How does Regenerative Braking slow you down??

[owlowlowlowl]

Interesting thread, as I was thinking about this while driving our new model 3 picked up this past Saturday.

When lifting my foot off the accelerator and feeling the regenerative brakes kicking in at a greater rate, I see the screen showing the Tesla with brake lights. My question: are the brake lights really turning on mainly as a courtesy to cars behind me, since I assume the physical brakes aren't actually engaging?

 

[swaltner]

This is correct on the brake lights. It’s a warning to people behind you know you are rapidly slowing down. No mechanical brakes are involved, but the reaction of the car is the same, so the brake lights are activated.

Note that the brake lights are activated by the rate of acceleration, not by the power of the braking action (in kW for example)? What’s the difference? If you let off the throttle here in Kansas, which is flat as a pancake (while Mt Everest is also flat as a pancake by the same analysis), the brake lights will activate because of the rapid slowdown. Do the same thing while descending down a steep mountain pass in Colorado and while the car is attempting to slow down just as much, the actual rate of slowing down isn’t fast enough to trigger the lights, so the lights stay off.

As far as I know, this is behavior that is unique to Tesla, partially because it is the only EV with regenerative braking that is strong enough to risk someone rear-ending you. For example, the Leaf has fairly strong regenerative braking compared to a hybrid, but is more like the engine braking on a vehicle with a manual transmission, so they don’t light the brake lights until you touch the actual brake pedal.

 

[owlowlowlowl]

Thank you for the explanation. Hard to observe not being able to drive behind myself.

This is correct on the brake lights. It’s a warning to people behind you know you are rapidly slowing down. No mechanical brakes are involved, but the reaction of the car is the same, so the brake lights are activated.

Note that the brake lights are activated by the rate of acceleration, not by the power of the braking action (in kW for example)? What’s the difference? If you let off the throttle here in Kansas, which is flat as a pancake (while Mt Everest is also flat as a pancake by the same analysis), the brake lights will activate because of the rapid slowdown. Do the same thing while descending down a steep mountain pass in Colorado and while the car is attempting to slow down just as much, the actual rate of slowing down isn’t fast enough to trigger the lights, so the lights stay off.

As far as I know, this is behavior that is unique to Tesla, partially because it is the only EV with regenerative braking that is strong enough to risk someone rear-ending you. For example, the Leaf has fairly strong regenerative braking compared to a hybrid, but is more like the engine braking on a vehicle with a manual transmission, so they don’t light the brake lights until you touch the actual brake pedal.

 

[T3forMe]

Seems like you've got it. The motor acts as a generator to put energy back into the battery. It's just the reverse of what's happening when you're accelerating.

The forward motion of the car produces kenetic energy. The motor becomes a generator when no electrical power is applied. The battery becomes a load to the generator when you remove your foot from the accelerator converting kenetic energy to electrical energy. The greater the load on the gererator, the greater is the force required to turn the generator. This force slows the car down. Hope this helps.

I have my dymanic braking set to high. I do love single peddle driving.

 

[animorph]

You might imagine a gas-powered electric generator. Now rip off the gas engine. In its place, put a big pulley with many turns of a wire cable wound around it. Stake the generator solidly into the ground. Drive a Tesla by it at 25 MPH. Magically attach the free end of the cable to the trailer hitch on the back of the car, still going 25 MPH, just as it goes by the generator. Now the Tesla is pulling the cable, turning the pulley and turning the generator.

If the generator is lightly loaded (say just a small TV), the Tesla slows down gradually due to the drag on the cable caused by the generator. It travels pretty far with that light drag.

If the generator is heavily loaded (say an active A/C unit), the Tesla slows down rapidly and doesn't get very far.

That's as close an example as I can get to regen braking. Your foot on the pedal controls the load on the generator, and the energy harvested goes into the battery.

 

[brucet999]

What i still don't understand is how any of that slows down the car. How is it exactly that the car slows down without me having to use the brakes??

Without going into the physics of electromotive force working on a generator, all you need to know to conceptualize regenerative braking is that when you are driving, the battery sends electricity through the motor, providing the force to make its shaft spin and move the car. When you take your foot off the accelerator, the momentum of the car causes the motor's shaft to continue turning, and that motor then acts as a generator sending electricity into the battery.

Why does it slow the car? It takes energy to run a generator, otherwise we wouldn't have to burn vast amounts of coal and natural gas to spin the generator shafts in electric power plants. In the case of regen, that energy comes from the momentum of 5,000 pounds of Tesla rolling down the road and spinning the motor/generator shaft until (most of) that kinetic energy is used up.

 

[EVChris]

So I basically understand regenerative braking. You're using the motor as a generator.

On the one hand, folks are saying their civilian brakes are lasting 'forever'. On the other hand, the Performance edition has upgraded brakes for track use. Furthermore, the Bosch video mentions torque blending.

If I'm doing One Pedal Driving with full regen when are the friction brakes engaged? Just the last 5 mph or are they torque blended even with normal driving?

 

[ohmman]

So I basically understand regenerative braking. You're using the motor as a generator.

On the one hand, folks are saying their civilian brakes are lasting 'forever'. On the other hand, the Performance edition has upgraded brakes for track use. Furthermore, the Bosch video mentions torque blending.

If I'm doing One Pedal Driving with full regen when are the friction brakes engaged? Just the last 5 mph or are they torque blended even with normal driving?

Regen is limited by the motor and the battery's capacity to take a charge. So there are number of situations where one will use the friction brakes, but most of them have to do with the limited amount of regen you have available at the time. If the pack is nearly full, or if it's very cold, it has limits on how much it can take, and you'll have to use friction brakes. If you're going down very steep grades, you'll often need to use friction brakes. Also, in situations of rapid deceleration (track, sudden stops in highway traffic, etc), you'll use them as well. And you'll need them to be powerful enough to stop the car even if regen is completely limited, which should help to explain why they are upgraded.

The only blending I'm aware of on a Tesla is when regen is active but you're not slowing fast enough and you manually hit the brakes. At that point you're using both friction brakes and regen to slow the vehicle.

 

[TMK26]

... What i still don't understand is how any of that slows down the car. How is it exactly that the car slows down without me having to use the brakes??

If someone asks me how this all works I'm just going to say "It's magic".

 

[jdbob]

As far as I know, this is behavior that is unique to Tesla, partially because it is the only EV with regenerative braking that is strong enough to risk someone rear-ending you. For example, the Leaf has fairly strong regenerative braking compared to a hybrid, but is more like the engine braking on a vehicle with a manual transmission, so they don’t light the brake lights until you touch the actual brake pedal.

It has been reported that the new Leaf will also turn on the brake lights when using the one pedal driving mode (E-Pedal).

 

[ThosEM]

Here's what may be a helpful question. In an internal combustion engine car, what slows the car down when you take your foot off the gas in a low transmission gear and before you apply any brakes?

In that case the energy of car is used to keep spinning the engine even when little or no fuel is entering it and no explosions are pushing the pistons forward. That energy goes into sucking in air, squeezing it in the absence of fuel, and pushing it out the exhaust system without having burned any fuel to heat it. All that makes quite a bit of drag in a low gear that spins the engine fast.

My first impression of strong regenerative braking was that it feels a lot like an old-fashioned car driving in a low gear. Downshift and you can generate quite a bit of drag. The electric motor/generator does it under the control of the inverter instead of a transmission.

 

[K10T5LA]

It’s called counter torque. When you let off of the accelerator you stop applying current to the stator. The magnetic field of the rotor continues to move because they are connected to the wheels and the momentum of the car keeps it moving forward. That rotating field of the rotor generates a voltage on the stator which is then rectified and regulated and then applied to the battery. When this happens current then flows through the stator which creates a magnetic field that opposes the rotors magnetic field. This interaction between the fields creates torque in the opposite direction of the rotor’s spin which will slow it down. The more current flowing through the stator, the more counter torque that is produced. The more current flowing into the battery, the better the regen rate. To understand this better, there are some other concepts that might help like: generator action, motor action, and faraday’s law. Hope this helps.

 

[arnolddeleon]

I don't know if the OP is old enough to have ridden a bike that had lights powered by a generator that were spun by the tire. Anyone has this experience will know how much harder it was to pedal when the generator was engaged and that should give an intuitive feel for how regenerative braking works.

 

[Chris L]

I wrote a three-page tutorial on single-pedal speed control which goes into everything about regenerative braking (I call it magnetic braking, because that is what it is).
SinglePedalShort.pdf

 

[BestHand]

Ok, thanks this is making some sense. but still hard to visualize how the process of generating electricity physically slows down the wheels.

It will be easier to understand if you will know that when there no regenerative breaking it is relatively easy to spin the wheel. As soon motor will start generate electricity it will be much harder to spin the motor (wheel). You will feel it like big resistance to spin.