In school I learned that braking is a waste of fuel as the kinetic energy transfers to sound and heat (which is effectively a loss, right?). So how does regenerative braking recoup some of that energy into the car’s battery?
In: Engineering
This isnt easy to ELI5, but I’ll give it a shot. Basically an electric motor can do two things. When you supply it with electricity, it spins. If you stop giving it electricity and spin the motor by hand, it creates electricity.
So when you’re stepping on the gas the battery is supplying power to the motor, but when you take your foot off the gas the forward momentum will drive the motor and create electricity. While its creating electricity it also creates physical resistance. That resistance works to slow the vehicle down or in the case of descending a hill, keeps the vehicle from gaining speed without using the brakes.
“Regenerative Braking” is misleading as most people think you have to use the brakes to get the effect when it has nothing to do with the brakes at all. Some people I’ve dealt with think it means their brake pads regenerate from use (I’m a mechanic btw).
Regenerative brakes don’t currently stop a car, when the brake is applied a small motor is engaged which converts the kinetic energy into electricity which can be used to charge the car battery. This slows the car down but is not yet able to stop a car so they are used in combination with your normal brake disks. It is not just a marketing gimmick. As the technology improves they could replace traditional brakes.
The way electricity is usually generated is you take an electric motor and spin the shaft. This causes the magnets inside to rotate next to the coils, which induced an electric current. So basically an electric motor can be used two ways: you can hook it up to a battery and use electricity to spin the shaft, or you can hook it up to a battery and spin the shaft manually and charge the battery. You might see where this is going.
Sine electric motors can be used in “reverse” to create an electric current, some engineer got the idea that since the wheels on an electric car are hooked to an electric motor, why not set up the cars battery to harvest the electric current created by slowing down? So when you press on the accelerator, the battery drains as the motors soon the wheels. But as soon as you let off the pedal, the car will begin to use the deceleration of the vehicle to recharge the batteries.
It’s not a lot, and like you say, some of it is lost to friction and heat, but it’s a clever way of extending the life of the batteries.
Braking is essentially wasted energy in most brakes. This will seem obvious, but the energy you put into your car to make it go fast (“kinetic energy”) will take your car a lot further if you don’t brake. One of the “laws of physics” states that energy cannot be created or destroyed, but only transformed – so where does this kinetic energy go when you brake?
Most brakes work with friction – friction happens when you rub one object against another. If there is a lot of resistance, energy is transformed into heat! Essentially, braking turns speed (“kinetic energy”) into heat(“thermal energy”). This is why cars can overheat if they are driving downhill and braking a lot – the engine isn’t making too much heat, the brakes are.
For a while it was impossible to get this energy back – it just isn’t reasonable to design something that can recreate fossil fuels from brake heat. But this changed when cars started using batteries because we know how to make electricity easily.
Now, instead of wasting energy by converting it all to heat, we can convert the speed back into electricity! And then reuse that electricity to make the car start again.
An electric motor is basically the same thing as an electric generator. One turns electricity into something rotating and the other turns rotation into electricity.
Regenerative braking is when you break by having the turning wheels of your car drive the generator to charge a battery.
You might be familiar with the concept if you ride a bicycle that doesn’t come with a battery powered light. Many bikes generate the electricity to power their lights with a small dynamo that rolls along the sidewall of your tire. Using this you can create electricity from just your bike moving forward, but it makes it a slight bit harder to pedal and if you just let your bike roll along without pedaling the dynamo makes it stop earlier.
If you scale this up enough you can have a dynamo that slows down the wheel so much that it acts as a break.
This way you charge your battery up a bit every time you break. It means less wasted energy as normally the energy would just be converted into heat when breaking and lost.
In city traffic where there is a lot of stop and go it can make a rather big difference.
Electric motors and generators are basically the same thing. They have a series of permanent magnets and a coil of wire inside. When you apply an electric current to the coil, it generates a magnetic field, which pushes against the magnets (opposites attract, likes repel) causing the motor to rotate, turning the wheels and making it propel your electric car.
However, this can also work in reverse. If you apply an external mechanical force to the motor, without applying power to it, the rotating magnets will generate a current within the coil. Causing the motor to become a generator. This current can then be collected and used to recharge your car’s battery. Of course there’s no such thing as free energy and no generator is 100% efficient. So some energy is lost along the way through forces like friction and heat. Which slows down rotation of the wheels, causing the entire vehicle to slow down in the process.
Regenerative breaking in EVs is used similarly to engine breaking in internal combustion vehicles. It’s no replacement for the mechanical breaks, but it does assist them. It’s a proven system that has been used on trains for a long, long time. That’s why diesel-electric locomotives have all those big fans on the roof. Since they don’t have batteries, the electric current is passed through huge resisters and converted into heat. It helps slow the train down while saving wear and tear on the mechanical breaking system.
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