Why You Can’t Put A Wheel Generator On Your EV And Drive Forever

Thermodynamics Generator Ts2
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There’s a certain post that floats around the Internet with some regularity, and it caught my eye. It shows us a Chevy Bolt with what appears to be some kind of generating apparatus attached to the rear wheel. The post states that the rotation of the car’s wheel is used to recharge the EV’s battery as it drives along, avoiding charging stops and paying for electricity! Genius, right?

And yet, we know that’s too good to be true. If slapping an alternator on the rear wheels was enough to make electric cars run forever, everyone would be doing it. We’d never burn a drop of petrol again.

You might be wondering, though, precisely why this doesn’t work. Or, you might have a rough idea, but you want to be able to definitively explain to family and friends why this isn’t the magical solution to all of America’s transport woes. Today, we’ll dive into thermodynamics and examine what’s really going on here. Don’t worry—it’s easy! Plus, we’ll even try and figure out why someone might have built this in the first place!

No Free Lunch

Thermodynamics Is Not a Dirty Word

Let’s start by examining what we have here. We have the rear wheel of the car connected via a belt drive to what appears to be an alternator or generator of some sort. When it spins, it makes electricity. That electricity could of course be put to use charging a battery.

That all makes sense. But does this mean we should all be putting generators on our EVs to drive forever? Well, no. Because no matter how much energy you get out of the generator, you’re spending more power to turn it using the car’s motor. It’s simply not possible for the generator to produce more energy than the Bolt’s motor had to spend to get it turning in the first place.

Let’s make a diagram. I’m using the terms “kinetic energy” and “mechanical energy” to mean basically the same thing—energy from motion. It’s a little simplified, but it should give you an idea of how this all goes down.

Infographic Gen
Consider the car as if it were an isolated system, up on jacks. The battery runs the motor, which turns the wheels, which turns the generator, which charges the battery again. How much energy ends up back in the battery? Less than it put out!

It all comes down to the thermodynamic principle of conservation of energy. In very basic terms, energy cannot be created or destroyed. It can be converted from one kind to another, but you can’t create new energy.

This is why perpetual motion machines, free energy machines, and this “wheel generator” concept don’t work, and simply cannot work. If you had some kind of rotating machine which rotated forever, and you could get some kind of work out of it – say, by having it turn a generator – it would be creating energy. That’s simply not possible, due to the laws of physics. For the same reason, you can’t put a generator on your car’s wheel, recharge the car’s battery via the electricity generated, and drive forever.

In the case of the Bolt, the electric motor converts the electrical energy from the battery into motion. It accelerates the vehicle and spins the generator. The generator then turns rotational energy back into electrical energy again. All the energy coming out of the generator originally came from the EV’s battery itself. The generator didn’t make any energy, it just converted energy from one type to another.

2013 Sema Chevrolet Zz5 350
An alternator puts out less electrical energy than it draws in kinetic energy from the engine. When you convert from one type of energy to another, you always face losses.

A further lesson that thermodynamics teaches us is that every time we convert energy from one type to another, we lose some. For example, a light bulb turns electricity into light, but we also lose some as heat. Combustion engines turn chemical energy into motion, but they also have losses through heat and noise.

It’s the same case here. The energy from the EV’s battery is converted multiple times, each time with some level of efficiency less than 100%. The EV’s motor doesn’t turn 100% of the battery energy into forward propulsion, and the generator doesn’t turn 100% of the rotational energy it harvests into electricity. Even if you feed its output back into the EV battery, you’d be getting less energy out of the generator than you were spending to turn it in the first place.

Simply put, turning the generator adds to the load on the vehicle’s drive motor. The EV’s motor has to work harder to push the car down the road because this generator is now on the back siphoning energy off the rear wheel. It’s thus drawing more energy from the traction battery than it otherwise would if there was no generator hanging off the wheel. Whatever energy you get out of the generator will be less than you’re spending to turn it.

But What About Regenerative Braking?

It is worth remembering, as well, that EVs have ways of turning rotational kinetic energy into electricity anyway. It’s called regenerative braking, and in itself, it perfectly explains why a wheel-attached generator won’t give you free unlimited energy.

When an EV engages regenerative braking, it essentially turns its motor into a generator that is attached to the wheels. What happens? Applying the load of the generator to the wheels slows the vehicle down. It turns the vehicle’s kinetic energy into electrical energy to charge the battery. The generator can’t run without slowing the vehicle down. The electrical energy has to come from somewhere!

Naturally, there are some losses involved in the conversion. You can’t accelerate up to speed, then regeneratively brake, and get all the energy back. Some energy is lost in overcoming rolling resistance and aerodynamic drag, and some is lost through electrical resistance in the electronics and as heat through the motor. Similarly, as the generator converts rotating motion to electricity, there are more heat losses in the electronics, motor, and battery in turn.

2020 Bolt Ev One Pedal Driving And Regen On Demand Chevrolet Canada 1 16 Screenshot

Sure, you might say that this rear wheel generator would generate electricity if the vehicle was going downhill, using no battery power to accelerate the vehicle. Yes, that’s true. But the Bolt’s electric motor is already capable of acting as a generator in that case anyway. Plus, you’re still not getting energy for free. You had to spend energy to get the Bolt to the top of a hill in the first place before you could reap the energy by rolling back down.

Many Such Cases

There are all kinds of “simple” and “free” sources of energy touted in snappy little videos on the Internet. For example, one touted the idea of putting turbines inside water pipes to get “free” electricity from the flow of water. Of course, this would be completely pointless for a great many water pipes, as their flow is generated by the city’s water pumps. The pumps would have to work harder to overcome the resistance presented by the turbines, using more energy than the turbines could generate.

Another example that is often bandied about is the use of an “HHO gas generator” to make a combustion-fueled car more efficient. This involves using a car’s electrical system to split water into hydrogen and oxygen gases. The mixture is then fed into a car’s intake to promote “cleaner” or “more efficient” combustion. Here’s the thing, though—even if the gas injection did improve combustion, you wouldn’t be getting out ahead. The energy required to split the gas would outweigh any potential energy you’d get back out of it when it passes through the engine.

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What’s Really Going On

The post shown at the start of this article is actually not the original. Somebody saw the photo of this vehicle and made an assumption about what was going on. Then, they spun their tale of free recharging that apparently Big EV was too stupid to implement.

The original photo was actually made by a user called rhinnaflor on Reddit, a full five years ago. Posting to r/whatisthisthing, they asked as to the use of this contraption. There are some compelling theories in the comments, and they don’t violate thermodynamic principles.

Belt contraption attached to the rear wheel of a Chevy Bolt
byu/rhinnaflor inwhatisthisthing

AJ_Mexico pointed out that the Chevy Bolt may be a “dingy” vehicle that is typically towed behind an RV. With modern cars, it’s often recommended to tow them while switched on. However, this can drain the vehicle’s 12-volt battery over time if it is towed for many hours. The theory was that the system attached to the rear wheel was a simple alternator that had been hooked up to keep the Bolt’s 12-volt battery charged while it was being towed with its rear wheels on the ground.  This wouldn’t do anything for the main traction battery, and thus it wouldn’t improve the vehicle’s range. It’d just keep the 12-volt battery topped off, though you’d normally expect the Bolt’s traction battery would do a fine job of that already.

Naturally, this energy still isn’t free. The mechanical resistance of the generator would make the RV towing the Bolt expend more fuel—marginally—than if the generator wasn’t connected. Of course, there are easier ways to handle this task, too. One could simply fit a 12-volt solar battery tender to the Bolt, or hook up some kind of power line from the RV’s own supply. These would be much simpler.

Others suggest it might be some kind of instrumentation for data collection, but I’m not sure I buy that. It’s a very janky install, and the belt drive wouldn’t be super great for accurately tracking the wheel’s motion without slipping. It’d only be worse in wet conditions and when there’s any amount of suspension travel.

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The generator seems to be hidden to a degree by some kind of black tape. And what’s going on with that thing in the middle of the bumper?

If we zoom in further, though, this case gets more mysterious. There appears to be a fuel filler neck, or maybe some kind of cabling, tucked under the rear bumper of the car. Commenters speculated as to whether some kind of standalone combustion generator might have been installed underneath as a range extender, but the jury is out as to the truth.

It’s very difficult to definitively pin down what is going on in this image. Our best guess is that it’s some kind of wheel-powered electrical generator, but for what reason, we can’t say. If you’ve seen anything like this before, or you know the car in question, perhaps you could shed some light on the matter for all of us.

Ultimately, I hope you found this article educational. You should have a better idea of why you can’t get unlimited range from slapping a generator on your car’s rear wheels. You might even feel confident enough to call out others who bandy about these long-disproven ideas. As much knowledge as there is out there on the Internet, free energy and perpetual motion ideas will seemingly never die. All we can do is call them out and have a chuckle when we see them.

Image credits: via Reddit, via AliExpress, Chevrolet

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172 thoughts on “Why You Can’t Put A Wheel Generator On Your EV And Drive Forever

  1. I mean….do we really need an article that goes THIS hard trying to explain a picture that otherwise just elicits a facepalm from anyone with a simple understanding of conservation of energy…..

    1. Do not overestimate the average citizen’s knowledge or underestimate their arrogance. Probably a depressing number of people think they are SO MUCH SMARTER than those thousands of engineers working in the automotive industry.

      1. I design engines for OEMs, my interior designer fiancée refuses to believe me when I tell her she’s in the wrong gear because she knows better.

        But then I also thought or dual aspect living room looked fine painted in just the one shade of green, so it’s not like I respect her professional knowledge either.

        It’s all Dunning Kruger Effect.

        1. I dunno, is there math wizardry or metrics to prove the best color for a living room?

          No?

          Then that’s just like, your opinion, man!

          On a more physical level perhaps you two might literally perceive colors differently (e.g. color blindness, tetrachromacy) so what looks good to you might look terrible to her.

          OTOH there are very provable, easily understood reasons why not to lug an engine or rev the crap out of it. Like you’ll blow it up.

          1. Sadly there is maths behind the two shades of green. The two walls with windows in them will always look darker from lack of light (plus sunlight is bluer than you think) so they get painted a lighter shade of a similar green so that all four walls look the same colour. If you’re custom made everything in the room to match or contrast with a particular shade then apparently it’s worth making your fiancé paint the same sodding wall four times until it’s the right colour.

            I still think it would have looked great in white with a motorcycle next to the TV. I thought offering to use my green Kawasaki was a fair compromise towards colour.

            1. Maybe you LIKE a more natural, slightly darker window wall.

              “plus sunlight is bluer than you think”

              Depending on the time of day, time of year, your altitude and how clean the air is. Unfiltered sunlight (as in seen from space) peaks in the blue green part of the visible spectrum (so it looks much greener in space) but is blunted towards red by the atmosphere and the particulate matter in it which is why it changes color depending on how much air its traveling through and how clean that air is.

              https://sunwindsolar.com/wp-content/uploads/2013/06/SunWind-WEB-Solar-Radiation-Spectrum-V2-1200×750.png

      2. Or the depressing number of people think they are SO MUCH SMARTER than those thousands of doctors and researchers working in the health care industry.

        That kids is literally how you get measles.

    2. Sometime, not that long ago, ignorance was something to be ashamed of; now it is celebrated, and the dumbest among us is revered and elected, instead of the best.

  2. I mean….do we really need an article that goes THIS hard trying to explain a picture that otherwise just elicits a facepalm from anyone with a simple understanding of conservation of energy…..

    1. Do not overestimate the average citizen’s knowledge or underestimate their arrogance. Probably a depressing number of people think they are SO MUCH SMARTER than those thousands of engineers working in the automotive industry.

      1. I design engines for OEMs, my interior designer fiancée refuses to believe me when I tell her she’s in the wrong gear because she knows better.

        But then I also thought or dual aspect living room looked fine painted in just the one shade of green, so it’s not like I respect her professional knowledge either.

        It’s all Dunning Kruger Effect.

      2. Or the depressing number of people think they are SO MUCH SMARTER than those thousands of doctors and researchers working in the health care industry.

        That kids is literally how you get measles.

    2. Sometime, not that long ago, ignorance was something to be ashamed of; now it is celebrated, and the dumbest among us is revered and elected, instead of the best.

  3. Glad you mentioned HHO generators. I have a friend who is convinced that they work despite me explaining many times that it’s simply not possible to create energy without expending even more energy.

    1. That was my queue to get off Facebook when I was just spending my time debunking BS links friends posted. I said even IF that HHO generator worked as is, the fact that the video had full jars of water in his car for months means that barely any (if any) gas was produced.

  4. Glad you mentioned HHO generators. I have a friend who is convinced that they work despite me explaining many times that it’s simply not possible to create energy without expending even more energy.

    1. That was my queue to get off Facebook when I was just spending my time debunking BS links friends posted. I said even IF that HHO generator worked as is, the fact that the video had full jars of water in his car for months means that barely any (if any) gas was produced.

  5. I always jokingly tell people that any energy source that puts out more than it consumes is essentially a bomb.

    The output feeding back into the input if somehow more than it started with will exponentially ramp up to infinity and consume the input entirely. In simpler words, BOOM.

  6. I always jokingly tell people that any energy source that puts out more than it consumes is essentially a bomb.

    The output feeding back into the input if somehow more than it started with will exponentially ramp up to infinity and consume the input entirely. In simpler words, BOOM.

    1. As a senior level design engineer, most often found in startups, I frequently found myself surrounded by non-technical executives whose non-existent technical expertise was overwhelmed by their self assurance, personal confidence, and organizational hammer. They usually surrounded themselves by a cadre of agreeable drones. Unfortunately, extensive resources usually needed to be expended to offset the ignorance.

      By far my favorite hammer science, was when our Pres DJT suggested that we inject or ingest BLEACH to solve the covid situation. The drones were silent.

      1. One of my favorites was an executive that asked if there was some kind of light that shot out… dark light… like, a bulb that you switch on that makes a room darker.

        1. Can’t make this stuff up.. This story is bringing out a lot of how engineers/scientists struggle with others. Usually our struggle is with technology, not interactions. I have often wished that part of the technical curriculum would have a class on “How To Deal With Buffoons”

      2. I worked on a project to create a new EV variant of an existing ICE vehicle, and one of the accountants insisted we’d save money if we carried over the gas tank, as the cost falls as the volume goes up.

    1. As a senior level design engineer, most often found in startups, I frequently found myself surrounded by non-technical executives whose non-existent technical expertise was overwhelmed by their self assurance, personal confidence, and organizational hammer. They usually surrounded themselves by a cadre of agreeable drones. Unfortunately, extensive resources usually needed to be expended to offset the ignorance.

      By far my favorite hammer science, was when our Pres DJT suggested that we inject or ingest BLEACH to solve the covid situation. The drones were silent.

      1. One of my favorites was an executive that asked if there was some kind of light that shot out… dark light… like, a bulb that you switch on that makes a room darker.

        1. Can’t make this stuff up.. This story is bringing out a lot of how engineers/scientists struggle with others. Usually our struggle is with technology, not interactions. I have often wished that part of the technical curriculum would have a class on “How To Deal With Buffoons”

      2. I worked on a project to create a new EV variant of an existing ICE vehicle, and one of the accountants insisted we’d save money if we carried over the gas tank, as the cost falls as the volume goes up.

  7. The simpler solution would be a deployable hook at the front, that could latch onto the rear bumper of a semi trailer. Once attached, just let the vehicle regen as it’s dragged along, and charges on the move.

    1. I have debated about getting something like that installed on one my vehicles especially for cross country trips. So many times I am going down the freeway thinking damn if I was just tied to the back of that semi I would be saving so much money.

      1. If you trust the truck driver, you could even crawl into the back and take a nap, full self driving for free…. In all seriousness, an actual controlled, engineered version of this would be my top choice for some sort of autonomous vehicle capability

        1. If you had a trained driver, and maybe a special system of straightish roads with very few stops, you could link up massive trains of linked vehicles, with everyone reading a book or sleeping.

    2. Years ago I tried drafting behind a pickup truck pulling a large trailer, which definitely increased my fuel economy. The person driving the truck must have known what I was doing, I don’t remember the specifics but I remember them driving in such a way that made it easier for me to follow close for well over an hour. I only tried it that one time, as it wasn’t exactly a relaxing way to drive, and not everyone would be as accommodating as the person I was drafting behind.

  8. The simpler solution would be a deployable hook at the front, that could latch onto the rear bumper of a semi trailer. Once attached, just let the vehicle regen as it’s dragged along, and charges on the move.

    1. I have debated about getting something like that installed on one my vehicles especially for cross country trips. So many times I am going down the freeway thinking damn if I was just tied to the back of that semi I would be saving so much money.

      1. If you trust the truck driver, you could even crawl into the back and take a nap, full self driving for free…. In all seriousness, an actual controlled, engineered version of this would be my top choice for some sort of autonomous vehicle capability

        1. If you had a trained driver, and maybe a special system of straightish roads with very few stops, you could link up massive trains of linked vehicles, with everyone reading a book or sleeping.

    2. Years ago I tried drafting behind a pickup truck pulling a large trailer, which definitely increased my fuel economy. The person driving the truck must have known what I was doing, I don’t remember the specifics but I remember them driving in such a way that made it easier for me to follow close for well over an hour. I only tried it that one time, as it wasn’t exactly a relaxing way to drive, and not everyone would be as accommodating as the person I was drafting behind.

  9. It doesn’t work stealing energy from the wheel to charge the batteries to turn the wheels. We all know this.

    Instead, you need to find a free source of energy. Anybody who has ever ridden in the back of a pickup truck knows that it is windy a.f. back there. Wind is renewable. This is known science.

    Therefore, we need to mount wind turbines in the back of our electric trucks. Or, if you are towing a camper with your electric truck, cover the top of the camper in small turbines. If done right, you will have infinite range.

    1. As a teenager I came up with the brilliant idea to cover the top of the car in solar panels and the bottom and sides in tiny wind turbines. This was before I went away to engineering school.

  10. It doesn’t work stealing energy from the wheel to charge the batteries to turn the wheels. We all know this.

    Instead, you need to find a free source of energy. Anybody who has ever ridden in the back of a pickup truck knows that it is windy a.f. back there. Wind is renewable. This is known science.

    Therefore, we need to mount wind turbines in the back of our electric trucks. Or, if you are towing a camper with your electric truck, cover the top of the camper in small turbines. If done right, you will have infinite range.

    1. As a teenager I came up with the brilliant idea to cover the top of the car in solar panels and the bottom and sides in tiny wind turbines. This was before I went away to engineering school.

  11. It’s just a function of gearing. If they gear it right, those losses become gains, just like how I can go further on a turn of the crank on my bicycle if use a small gear in back and a big one in front. Same turns = more distance. If you get the ratio right, you don’t even have to turn it. /S/ in case it’s not obvious.

  12. It’s just a function of gearing. If they gear it right, those losses become gains, just like how I can go further on a turn of the crank on my bicycle if use a small gear in back and a big one in front. Same turns = more distance. If you get the ratio right, you don’t even have to turn it. /S/ in case it’s not obvious.

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