Some bikes must be pedaled before they can start or run. Know how yours works, and check that they are correctly aligned. If the wheel is rubbing the frame, or a brake is closed, the motor may not be able to push the bike. Recently, disposable Zinc air batteries have become available for cell phones. Zinc air batteries have been used by the military for some time, and the power to weight ration is impressive.
Military batteries have replaceable elements that are pulled out and new ones are inserted.
And the cost is quite high to replace a battery.