Sounds like a normal eBike. Your car will really accelerate quickly when it's sitting on jacks. No weight to move, no aerodynamic resistance, no road friction. There are speed limits set in the controller. If class 1 it will loose power around 19.5-20 mph. Are you sure the controller isn't set to European laws? There it would fall off around 15mph. There is a thing called voltage sag in a battery. The display can show 2/3 left, but under heavy load and high speed that quickly falls off. Reading voltage and not bars on a display will give you a more accurate battery capacity. eBikes are not motorcycles, they are electric assist bicycles. Now providing numbers will help. What size is the motor, controller, and battery. At what certain speed does it fall off. What is showing on the display or some details why you think it's showing that the battery is dying or dead?
