why can’t electric cars be charged by their own movement?


why can’t electric cars be charged by their own movement?

In: 14

Some of them are, it’s called regenerative braking. You usually still lose more energy than you gain, though

I think some of them are to an extent. They do use the motion of the car to recharge. However, due to friction, no machine is perpetual. If the engine expels 100 kw of power, then perhaps only 95 kw actual transfers to moving the car forward. That 95 km could then (at best) charge to battery, but again power is lost so now the battery as 90 kw. So now the car engines has 90 kw of power, but only 85 kw goes to motion. That 85 kw of motion charges to battery to 80 kw. This will continue to decrease until the car runs dry.

Because in order to turn the kinetic energy of the car into electrical energy, you need to take away that kinetic energy – slowing the car down. And with everything else in physics, nothing is 100% efficient much less > 100% efficient.

Regenerative braking DOES provide some electrical energy to electric cars and hybrids, but it’s just a small supplement.


Energy conversions are never 100% efficient. Taking X amount of electrical energy stored in a battery and converting it to kinetic energy in a motor leaves you with X-Y. Taking some of that energy and converting it again to electrical energy would leave you with Y-Z, and the cycle would continue.

Why don’t they just have a generator connected to a gear that’ll spin the generator?

Energy in may never be equal to energy out.
It’s a law of thermodynamics.
Converting electric charge to mechanical rotation isn’t perfect. There are always losses.

Reversing it also has losses.

100% power in battery. Only 70-ish% ends up being rotation.

If you tried to then turn rotational energy back into a charge, the car would have to stop moving. Because your rotation would go to 0, as charge comes back.

You start with either charge, or rotation. You MUST end with

either charge OR rotation, or half and half, or 70/30… something.

You may not have both. If you started with one, and ended with both, you’d need to have made more energy for no cost.

Add in those losses we mentioned earlier.

And 100% battery charge becomes 70% rotation 0% battery as the other 30% energy turns to heat, which you can’t get back.

Then that 70% goes to 49% battery, 0% rotation, as 30% goes ro heat again. You always lose.

Gas powered cars do this by consuming chemical fuel.

Most electric cars do take advantage of converting their movement into energy, but that can only be done while braking (or going down hill – which is still effectively “braking”). Instead of using regular brakes to convert the movement into heat (and thus slowing down) they use the movement to spin the motor and use it to generate a little bit of electricity. It is known as “Regenerative Braking”.

It is not very efficient – but it does add some extra range to the vehicle that it would not otherwise have.

Conservation of energy. You could do that, but you’d still lose energy due to air resistance and friction with the road

Let’s say they do charge while they drive. Perpetual energy machines are literally physically impossible (Literally violates the laws of physics). You will never get more energy out of something than you put in. There’s a reason why patent offices will basically reject perpetual motion machine patents outright.

So that means at best you will get back the energy you put in. Which means at best you can stop and go again with the energy, but you would never be able to go faster than you started. Which is also impossible because it’s a physical impossibility to 100% efficiently convert the energy in a car (Literally violates the laws of physics). Which means doing this will always result in you slowing to a stop unless you’re on a hill.

Now electric cars do have regenerative breaking. This lets them charge their batteries when they slow down/stop. This works because the electric motor also works as a generator. In fact, nearly all non-solar power plants (and even some solar plants) essentially use the equivalent of giant electric motors to produce power.

They can charge through deceleration but the kilowatts produced from that are minimal. They could charge through mini wind turbines, but the drag they would create would offset the energy output. They could charge through solar panels but the surface area on a car isn’t enough to reliably keep a vehicle working.

Some do. They use “regenerative breaking,” which takes the cars momentum and turns that into energy through magnetic breaks. It’s not 100% efficient though (and far from it).

Due to the “laws of thermodynamics,” no system can be 100% efficient. This means that if the electric motor produces 1 unit of force to move the car forward, some of that energy will be lost as heat (friction). Further, more energy will be lost as heat when we try to capture that energy back in the regenerative breaking.

This is also the same reason perpetual motion machines can never work. It would mean the PMM would not only have to be 100% efficient, but actually produce *more* energy than was used to make it move, which is impossible.

cars with that type of charger built-in can top up the battery by rolling steeply downhill for tens of km. a lot of movement to produce a little current. some people aren’t connected to a reliable high voltage grid. they have to generate their own electricity and they appreciate just how inefficient it really is.

only with modern engineering do we get some current with relatively little effort. and even then, it takes a lot of movement to get a little current.

we can ask a related question; “why can’t i keep my cellphone fully charged using a compression device built into my shoe-heels”.

and the answer is; “we can! but, just like the gravity powered car that must roll downhill constantly, so we would have to walk everywhere constantly.”

When you spin the shaft of a generator/alternator, the magnetic fields inside the generator push back against your effort. The more electrical resistance between the terminals of the generator, the more the magnetic field resists in turn. The end result is that you can convert electricity into motion (a motor) or motion into electricity (a generator), but if you try to convert electricity to motion to electricity, you end up with no more electricity than you started with. This is so fundamental and unavoidable that we think it’s one of the rules written into the fabric of reality itself, which we call the Law of Conservation of Energy.

Imagine you adda 5th tire to an EV car and collect power from it by an electric motor connected to the battery. There will be ton of power lost due to friction, and power lost from engine trying to move than 5th power because there is also friction from this 5th tire effecting the engine.

Because the real original Tesla died all of a sudden and we were stopped from using the electric car as the most popular form of private vehicle in London during the 1920’s. We wouldn’t burn fuel to charge them if they did that 😀

The car’s functions use more energy than the amount of energy that can be made by the car. Charging a battery would actually take some energy itself. Since you mentioned “by their own movement” let’s look at an aspect of that. A car has wind resistance. If you wanted to harness the energy of the wind rushing over the car, that would cause even more resistance (just as some of wind’s energy is used to turn the blades of a turbine before the rest is converted to electricity) causing the car to have to expend more energy to compensate and that would be more than the wind would generate. Say the normal energy required =5. Wind capture gives 1 back to battery (4). Increased wind resistance causes car to work harder to and takes 2 from battery. End result car now needs energy 6 to run.

Add that to the energy the car uses to just keep running (friction of moving parts, friction with the road/ gravity) there is no way, even if a wind capture system *wasn’t* a net loss, that it could power the whole vehicle. All other types of “car powering itself by it’s own functions” run on the same principle. You lose energy by friction or resistance capturing the energy you get to use. Obviously regenerative breaking can capture energy, but that is because it is actively working against the car’s movement, which of course you want to do when braking, but not when you are moving forward.

So much misinformation and good information followed by terrible corrections and vise versa in this post. As a life-long car guy and a mechanical engineer, I’m restraining myself from replying to almost everything. Those of you out there who are experts on this subject and setting people straight, keep up the good work. OP, good luck sifting through the info.

As others have already said, they can be and many are – our former Toyota Highlander Hybrid SUV would start to recharge every time we coasted down a hill rather than using the gas or used the brakes (the “regenerative braking” Fallen-Halo mentions).

Not that the battery part was worth much given you’d eat through the charge in a matter of minutes in stop&go traffic or going slowly in a parking lot….

We use the word “generator” to describe anything that uses movement to create electricity.

An electrical load on the generator’s electrical parts *creates* a mechanical load on the mechanical parts.

In other words, when you have an electrical current that can do something like charging a battery, it *acts as a brake on the generator*.

This is good when you’re in a situation where you want the generator to stop. For example the user is indicating they want the car to stop by putting their foot on the brake pedal. A lot of electric cars actually do charge the battery in this situation, it’s called *regenerative braking*.

However you can’t get more energy out than you put in. If you start with the battery at say 60% charge, then discharge the battery to 59% charge to make the car go, then use the car’s motion to charge the battery, you’ll discover that the car comes to a complete stop when the battery’s only recharged to 59.7% or so, considerably less than its original level.

As other posters have noted, you can sort-of get around this by driving downhill. That’s because you’re adding an additional source of energy, the car’s elevation above sea level. If you’re up somewhere in the mountains, and go downhill while engaging the regenerative braking, you could charge the battery from 60% to 61% while driving downhill, let’s say you dropped 4000 feet in altitude. That energy came from somewhere: You gave up potential energy by getting 4000 feet closer to the center of the Earth.
If you turn around and use the battery to drive up 4000 feet in altitude back to your original location, you’ll find that the battery goes down to below its original level — 59.5% or so.