Regenerative braking is a feature found on electric vehicles, which can recover some of the vehicle's energy when the accelerator pedal is released, regenerated power back through the powertrain rather than causing waste energy from braking.

Some cars, like the Chevrolet Bolt, function well with "one pedal driving." The regeneration does enough that you can avoid the brakes, except in obvious circumstances.

We have seen different "experiments" regarding the technology. One of which is towing an electric car with all wheels on the ground.

Another is letting the vehicle coast down a hill. Engineering Explained on YouTube attempts to see how this will work with a Tesla Model S. With everything considered, unless the mountain was about twice the height of Mt. Everest, you could never really fully recharge the car.

Video Description via Engineering Explained on YouTube: 

How big of a hill is needed to recharge a Tesla? How about A Nissan Leaf? Ultimately, it's a matter of potential energy, and how much energy the Tesla could recover, based on its regen efficiency. There are gearboxes, motors, inverters, controllers, and finally the battery which the energy must transfer through, which all have efficiency losses. This is to mention nothing of rolling resistance or air resistance. Energy = force times distance, so this will allow us to determine what height is needed to recharge a Tesla. And finally, it's imperative to put things in terms of Mt. Everest units. It's a tall hill.

For more on regenerative braking, check out the video below.