We already have a magnetic core, why can't we use it to recharge the batteries? The only problems I see with it are potentially wiping magnetic data, but doesn't the electromagnet have to be revolving around the damageable device?

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    $\begingroup$ How would an electromagnet help charge the batteries? And you do have the alternator that does exactly this, recharging the battery, by using some of the energy of the engine. $\endgroup$
    – nasu
    Commented Jan 23, 2021 at 2:39
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    $\begingroup$ Most of the answers assume an internal combustion engine vehicle, are you talking about that or electric? $\endgroup$ Commented Jan 23, 2021 at 10:00
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    $\begingroup$ This is a "cargo cult question". It looks and sounds like it might be a question, it has interrogative grammar structure, and it even has a couple of question-marks -- but it refers to a bunch of sciency-sounding things and doesn't explain how they might be related or why you should think of them at the same time. $\endgroup$ Commented Jan 24, 2021 at 17:01
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    $\begingroup$ Probably interesting info for the OP: Electric cars do convert some of the energy from cars wheels back to electricity during braking. $\endgroup$ Commented Jan 24, 2021 at 22:58
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    $\begingroup$ Sounds like OP has invented the hybrid car. $\endgroup$ Commented Jan 25, 2021 at 0:48

7 Answers 7


This is basically what happens in the alternator. The car's engine, which turns the wheels, also turns the alternator's rotor. The magnetic rotor is surrounded by coils of wire, and induces a current that charges the battery. It is important to note, though, that this does take energy from the engine: nothing is for free.

  • $\begingroup$ It takes energy - yes, actually they taking off energy works like a brake. Therefore the whole idea wouldn't work. $\endgroup$
    – DonQuiKong
    Commented Jan 24, 2021 at 17:11
  • $\begingroup$ Electric cars actually can and do (at least to a certain degree - but that is question of safety rather than anything else) break in this manner. $\endgroup$
    – peterph
    Commented Jan 24, 2021 at 21:50
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    $\begingroup$ @DonQuiKong I think you and others might be misinterpreting the question. The OP didn't ask whether it's possible to create a free energy machine. The question is whether we can recharge car batteries using the spinning of the wheels. The answer is yes. In fact, this happens in all cars. It's a zero-sum game, so you couldn't use this to generate more energy than you had starting out, but you can definitely use this convert some of the mechanical energy into electrical energy. $\endgroup$ Commented Jan 24, 2021 at 22:17
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    $\begingroup$ @JoshuaTS interesting, I read it again and that's the only interpretation that makes sense to me. If it's not about free energy, then what? Not needing the alternator? Anyways, one of us is interpreting the question incorrectly, I agree. $\endgroup$
    – DonQuiKong
    Commented Jan 25, 2021 at 10:29

Yes. This is how regenerative braking works.

There is a style of brakes that is used in many electric or hybrid cars that, rather than converting the motion of the wheels into heat energy the way conventional brakes do, instead convert the motion of the wheels into electrical energy that recharges the car's battery. By doing this, they can improve the performance of the car and extend how far it is capable of traveling when driven in environments with frequent braking, like most cities.

There's no technical reason why such a braking system couldn't be installed on a car with a purely internal combustion engine, though I expect that the energy that they produce might be wasted once the battery has been fully recharged, and the extra cost probably isn't worth the marginally increased battery performance.

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    $\begingroup$ It's a matter of scale. In a car with an IC engine, very little power is needed from the battery, and the fraction of the energy drawn from the IC is insignificant relative to the energy used to turn the wheels. In an all-electric car, trying to charge the batteries from the battery-powered engine is just a cycle that wastes power. Regenerative breaking essentially uses the momentum of the car as an energy source. $\endgroup$
    – chepner
    Commented Jan 23, 2021 at 16:19
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    $\begingroup$ Your suspicion about wasted energy is correct. From my years of experience driving a hybrid in mountainous country, about 2000 vertical feet of steep downhill would fully recharge the battery, after which the regen braking function shuts off. $\endgroup$
    – jamesqf
    Commented Jan 23, 2021 at 18:22
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    $\begingroup$ Many years ago I rode a shuttle bus that went up and down a steep hill. The vehicle pool got tired of replacing brakes and mounted a generator on the axle. The driver was provided with a separate hand lever for braking. The output of the generator was just routed to a resistor so the bus could be slowed without using up the brake pads. $\endgroup$ Commented Jan 23, 2021 at 20:49
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    $\begingroup$ @Dmiters That system (phone charging) is considerably more complicated since you wouldnt want to provide intermittent power. Also note that the energy was being wasted before the addition of the generator too. Adding a more efficient (due to less wear on disposible parts) braking system is an improvement, not a waste. $\endgroup$
    – Matt
    Commented Jan 24, 2021 at 15:01
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    $\begingroup$ Railroads in the US have had a system called dynamic braking (en.wikipedia.org/wiki/Dynamic_braking) since the 1930s, where current generated from axle generators is routed into large resistor banks to help slow trains. There is typically a separate radiator and cooling system just for these resistor banks. It is not unheard of for these resistor banks to get hot enough to damage the paint on the locomotive. $\endgroup$
    – Dan Delany
    Commented Jan 25, 2021 at 15:22

This is a classic "Free Energy" setup, which sounds great...until you realise that crummy ol' conservation of energy.

The problem is the induction principle. It's what makes electromagnetic motors possible, but it's what stops us from free energy. The problems is that the faster the wheel spins, the harder the magnet is going to resist that spin as it generates electric current.

Because the magnet is going to resist the spinning of the axle, which would be what makes wheels rotate, you would always need to have more power from your engine than the power which the electromagnetic axle generates. In essence you would generate some electricity in your axle, sure...but you would always have to use more power from your engine.

You can run through the math and whatnot, but that is the basic idea.No such thing as a free lunch, especially when it comes to energy!

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    $\begingroup$ But, since vehicles often need to brake, the resistance can be used to avoid dissipating that braking force as heat. Which is why many hybrid or electric cars have regenerative braking which recharges their batteries. Improving efficiency but not providing free energy. $\endgroup$
    – matt_black
    Commented Jan 25, 2021 at 11:50

Because of how electromagnetism works, plus a lot of practical issues

Other answers have focused on what is currently available and why that works. I'm going to aim at the deficiencies of your exact idea.

The first problem is that simply wrapping wire around the axle won't achieve anything, because in order to generate electricity, the coil needs to be perpendicular to the direction of motion. So we're looking at coils laid along the axle somehow. Not only is this going to be difficult to achieve, but it'll create all sorts of problems with driveshaft balancing.

Then we have the problem of how a generator works. You don't just need a coil, you also need a separate magnet as well. This could be a fixed magnet, or an electromagnet. One part of this is fixed (the "stator") and the other (the "rotor") is attached to the spinning axle. You can choose whether you want the generating coil to be the rotor or stator, or the magnet, depending on how you want to build this. So we need something big and magnetic attached to the axle - which in a front-wheel-drive car is part of the unsprung weight of the car, by the way, so that's undesirable. Let's skip that detail and say we've got a rear-wheel drive car with a driveshaft though.

Then you hit practical problems. Axles and driveshafts are in an area of the car which is very prone to flying stones and other debris. Not a good place for delicate electrics. They also twist substantially under load, which is not good news for anything fastened to them.

But the real killer is electrical. The efficiency of magnetic coupling depends on how close together the magnet and coil are. Driveshafts will inevitably wobble under load, so you'd need the stator to be spaced away to allow room for that. This is a major blow to your efficiency.

To make matters worse, electrical requirements mean the coil needs to be a lot of turns of very fine, very thin wire. I mentioned stone strikes earlier. These are going to be "very* vulnerable. Sure, you could put a big casing around it for protection - and this is all starting to get not very practical.

Compare and contrast to an alternator. Because it doesn't have the same mechanical constraints, it can be built to put the rotor and stator close together, with fine wire, all inside a conveniently sized package. That's why we do it that way.

And finally, it'd only work when the car is moving. Unless you have an alternator connected to the engine as well, you're going to run out of battery if you're stuck in traffic. And if you've already got an alternator, this is redundant.

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    $\begingroup$ +1 for this, which is the key issue with the OP's concept -- plus the next paragraph too. «The first problem is that simply wrapping wire around the axle won't achieve anything, because in order to generate electricity, the coil needs to be perpendicular to the direction of motion. So we're looking at coils laid along the axle somehow.» The other issues are engineering/practical constraints, so not really physics, but relevant too. :-) $\endgroup$
    – Noldorin
    Commented Jan 24, 2021 at 18:08

Not stated in your question is the type of vehicle we are discussing - I am assuming this is about vehicles powered by internal combustion engines.

Attaching some sort of electricity generating device to the car's final drive to recover kinetic energy during braking comes down to cost/benefit.

Every piece of equipment you add to a car increases its mass, which reduces its overall efficiency - more fuel consumed to haul it all around.

Car engines are equipped with alternators to provide electrical power whenever the engine is running, in part because the car doesn't have to be moving in order to power lights and accessories, charge the battery for the next start, and so forth.

The amount of power moving around the electrical system and the amount of energy stored in an automotive battery is relatively small compared to the kinetic energy of a moving vehicle and power required to get one up to highway speed in an acceptable amount of time.

An energy recovery system would need to be rather sophisticated to capture a useful amount of energy from a decelerating car - the slower it is moving, the less efficiently it will work.

Electric cars use regenerative braking, it works, and is worthwhile because the motors used to drive the vehicle are operated in reverse; the same strategies which allow the motors to efficiently accelerate the vehicle over a wide range of speeds allow for efficient energy recovery when decelerating. It's a matter of re-using equipment that is already there.

In summary, the existing alternator meets the need and is well-matched; attempting to recover kinetic energy during braking as electrical energy would be too poorly matched to the requirements of the electrical system to be of practical benefit for the cost incurred.


The car's alternator would be more efficient and economical way to charge the battery. If you had generators or alternators on the wheels the engine would have to work harder to turn the wheels as there is no way to receive more energy from a machine than is put into it.


We already have a magnetic core, why can't we use it to recharge the batteries? The only problems I see with it are potentially wiping magnetic data, but doesn't the electromagnet have to be revolving around the damageable device?

This is precisely how cars both charge their batteries and produce electricity to run things like lights and the radio. The magnetic core can be anywhere in the power train between the engine and the wheels. Generally, it's easiest to connect it to the belt that already runs between the engine, the air conditioning compressor, the power steering pump and sometimes other pumps.

The device that does this is called an "alternator".

More modern cars also use this to convert excess energy to electrical energy during deceleration or going downhill.


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