Brake pedal effect on regen in various vehicles?

E

ekutter

Member
Seems like using the brake pedal in various electrified cars behaves differently in each one. Anybody know of a site that might keep track of exactly what the braking behavior is for various models? It would also be interesting to understand what the various one pedal modes do in each vehicle. "B", for example, seems to have many different meanings. In BEV's it clearly just increases regenerative braking when you ease up on the gas. My 2010 Prius actually does less regen in "B" mode because it down shifts, causing more engine friction.

I'm specifically referring to when do the mechanical brakes actually kick in vs braking via regeneration. I was recently surprised to learn that in the Tesla Model S, the brake pedal does not cause any regeneration but rather purely activates the mechanical brakes, almost making more aggressive One Pedal a requirement. My 2010 Prius, as far as I can tell, is 100% regen for the first (very) small amount of braking and then adds in the mechanical brakes. So I wrongly assumed this was how all of these vehicles work.

I test drove a 2018 Leaf the other day and it seems to have 3 different modes that cause different levels of regen when you lift up on the gas. "D" gives very little regen (almost coasting). "B" mode increases the regen. E-Pedal greatly increases the regen. But I have no clue what the balance between regen and mechanical braking happens when I touch the brake pedal.

Similarly, in my 2018 Volvo XC60, there is a "B" mode which also increases regen when you lift off of the gas. But I don't know if this is identical to slightly depressing the brake pedal or not. Using the brake pedal lightly definitely increases regen but does it also add mechanical braking? I can't tell. Also can't get info on whether leaving it in "B" mode all the time has any negatives on fuel efficiency.
 
In my Spark EV, “D” mode is almost like a regular car but with some regen when you lift of the accelerator. If you lightly press the brake pedal, the regen amount increases. There seems to be a definite point at which you can feel the mechanical brakes engage.

In “L” mode the regen is much stronger as you lift the accelerator. And you can feather it for various levels. Can almost, but not quite use it for one pedal driving. And again, you can just tell the difference when the mechanical brakes engage.
 
Battery state of charge also affects this: if the battery’s full it can’t take any regen but I think most EVs are designed to blend regen and brakes, and are set to to get as much regen as possible in B mode. The Nissan e-pedal apparently blends in mechanical brakes to achieve the low speed stopping, and probably does that when the battery’s over 80% too, since the regen wont slow you down enough. My 2017 Leaf just coasts in D or B mode when the battery’s over 95%, so maybe e-pedal uses both to make the response consistent?
 
Most EV's integrate regen on the brake pedal. The only ones that do not (as far as I know), are Tesla's.

Most EV's have some regen when you lift your right foot off the accelerator pedal - only the e-Golf and Ioniq have coasting by default, and both have optional mode with regen when you lift your right foot. The e-Golf has four levels selectable by the shifter, and I think the Ioniq has one level.

Some EV's offer versions of 1-pedal driving, and usually it is optional. Only the i3 has it by default - and no other options are available. They also have the best implementation - it has an easy spot to allow coasting, when you hold the accelerator pedal in the right position.

Oh, and the Bolt EV has a charging mode called Hilltop Start, that leaves "room" in the battery for normal regen even right after you unplug.
 
Neil, all good info and similar to what I've experienced. The one piece I'm very vague on is whether the mechanical brakes engage immediately, in addition to regen, when you press on the brake pedal, or if like in my Prius the first little bit is purely regen. If it is purely regen as long as the electric motor can handle it, then pressing slightly on the brake pedal should have the identical effect on battery efficiency as using a one pedal mode. Like in my XC60, does slowing just with "B" mode actually regen more than slightly pressing the brake pedal, assuming the same rate of "braking"?

I'm guessing that without AWD, slowing down through regen is only using two wheels as there wouldn't be an electric motor connected to the other two wheels. So to get 4 wheel "braking", the non drive wheels would require mechanical braking. But again, when does this kick in if you are careful with the brake pedal. Maybe current anti-lock braking tech is smart enough that it would immediately engage the other two wheels if one of the drive wheels started slipping so that two wheel braking wouldn't be a safety issue.
 
Here is what the ABS section of the C-Max HEV/PHEV manual.

Brake Modes

Because much of the braking torque required to stop the vehicle can be achieved through regenerative braking, it is not always necessary for the conventional friction brakes to be applied. The brake pedal uses a pedal feel simulator that acts against a curved bracket to give the driver a simulated pedal resistance as the brake pedal is applied. The ABS module uses the driver input on the brake pedal, wheel speed sensor input and the lateral deceleration messages from the RCM to determine the amount of deceleration the driver is requesting and which brake mode is necessary.

Regenerative Braking Mode

The ABS module determines the amount of braking torque necessary and sends that information to the PCM along with the current threshold for ABS intervention. The PCM determines how much braking torque the electric motor can provide based on the High Voltage Traction Battery (HVTB) state-of-charge and sends this information back to the ABS module. At the same time, the electric motor is switched to a generator which supplies the determined amount of braking torque. Based on the amount of braking torque the PCM can deliver, the ABS module determines whether it is necessary to apply the friction brakes or if the brake torque from the electric motor is sufficient to safely stop the vehicle.

Friction Braking Mode

Under some circumstances, the braking torque generated by the electric motor is insufficient to bring the vehicle to a safe, controlled stop and requires the assistance of the conventional friction brakes. Additionally, the state-of-charge in the High Voltage Traction Battery (HVTB) may not allow for regenerative braking to take place or the vehicle may be experiencing an ABS, stability control event. In these instances, the ABS module activates a mechanical relay which supplies power to a solid state relay that activates a vacuum pump. At the same time, the ABS module sends a PWM voltage to a solenoid mounted on the brake booster. The solenoid allows vacuum from the vacuum pump to enter the brake booster which moves the booster push rod and applies the conventional brakes. During certain braking events, the friction brakes can be applied directly by the driver. The brake booster push rod is equipped with an adjustable stop, once the brake pedal travels far enough to engage the stop, the brake booster push rod is forced into the master cylinder and the conventional friction brakes are applied.
 
I do find it surprising that there is so much variation in how the brake pedal does (or does not) engage regenerative braking, across different models of plug-in EVs.

It would be great if someone would come up with a comparison chart, but I don't recall ever seeing such a thing.
 
ekutter said:
I'm specifically referring to when do the mechanical brakes actually kick in vs braking via regeneration. I was recently surprised to learn that in the Tesla Model S, the brake pedal does not cause any regeneration but rather purely activates the mechanical brakes, almost making more aggressive One Pedal a requirement.
Click to expand...

Unfortunately I can't speak from personal experience, not owning a Tesla car :(, but here's what one owner had to say on the subject. Please note this refers only to Tesla cars, and not other EVs:

After driving it for a day or two, you realized how much better Tesla's implementation is. In normal city driving, you are not constantly switching back and forth between accelerator and brake; it's basically one-pedal driving for the most part. At constant speed, you can either find the sweet-spot (just like every other vehicle) or use cruise control which seems to be pretty energy efficient to me. You do control the speed of deceleration by feathering the accelerator instead of feathering the brake, but either way you would be feathering a pedal, and I would rather feather the pedal that my foot is already on.

This article (http://everydaydriver.com/exotic/tesla-model-s-review/), which is one of the more well-balanced reviews, also mentions the braking system:
"Of course all this power and weight requires good braking and Tesla pioneers again by placing the regenerative feature on the accelerator pedal instead of the brake. Lifting off the go pedal begins the regenerative cycle and even illuminates the brake lights to alert other drivers. As a result the brake pedal feels like any other well-tuned automobile instead of the difficult to modulate fight so often associated with regenerative brakes."
​
(source)
 
jdbob said:
Here is what the ABS section of the C-Max HEV/PHEV manual.
...
Click to expand...
Wow, I'm impressed with the C-Max manual. By far the most detailed description I've seen. Why can't they all be that clear. Not to mention that is exactly how all electrified cars should behave in my opinion, and how I assumed most of them did.
 
ekutter said:
Neil, all good info and similar to what I've experienced. The one piece I'm very vague on is whether the mechanical brakes engage immediately, in addition to regen, when you press on the brake pedal, or if like in my Prius the first little bit is purely regen. If it is purely regen as long as the electric motor can handle it, then pressing slightly on the brake pedal should have the identical effect on battery efficiency as using a one pedal mode. Like in my XC60, does slowing just with "B" mode actually regen more than slightly pressing the brake pedal, assuming the same rate of "braking"?

I'm guessing that without AWD, slowing down through regen is only using two wheels as there wouldn't be an electric motor connected to the other two wheels. So to get 4 wheel "braking", the non drive wheels would require mechanical braking. But again, when does this kick in if you are careful with the brake pedal. Maybe current anti-lock braking tech is smart enough that it would immediately engage the other two wheels if one of the drive wheels started slipping so that two wheel braking wouldn't be a safety issue.
Click to expand...

As far as I've seen, if you brake lightly to moderately, it is all regen. As far as the non-driven wheels go, it is fine to just use regen. Front wheel drive has a theoretical advantage to generate more energy, since the front wheels provide ~70% of the brake power. But, the i3 is rear wheel drive, and yet it has excellent 1-pedal driving, and only needs the brake pedal for extreme stopping.

The thing to remember, is that coasting is the most efficient, and regen should only be used to slow down. If you need a lot of regen, then you are accelerating too much. Regen is only a less bad way to brake, than the friction brakes.
 
NeilBlanchard said:
The thing to remember, is that coasting is the most efficient, and regen should only be used to slow down. If you need a lot of regen, then you are accelerating too much. Regen is only a less bad way to brake, than the friction brakes.
Click to expand...

That is exactly why I started this thread in the first place. I really hate one pedal driving as it really is quite inefficient, assuming using the brake pedal does try to maximize regeneration. It encourages way too much braking. You get energy back from regen, but not nearly as much as it takes to accelerate. After needing to perfectly feather the gas pedal for 9 years in my Prius to maximize mileage (I usually get close to 60mpg without crazy hyper mileing, just being careful), I really prefer a car that just coasts when you take your foot off the gas pedal. I really don't mind moving my foot to the brake. Plus your foot gets to rest while your coasting compared to perfectly feathering. That was one of the biggest negatives after driving a Tesla Model S for me. Most people seem to argue that you don't need to care about efficiency because it's just electricity. It's still energy. And I'd argue efficiency is much more important in a BEV because of the limited range issues, at least for now.
 
I totally agree. The e-Golf gets this nearly perfectly. It coasts by default in D, and has 4 levels of regen - D1, D2, D3, and B in increasing order.

It also has adaptive creep, which I also really like. Normally, there is no creep; but if you get into stop and go traffic, and roll forward a couple of times, the car then has a gentle creep. It goes away again after you get above a certain speed.
 
ekutter said:
Wow, I'm impressed with the C-Max manual. By far the most detailed description I've seen. Why can't they all be that clear. Not to mention that is exactly how all electrified cars should behave in my opinion, and how I assumed most of them did.
Click to expand...

To be fair, that's from the service manual, not the user manual.
 
NeilBlanchard said:
I totally agree. The e-Golf gets this nearly perfectly. It coasts by default in D, and has 4 levels of regen - D1, D2, D3, and B in increasing order.

It also has adaptive creep, which I also really like. Normally, there is no creep; but if you get into stop and go traffic, and roll forward a couple of times, the car then has a gentle creep. It goes away again after you get above a certain speed.
Click to expand...

I wouldn't mind seeing some of these features in my i3. I'm a big fan of the one-pedal experience, but a lesser regen mode might be worthwhile while carting around passengers. I pay special attention to giving them a smoother ride than allowing heavy regen to take place. If I let off too soon, everyone goes dipping forward. So careful modulation is the key. Since there isn't any creep mode, a slightly smoother throttle tip-in would be appreciated in stop-and-go traffic. A little too much pedal and we're off to the races!
 
I'd be interested to learn exactly how much energy saving you get from regeneration. I suspect it won't be much unless you use the brakes a lot. Personally, I use them as little as possible, coasting up to lights rather than hurtling at them and slamming the brakes on etc. Its main advantage is to save wear on conventional mechanical brakes I expect.
 
NeilBlanchard said:
Martin, I agree - coasting is much more efficient than regen. And regen is much better than friction braking.
Click to expand...

Neil, do you know of any published studies that looks at this? I've heard it alleged, but many people believe differently. I'd like to see some data.
 
I would expect most of the energy used in driving a car to be expended on overcoming rolling and air resistance, both of which are irrecoverable.

You can calculate the kinetic energy in a car of mass M kg moving at V metres a second using the formula (MxV^2)/2 Watt seconds. So a car doing 60 mph (100 kph) will contain a bit over 210 Watt hours - 0.2 of a kWh - of kinetic energy. That's the most you can recover with no conversion losses. Unfortunately the energy is proportional to the speed squared, so if the car is doing only half this speed, the kinetic energy is about 53 Watt hours 0.05kWh. This is only rcoverable when you want to stop, and if you are on a long journey, I imagine you would not want to stop anyway.

These figures are wildly optimistic of course. Conversion losses would cut this down significantly, and I have assumed that none of it will be lost to air and rolling resistance which will gobble up some of the KE available too

Capacitors are better than batteries at this by the way. They will tolerate very high charge and discharge currents. For a battery, you would need hefty DC-DC conversion. I would doubt it is worth the trouble. Remember the more of these bells and whistles you add the less reliable the car becomes as well as being more expensive.

If you want to save energy, drive slowly. The savings will be much bigger. Enjoy the savings on brake pad wear and forget about improving the efficiency. Dump the recovered energy into a load resistor and forget it!
 
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