Regenerative Braking & Coasting - Why/When These Are Good Things

[NeilBlanchard]

Regenerative braking aka regen - is using the electric motor in the opposite way it is used to move the vehicle. The motor can be powered by energy from the battery to move the car - or the moving car can be slowed down, by using the motor to *generate* energy, that recharges the battery a bit.

So, instead of using the friction brakes to slow the vehicle - which simply turns the kinetic energy of the moving vehicle into HEAT - it instead is able to regain a portion of that stored kinetic energy of the moving vehicle, and turn it back into electricity that can be put back into the battery. Some energy is still lost (to heat) - but a good portion of it is regained, to then be used again. Regenerating ​electricity is an important way to make electric cars even more efficient than they already are.

While I am on the subject of the stored energy of the moving vehicle - the MOST efficient way to use this energy (that you have "invested" by accelerating the mass of the vehicle) - is to *coast* whenever possible. So, backing off the accelerator when you are anticipating a stop, or when you see that you don't need to keep accelerating - means that you stop using any more energy, and then use the stored energy of the moving vehicle - to continue to move the vehicle forward *essentially* for free.

This is why some electric vehicles - VW and Hyundai and Kia, among others - can coast by default. An electric car doesn't need to idle the engine when coasting (like an internal combustion engine (ICE) vehicle - so when they are able to coast (without any regen) when you lift your right foot, you can move the vehicle forward *for free*. This can vastly improve your driving efficiency.

Once you drive an EV that does this, you intuitively just learn how to do it. You will find that coasting opportunities are much more common than you would have expected them to be. And when you need to slow the vehicle down - use regenerative braking to do so.

BUT - this is why 1-pedal driving by default is problematic: just because regen is better than friction brakes, does NOT mean that regen should be implemented when you lift your right foot. Please understand that many EVs are designed to have some regen integrated onto the accelerator pedal - mainly this is done for an understandable reason - but in my opinion, this is short sighted - it makes a vehicle behave like an ICE vehicle.

But it negates the ability to coast.

One pedal driving takes the idea that if regen is a good idea - and it IS a good thing - then it leaps to the idea that MORE regen is better. I think that 1-pedal driving BY DEFAULT - or especially as the only option - is too much of a good thing, so to speak.

Here's why I think that having 1-pedal driving is not necessarily a good thing - you can try to "feather" the accelerator, to just keep the vehicle moving forward at a constant speed - but this takes a lot of effort, and you will inevitably accelerate (a bit or a lot) more than you need to, and then you will use regen to slow back down - which by definition, is less efficient than coasting.

Let's state the obvious: ​we are driving a vehicle (most of the time) to move ourselves / things from point A to point B.​ And the most efficient way to do that - is to accelerate only as much as needed, to accomplish that.

One-pedal driving is more likely to cause the driver to accelerate more than is needed, to just get from point A to point B. One-pedal driving tends to "encourage" a racier mode of driving - to accelerate right up to point that you need to stop. SINCE regen can only regain some of the energy "invested" in the moving vehicle - and since 1-pedal driving makes it essentially impossible to coast freely - it removes the best way to move the vehicle forward i.e. coasting. Coasting is the most efficient way to use that stored kinetic energy - so EVs that don't even have the option to allow coasting - is less than ideal.

If you live in a mountainous area, then having strong(er) regen modes when you lift your right foot ARE a good thing. We should trust the driver, and make it possible to choose whether you coast by default, or have mild/medium/strong level of regen, when you lift your right foot. I do think that regen should always be integrated onto the BRAKE pedal - and MOST EVs do this.

The big exception though - is Tesla. While most other things are very well designed on Tesla vehicles - and in the case of having very low aerodynamic drag, Tesla is better than most, if not all of other EVs available today - this design choice is a puzzling exception.

So, to sum up my points - we should have coasting by default on all EVs (at least as a set option), and we should have driver selectable regen modes, for having regen integrated onto the "go pedal" - and regen should always be integrated onto the "stop pedal".

Please discuss!

[jakobnev]

Safety is another concern I have with 1-pedal driving: If you always need to have your foot on the accelerator to not lose speed, then you are going to lose the habit of hovering the brake pedal when approaching situations with potential for danger.

[freebeard]

I drive my Superbeetle with the right hand resting on the pull-up hand brake. If the parking brake were electric (like an FUV), then the hand lever could be dedicated to regen with the foot pedal having it's normal function.

[Isaac Zachary]

I've always thought that there should be a coast/regen option.

When I had the Nissan Leaf I would constantly be shifting into neutral. This allowed me to coast to slow down.

With my hybrids, if the engine stops I will do the same thing and shift into neutral. But if the engine is running I don't want it to run just because and not do something productive. So I will feather the accelerator pedal to where the vehicle is close to neutral. It's sure hard to feather it exactly in a neutral position though.

I'd prefer a way to chose between coasting when I take my foot off the pedal and several levels of regen. I think this could be easily integrated into the shifter, from a manufacturer's standpoint. I wonder if I could do something to my hybrids to do the same.

When to coast vs When to regen

The first thing that comes to mind is safety. The more you coast the sooner you have to let off the pedal before a turn or stop. In a town where there's one stop sign and then another down a block or two, you may have to barely get any speed in order to coast to the next stop, even if you're going to use your brakes a little. Driving around at 10-15mph on 25-35mph streets not only causes everyone else to want to kill you, but also kind of defeats the purpose of having a faster-than walking/biking vehicle.

Since there are a lot of hills aroud where I live, also taking into account how much speed can be lost/regained off of the next hill affects my decision to coast or not. A lot of times I will start to coast, even though I start going slower than the speed limit. Then, as I get speed back up I'm back at the speed limit by the time I'm at the bottom. It takes me longer to get to that speed, but at least I'm not accelerating than braking, even if it is regen braking. I do take into consideration if there's someone behind me and if there's a passing lane and if there's traffic coming that prevents those behind me to be able to pass.

[euromodder]

It's the same with ICE cars, innit ?

Lifting the accelerator gets you 0 fuel use and braking
That's the part being taken over by regen - when you NEED to brake

But coasting is more efficient to cover the distance @ decaying speed


Isn't there the option to not lift off the accelerator completely in 1 pedal driving, and thus to be not accelerating nor braking ? (ie coasting)

Originally Posted by jakobnev

Safety is another concern I have with 1-pedal driving: If you always need to have your foot on the accelerator to not lose speed, then you are going to lose the habit of hovering the brake pedal when approaching situations with potential for danger.

Then again, approaching situations with potential for danger could just as well be a good time to lift off the accelerator

[euromodder]

Originally Posted by Isaac Zachary

I'd prefer a way to chose between coasting when I take my foot off the pedal and several levels of regen. I think this could be easily integrated into the shifter, from a manufacturer's standpoint.

It sure can

BMW does that with their ZF 8 speed auto, in Eco mode

That's a popular tranny, so other car makers could also have it programmed that way

[Hersbird]

The Pacifica Hybrid has the dial shifter like in the gas version but when in neutral it coasts no regen, in drive it regens about 4-6 kW and in low it regens about 35 kW. Of course with any break pedal application it increases regen up to about 85 kW.

I never go to neutral, but I do put it in low all the time as that slowing amount is like a downshift on a manual going down a long hill or normal brake application approaching a stop. Maybe I'll try neutral more when I really just want to coast.

[jakobnev]

I'd prefer a way to chose between coasting when I take my foot off the pedal and several levels of regen. I think this could be easily integrated into the shifter, from a manufacturer's standpoint. I wonder if I could do something to my hybrids to do the same.

I like Hyundai's solution with having regen adjusted with the flappy padles by the steering wheel.