Why Don’t Electric Cars Have Alternators?

A common question we get about D.I.Y. Electric Car projects is “Why don’t you just add an alternator to charge the car as you drive?”

So, how about it? If fossil-fueled cars have alternators to charge their battery, why don’t electric cars just do the same?

To find the answer, we need to know how an alternator works and where the energy comes from.

An automotive alternator is a type of generator which creates Alternating Current (AC). A belt from the vehicle’s engine spins a pulley which is connected to the rotor of the alternator. On the shaft are either permanent magnets or copper wire coils with a small DC current flowing through them. The flow of current creates a magnetic field. As the shaft rotates, this magnetic field induces current to flow  in a matching stationary set of coils. Because the magnetic fields on the rotor alternate polarity (north/south), the direction of the flow of current induced “flip-flops” many times per second, reversing direction as well. We normally call that “Alternating Current”.
The alternating current is converted to direct current (DC) with diodes, and a voltage control circuit makes sure that the output voltage is appropriate for charging the vehicle’s 12V battery.

Where does the original source of energy for the alternator come from?
Scientists such as Galileo, Newton, and Bernoulli studied the natural world and came to realize that there are certain “rules” that are always followed. This eventually came to be known as the Laws of Thermodynamics.
According to the Law of Conservation of Energy, “Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.”

In the case of the alternator, the fossil fueled engine (connected by a belt and pulley) provides MECHANICAL* energy to the alternator which converts that energy to ELECTRICAL energy. The electricity isn’t “created”, it’s just some other form of energy which is converted. That means an equal amount is given up from the engine. The engine has to work harder and burns more fuel to be able to provide this additional energy.

Another important concept to remember is that the mechanical energy of the engine is not PERFECTLY converted to electricity. Some of it is converted to noise, heat, and vibration. Although efficiency is far from 100%, it’s still very useful to convert mechanical energy to electricity to charge the battery and run the electrical systems of a vehicle.

If an alternator works fine to charge a battery, why don’t we use one in an Electric Car?
The two main reasons are:
1) An E.V. doesn’t have an engine
2) There’s a BETTER device to use than an alternator.

Remember, the ENERGY that powers the alternator is actually the mechanical energy created by the engine through combustion of fossil fuel. An electric car doesn’t have an engine or gas/diesel fuel. Instead, it has an electric motor and rechargeable battery. We COULD use the battery to spin the motor to spin the alternator to generate electricity. BUT at best, we would simply be using electricity to make electricity. Even worse, there are still losses on the way. Conversion losses often show up as heat, but noise and vibration as well. Essentially, using an alternator powered by an electric motor, we could only ever make a smaller amount of electricity than we started with. (If we could create an equal amount or more, we would have a Perpetual Motion Machine.) However, we COULD make a different voltage. An electric car battery pack is typically over 300VDC, whereas we only need a 12V system for headlights, radio, and other accessories.

DC/DC – A Better Way:
If we only want to convert direct current from one voltage to another, there’s a much more simple and effective way to do it. That’s through a device called a DC to DC Converter. This is an electronic device which takes direct current at one voltage and converts it to a different voltage. It’s still not “free energy”. Converting to a higher voltage means also getting a lower current. Getting higher current means also getting lower voltage. Either way, it’s still the same amount of energy, minus losses, which show up as heat. Basic DC/DC converters dissipate the heat through a metal case. Higher power converters have heat-sinks, fans, or even active liquid cooling.

DC/DC Converters have several advantages over alternators. They are compact. They save weight. They are more efficient. There are no moving parts – no belts to replace, no pulleys, no bearings. That also means they are maintenance free.

If you purchase a DC/DC converter for your own DIY Electric Car project, keep in mind the input voltage and the maximum power. You want the input voltage it’s designed for to match the voltage of your battery pack. Many converters allow for a range of input voltage and are listed as so. For example, one might be listed as “48-96V IN / 13.5V OUT”. Power is rated in the maximum number of Watts that can be produced by the converter. In a car, you need enough power for the headlights, radio, and other accessories. This might be as much as several hundred watts. Remember to fuse the output of the DC/DC converter, and use appropriate gauge wiring for the output current. With rare exceptions, a DC/DC converter is always still combined with a 12V battery, just as an alternator is.

Electric Motor AS Generator:
Lastly, the AC motors used in commercially-built electric vehicles make excellent generators. But you still can’t magically create power while driving the car. Remember, we can only CONVERT energy from one form to another. There are still times when this is useful. Converting the KINETIC energy of a car to some other form (electricity to charge a battery, or heat of friction from traditional brake pads) will remove the kinetic energy and thus SLOW DOWN a moving vehicle. This is perfect when you want to slow down anyways! On an EV, we exploit this and call it “Regenerative Braking”. It’s a great way to recapture some of the energy used to originally accelerate the car, and increases the total efficiency of the vehicle.

In another scenario, a vehicle can be coasting down a large hill or mountain. The POTENTIAL energy is converted to KINETIC energy. In a manual transmission fossil fuel vehicle, the driver might downshift and use the compression of the engine to help slow the vehicle. In an electric car, the motor can charge the battery to do the same. It should be noted that the battery needs to be at least partly discharged to have a place for the electricity to go for regenerative braking to work. Better to live at the bottom of a mountain than the top of a mountain, as a fully charged battery won’t be able to accept any more charge through regenerative braking.

Alternators are neat technology, allowing us to convert mechanical energy into electrical energy. Similar technology is used on a large scale to generate electricity at power plants. Alternators work great for fossil-fuel-burning vehicles, but are complicated, inefficient, and high-maintenance compared to DC to DC Converters used on Electric Vehicles.

We hopes this give you a better understanding of alternators and electric vehicles for you to be able to work on your own Electric Vehicle Project.

-Ben Nelson and the 300MPG.org team



*A fossil fuel engine actually takes CHEMICAL energy (in the form of gasoline, diesel fuel, etc.) and ignites it, converting it to THERMAL energy which then moves a piston (LINEAR MECHANICAL ENERGY) which then turns a crankshaft (ROTATIONAL MECHANICAL ENERGY) which only THEN goes to the alternator via a belt. Energy is lost at every step of the conversion process.
Even worse than that, we need to get the fuel for the vehicle to start with. It takes a tremendous amount of energy to search for oil, drill for it, transport it, refine it, ship it to a gas station, and then finally get it in our fuel tank.
When we look at a total “Well-to-Wheels” scenario, a modern passenger car is possibly the most INEFFICIENT machine in human history.

{ 26 comments… read them below or add one }

1 Mark August 7, 2019 at 12:38 pm

Why not apply an alternator to the hubs or spindles of the wheels. The motion of the car rolling is your energy being transformed into energy going back into the battery.

2 admin August 8, 2019 at 11:25 am

Most electric cars have regenerative braking. When slowing down, the motor becomes a generator and recharges the batteries. No alternator needed.

3 John M WOOD August 29, 2019 at 9:06 am

An alternator takes very little energy to spin it. To say that you would be loosing more in the transfer is ridiculous. I have made homemade water wheels that I attached and alternator to and was able to have electricity to run lights. This was simy using tg h ed flow of a small stream. This shows how little energy need to spin the alternator, which on turn produces twice the energy put in. ⁷ My belief is that we could have very efficient electric cars, if and only if the capitalists can figure out a way to profit from it. As long as those in power are making money from a renewable resource, but convincing everyone that its comes from fossils and that there is limited supplies, then we are screwed. Just saying.

4 admin August 29, 2019 at 9:27 am

An alternator does NOT put out MORE energy than it takes in.

For a water wheel or hydro-generator, the source of the energy is the weight of the falling water. Now imagine that the energy made by the water-wheel generator has to pump the water back uphill to power the generator. That’s exactly what you are trying to do if you want to spin an alternator from an electric motor and then use that electricity to charge the battery to run the motor.
It’s like trying to pick yourself up by grabbing your own belt and lifting.

Spinning an alternator is very different than getting a significant amount of power out of it. The more power you are getting out of it, the more the electro-magnetism resists and the more drag the alternator puts on the engine. An alternator is a lossy way of converting mechanical power to electrical power

Energy needs to come from an external source, like charging a battery from wall power, from the movement of falling water, or filling a fuel tank with gasoline. You can never get more out than what you put in.

5 Andy September 19, 2019 at 11:32 am

I think you are skipping Mark’s point.an alternator/ Dynamo on each hub would have very little rolling resistance and should be able to add to the regen system while driving and not braking.I really cba to do the calculations, but his idea does hold up as a constant top up.

6 Johnney Dean October 2, 2019 at 3:02 pm

I know EXACTLY how to install an alternator that almost continuously charges the battery. It’s very simple…But I will talk to Mr. Elon Musk only. IF there’s anyone who could get me such a meeting. I would certainly compensate them. #(321)216-7114… Call ANYTIME.

7 Devin October 23, 2019 at 4:33 am

I seen the above response now and your explanation makes sense. The electric motor would still have to produce the power to spin the alternator thus cancelling each other out.

8 Bashier Sinclair October 28, 2019 at 2:14 pm

Why not use four wheels as pullies for four individual alternators to charge a complete free set of rechargeble battries, waiting for the cars clever system to select/replace new pack when it’s almost run out with curret pack, and then alternate to use the new batt pact whilst the current one will be charging….Two system to run as one …there will be no conflics and all electric cars can probably work to full potential without having to stop to charge.

9 admin October 28, 2019 at 2:59 pm

Perhaps you should read this article: https://300mpg.org/projects/electro-metro/perpetual-motion/

10 Kenneth November 29, 2019 at 3:20 pm

Permanent magnet alternator, without iron core, less drag. Pancake 2 or 4, gets same job done.

11 Landon December 17, 2019 at 11:20 pm

Listen. The electric motor is already having to spin for the car to move. If u add an alternator it will be able to power the alternator no problem. It just needs to be charged manually one time. And if the alternator doesn’t completely charge the battery. It WILL get more range.

12 admin December 18, 2019 at 8:06 am

Hi Landon,
The electric drive motor in an EV certainly can spin an alternator no problem. The issue is that it will be HARDER to spin because of the resistance from the alternator. (Not just the bearing, the belt, etc. An alternator is HARDER to spin the more current it is producing as the mechanical energy gets converted to magnetism to electricity.)
The alternator is never going to produce more power than it wastes.
The electric drive motor will need to use MORE extra energy to spin than what the alternator can make.

If you COULD get more out than you put in by adding an alternator, why wouldn’t we want to add as many alternators as possible? Maybe add 8 or 10 for even MORE range?
We don’t because it doesn’t work that way.

If you only mean to have the drive motor to spin an alternator to charge a 12V battery – to change some high voltage main battery power to lower voltage accessory battery power, that can work. (You STILL don’t get more power out than you put in!) However, an alternator is inherently lossy, because it’s a mechanical device. A DC/DC converter is a smaller, more efficient, electronic device with no moving parts, which performs the same duty.
With rare exceptions, an EV will have a DC/DC converter to keep the 12V system topped off.

13 Ron January 16, 2020 at 1:03 am

Why can’t you have a small gas engine in an electric car that could trickle charge the batteries?

14 admin January 20, 2020 at 9:13 am

You CAN have an engine slowly charge an electric car!
But at that point, it’s a plug-in hybrid, not an electric car.
The BMW i3 with Range Extender does exactly that.

At one point, I also played around with my Geo Metro Electric conversion turning it into a hybrid using an LP generator.

15 Matt Carrell January 23, 2020 at 11:39 am

All those thinking an EV can recharge its own batteries and not need to charge from external power while its driving using dynamos or alternators attached to the EV Motor or drive shafts or wheels fail to understand how generators work when they are under load. Sure they seem to spin fairly freely when nothing is connected but when you place a load upon them, the loss of electricity to the load puts strain on the electromagnetic field (back EMF) in the generator which thus makes it harder for the generator shaft to rotate at the same speed it was when there was no load placed on it by a charging system. The alternator clamps down with backward mechanical resistance due to the increased loading of having to charge the drive batteries which backfeeds resistance to mechanical rotation on the drive system and thus puts higher demand on the EV motor to push harder to maintain the required speed thus the drive electronics have to pull increased amounts of power from the drive batteries and thus your charging system sees more drain than before the alternator kicked in on the batteries and accordingly now charger has to pull more power from the alternator again to cover the increased rate of battery drain and that puts more mechanical rotation resistance back feeding from the alternator covering the elevated power use to the charger through the drive train to the EV motor which then again needs slightly more energy to maintain the current drive shaft rotational speed (MPH) due to increased exertion needed to overcome additional mechanical resistance backfed to it from the alternator which requires the drive electronics to again up their pull from the drive batteries and the whole thing loops over and over again steadily pulling slightly ever more and more power from the drive system and thus batteries overtime to supply more charge needed by the charging electronics charging those same batteries to compensate self-created rising levels of power drain from the batteries. This trend to lose more and yet more still over time between two interconnected opposed-purpose systems is what is called a negative reinforcement feedback loop. This will run your car down faster because of all the additional losses you wouldn’t have additionally imposed if you hadn’t added the alternator and its associated drain upon the drive system in an attempt to recharge the batteries while those same batteries drive the car because the recharging energy will due to inefficient of conversion and fictional losses always creates a slightly higher demand upon the batteries to cove what it can deliver back to the batteries to recharge them, thus spiraling the battery energy down faster and faster to maintain the same road speed and much more quickly running the batteries down. The attempt to self recharge the batteries in a quest for additional mileage thus actually results in less overall mileage because all the difference between returned energy which is always less due to additional losses to conversion inefficiency and fiction imposed which are significant and unavoidable in terms of added heat resistive loss in controller electronics, internal wire resistance lost as heat throughout the connections from battery to controller and controller to battery and alternator to charger and charger back to batteries wire losses, additional inefficiency in battery losses of additional electrical charge conversion between mechanical and electrical energy due to batteries working increasingly less efficiently the higher the current demand is upon them, more loses in both motor and alternator {due to colliding field edge cancellations, stray Eddie current oscillations, heating of the metal coil cores internal electrical flow resistance, and air-gap magnetic flux variably wasted by radiating away not fully utilized for magnets to induce current in pickup coils or to counter magnetics from electrified coils as they move closer or rather away from each other and thee is always a bit of an air gap between rotating parts which dilutes the flux field} and loss to energy as mechanical fiction slowing down everything connected to the the drive shaft in resistive rotation loss due to metal to metal contact drag and heating in bearings, motive force lost inefficiently to mechanical vibration, any light grab from barely touching imperfectly functioning disk breaking system alignment or calibration (pads skating on rotors which breaks are not technically engaged but the driver), tire to road traction loss to.road gripping and tire heating, and any acting wind drag upon the car creating additional resistance counter to any force exerted in the drive train to propel the car though force backfeed and additional friction put to tires by increased counterforce, etc.)

Now as for charging the 12V battery instead of the mains by using a DC DC converter, it does remove the negative feedback loop because any increased drain on the 12V battery will not cause additional drain upon that battery because its charge is not used to charge itself thus not requiring higher usage from this battery to cover higher charging needed under higher usage an avoids all the mechanical and electric to mechanical and back conversion penalties additionally incurred if it was charged using an alternator. The DC DC converter penalty only involves one loss through transformer inefficiency and some electronic heat and much less wire resistance heating loss and no mechanical friction losses. You will loose far less power charging 12V directly electrically from the drive batteries.

You will always loose some energy moving power from any system to any another, including directly from drive batteries to 12V battery (for example) through the resistance losses of the wiring and internal inefficiencies of converting chemically stored electricity to raw electrical energy from the main batteries (heat loss) and the opposite converting electrical energy back to chemically stored charge (heat loss) in the 12V battery and the step-down voltage and regulator and charge electronics (heat losses, and step-down voltage transformer magnetic field loss inefficiencies moving power through the air or metal core from one coil to another). Those losses are much less though than if you tried to pull the addition of charging power from a mechanically connected alternator on the drive train. True the alternator would avoid small penalties of a DCDC converter (which are only like 95% efficient at best) but it adds much large additional electronics heat losses and mechanical heat losses due to the linked resistive friction of tires, superficial imperfections and friction between the bearings and their housings, mechanical drive transmission inefficiency (if one is used) and all additional vibrational and rotational friction and heat losses to U joints/differentials and mechanical matings of the motor to the drivetrain and drive line to wheels, wheel hubs if even slightly imperfect (off center or slight difference in angle of rotation between mated couplings), imperfectly balanced or align tires causing more rotational resistance due to lost traction, tire heating and vibration losses, etc etc.. you get the picture. The more junk connected, the more you are going to lose no matter what so the less stuff is connected in a system, the less your losses will be. And any kind of feedback loops should be avoided because the will always become negative feedback loops because you never gain energy to inefficiency and complexities added, you only lose….

16 Michael Wurtz April 12, 2020 at 2:46 am

It seems as if you install alternators inside all four wheels and allow the alternators to spin feely whilst your foot is on the gas and provide what is it 3 amps per alternator so 12 amps (from the battery not the motor) when your foot is not on the “gas” then wouldn’t the rolling weight of a 2000 pound vehicle be enough for 4 120amp alternators to keep the batteries topped off indefinitely 🤔 or maybe I just have too much common sense to be taken seriously.

17 Jake C April 16, 2020 at 12:30 am

Seems to me that recharging the battery is certainly something that CAN be done and probably achieved already. There is no monetization in a car that can recharge itself. The oil/energy industry is literally the backbone of the global economy. I think we we could never expect a manufacturer to offer that, however, we could hack it ourselves!

18 Oshé May 17, 2020 at 7:03 pm

You could just use ac motors instead. If you invert the power to ac to power ac motors and wire in an electric generator (switch out the gears and lube for a near frictionless turn) and for every rotation the motor makes the generator will make 5 and you can use that energy to charge the battery. AC is way more efficient and easier to convert.

19 admin May 18, 2020 at 9:24 am

That’s still just a mechanical process of converting high voltage battery power to lower voltage 12V power for the accessory battery. Belts, gears, and any other mechanical connection is still going to be less efficient than simply using electronics (DC/DC Converter) to perform the same task.
You won’t see ANY commercially built electric vehicles that use an alternator or generator for charing the 12V system.

AC motor are great, and offer almost zero maintenance and regenerative braking, but they don’t give any special advantage to generators or alternators.

20 Anil June 4, 2020 at 10:34 pm

Can anyone please answer me..
What if we transform the wheel motion of ev to charge the battery by means of alternator?
What if we consider wheel motion in ev as ic engine for alternator to regenerate the electricity for charging battery?
Can it be done?

21 admin June 5, 2020 at 9:10 am

The electric motor in a modern EV already does EXACTLY THIS!
It’s called regenerative braking. When slowing down, the AC motor converts the energy of the car movement into electricity to charge the battery.

22 tilemachos June 11, 2020 at 7:05 am

seems to me a combination of technologies can work. we can’t make perpetual energy but we can at least extend the range of an ev. Solar bonnets, solar roofs and air turbine at the front of the car (inside the bumper of course) and alternators inside the wheel, can extend the e.v range. Say from 300 to 320 miles. That 20 mile increase will increase the odds of finding a charger.

23 John B. Hite September 11, 2020 at 4:40 pm

I understand that electrical to electrical energy will not work due to power demand being greater than output, but what about rotational to electrical energy? They use regenerative braking so why not belt driven at every axle during acceleration?

24 admin September 11, 2020 at 5:12 pm

Regenerative braking SLOWS the car down.
It converts the kinetic energy (movement of the car) to charging the battery.

Battery charge goes up, but speed goes down.

It’s all about exchange.

Trying to charge the car from the wheels while accelerating would be the equivalent of hitting the gas and the brakes at the same time. It just ends up getting you nowhere fast.

25 admin September 11, 2020 at 5:13 pm

Solar would work, as it is bringing in energy from external to the system (sun shining on solar panels.)

Other things, like alternators on the wheels, are just trying to rob the vehicle of the energy needed to push it down the road. Likewise, wind turbines make it HARDER to push a car down the road. (Wind turbines while PARKED would work, although impractical..)

26 JD September 16, 2020 at 1:45 am

make the wheels the alternators

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