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.

{ 11 comments… read them below or add one }

1 Ben N November 29, 2022 at 12:00 pm

The thing to remember is that it takes energy to GET UPHILL in the first place!
For all the “free” energy that you get coasting downhill, you had to put in EXTRA energy going uphill before that!

Gravity actually does not PROVIED energy. The energy is put in to the system by fighting the gravity in the first place.

2 Sal December 9, 2022 at 12:23 pm

It can be done. no one want’s too. drive the alternators of the wheels and tires with some gearing. It’s not brain surgery.

3 Ben N December 9, 2022 at 12:55 pm

So, if the alternators get the energy from the wheels, where do the wheels get their energy from?

4 Ben N December 9, 2022 at 12:58 pm

It’s always shocking how many people don’t understand simple scientific concepts.
I try to educate as best I can and hope that’s a positive influence on the world.

5 Ben N December 9, 2022 at 1:01 pm

If the energy comes from the motion of the axle, that means that a rotating axle will SLOW DOWN as energy is removed from it and transferred somewhere else – such as into electricity through a generator or alternator.
There’s no free lunch.
For energy to go somewhere it has to be removed from somewhere else. In the case of rotating masses – it means slowing down.
Essentially, trying to power alternators from the wheels or drive shaft(s) would be like trying to drive with the parking brake on. You can only end up LOSING energy!

6 coy Fuller December 18, 2022 at 4:00 pm

Ben, Your stuck on the first step of the big picture. Look further.
On an IC engine the alternator/generator is driven by pulley/belt from the crankshaft. The effect of the energy from the crankshaft to the driveshaft exists, but is minuscule in comparison.

The power from the generator could then provide power for the sight and sound features of the car as well as provide at least some degree of charging to the batteries, thereby extending the range at least some amount.

Actual design calculations may prove the benefits too small to be worth the added cost. However, it is a cost feasibility determination. not a technical assumption only as you seem to insist.

7 Ben nichols January 17, 2023 at 12:33 pm

I have the answer I was an electrician for 17 years for an electric car what would charge an electric as u drive it I’ve known for 30 years or so but don’t no who to see about it tesla was interested in giving me a job in calafiona to far for me to go to work as I am from hull I could make it with the help of a car manufacror of a team of engineers I no I can build the device I need but would need a lot of time and money to see this through cos it would have to be specially made not summat u could just make in your garage am nor bothered about money I would like to build it cos I want to make a proper all electric car that charges the battery’s while u drive it can’t believe how fucking thick these people like testa can’t do it I can and am just a normal guy

8 Ben N January 17, 2023 at 2:43 pm

We’d love to see your plans for a “self-charging” car!

Please share the details!

9 Ben N January 17, 2023 at 2:45 pm

If you are talking about a car with an engine/generator AND an electric motor for propulsion, that’s just a HYBRID car! We already have them, and they work well.

I’m talking about a situation in which people insist that a fully-electric car can charge itself by means of adding an alternator to it.

10 Smith January 18, 2023 at 7:44 pm

Why could you not have a gear reduction Box from each of the wheels to a alternator? You could have it geared were it created virtually no resistance, I understand it will still need charged eventually but you could theoretically double if not triple you battery charge.

11 Ben N January 18, 2023 at 8:27 pm

The problem is that you simply CAN’T get more energy out than you put in.
It doesn’t matter how much you gear it.
If you have a bicycle with a light on the front wheel, powered by a small dynamo, you need to pedal to power it. It doesn’t matter which gear the bicycle is in. It will ALWAYS take more energy to pedal the bike powering the dynamo than without it.

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