that it may be because the "battery for the brake" had not received a charge from the main battery. Very dangerous and never disclosed. If I had not taken evasive action I could have easily been killed. Car is being dropped off at collision center today but is available for inspection. There were no warnings or other symptoms.