One of the benefits of electric cars is that slowing down becomes a useful activity.

Regenerative braking recovers energy normally lost as friction heat, as it does when the accelerator pedal is not pushed and the car is coasting.

In some cars, regeneration is aggressive enough to allow "one-pedal driving," where the friction brakes are only needed for hard deceleration, or to bring the car to a complete stop.

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But how much energy do these systems really recover at any given time?

Would it be possible, say, to recharge an electric car by simply rolling it down a tall, long hill?

That's what Engineering Explained sought to investigate in the video above, using a Tesla Model S 60 as the basis for its calculations.

2017 Tesla Model S

Answering that question involved converting the storage capacity of the Tesla sedan's 60-kilowatt-hour lithium-ion battery pack into joules, and taking note of the car's weight.

The Tesla website lists that as an oddly-specific 4,647.3 pounds.

Some calculations needed to be done to determine the required potential energy that matched the battery pack's capacity in joules.

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With the car's weight known, the variable in question was the height of the hill.

It turns out that to recover the requisite energy, the hypothetical Model S driver would need a downhill slope 10.4 kilometers (6.4 miles) high.

However, that assumes the Model S can convert 100 percent of the recovered energy into electricity that makes its way into the battery pack, which is not the case.

2017 Tesla Model S

In reality, the system is subject to efficiency losses that limit the amount of usable electricity produced.

Engineering Explained quotes a 60-percent efficiency figure for energy recovery in the Model S.

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Factoring that in, the driver would need a hill 17.4 kilometers (10.8 miles) tall—just under twice the height of Mount Everest.

So while regenerative braking on downhill stretches can help increase the remaining range of an electric car, it can't do the job of recharging it all by itself.

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