My Prius has about 45000 miles. About 3 months ago the battery started having issues. It suddenly drops from fully charge to 1 bar and looses a lot of power. It takes me driving very slowly to start charging the battery again. I've notice this happens in the afternoon when I leave work and it's very hot outside. I'm in Florida. It also happened twice while driving at high speed on the highway which almost cause me to have an accident. I've taken the car to the dealer 5 times and they say there is nothing wrong with the car.
When you are driving at high speed (what is your definition of high speed: 70 mph?) and the traction battery has a problem, what exactly happens? Do any warning lights appear? If not, then the dealer won't take any action.