Regenerative Braking - Best Practices?

Congrats on your new SE!

When you use the brakes, you still get some regen braking, but some of your kinetic energy is turned into brake heat. When you use regen braking only, you maximize the amount of energy going back into your battery. And it's more fun, too.

 

Coasting > regenerating > braking.

You'll use the least energy if, when it comes time to slow down or stop, you feather the throttle until the needle shows neither acceleration nor regeneration and allow the car to coast as far as possible.

If you need to stop sooner, regenerating is worse than coasting, since you have fewer losses coasting (letting that kinetic energy create distance and a little friction heat -- tires, air, and bearings) than you would regenerating and turning that Ek into electrical, then chemical, then back into electrical, electrical back into mechanical, and mechanical back into Ek.

Of course, braking is the worst, since it turns Ek into heat, faster.

 

We love debating these topics, so keep asking. What @F14Scott wrote is right on. If you must use some kind of braking and it's not too late to use regen braking, I don't think it matters much if you apply it gradually or all at the end of your deceleration. I think it's fun to see if I can use full regen braking as late as possible to see if I can accurately guess the braking distance.

An interesting observation one forum member made (which I choose to believe because it's what I would prefer to be true) is that you don't save much energy by accelerating slowly in a BEV. So tromp on that one pedal and then plan ahead so you can coast to a stop using no braking of either variety.

 

My take on this subject is the regenerative braking will recover as much energy as it can, so the amount of stopping power is limited by how much regeneration is taking place. This seems to be reinforced by the two regenerative modes, since it takes a lot longer to come to a stop in low regen. I do what @insightman says, and take my foot off the "go" pedal to let it brake at maximum trying to time it to come to a complete stop at the right point. I think "feathering" the regenerative braking is self-defeating, because by slowing down the braking effect you're actually expending energy.

I avoid using the friction brakes, since that loses energy and wears out the brake pads. Of course it can't be avoided in some circumstances, so it's good there are friction brakes.

As for coasting, it's not clear to me if it is beneficial or not. The idea behind coasting is you're using the energy you've already expended accelerating the car (assuming a flat surface), which could indicate too much energy was spent getting to that speed. In the SE the only way to effectively coast is to switch into neutral since the neutral drive mode is such a tiny window and would take great skill to hold. I have noticed using the cruise control seems to be a way to optimize that, and I've thought of testing cruise control versus using neutral for hypermiling. I've also wondered if an app to indicate the slope of the vehicle would be useful for hypermiling, but that's a whole other thing.

 

With practice, you can modulate the accelerator to hit that sweet spot for coasting. Note that driving mode has a significant impact on how easy it is to achieve that. Green mode is easiest since the accelerator response is much slower, whereas Sport mode is hardest due to much faster response to your accelerator inputs.

The past month or so, I have been honing my skill (in Mid mode) such that I can stab the accelerator to quickly get up to my desired speed then quickly back off just the right amount to land in the coasting section without dipping into regen. Much like learning how to throttle the pedal to get the right amount of regen as you approach a stop, this is something that takes some practice to build up the muscle memory. I'm getting there...

 

My last car was an old manual ICE with some drivetrain slop and elasticity (and a cable actuated throttle). Trying to drive at a slow constant speed in traffic without using the clutch a ton required super precise throttle position to prevent lurching and jerking.

That plus rev matching was a great 5 years of training for the one pedal driving in the SE! Not to brag, but I can quickly hit the coasting zone even in sport mode

I still haven't quite gotten use to matching the accelerator pedal position to my current speed when I have to abruptly turn my cruise control off though! I'm usually still a little bit off. The first time it didn't even cross my mind that the car would suddenly regen at full strength... definitely a unique problem to EVs with one pedal driving.

 

Same! If I can anticipate turning off the cruise control in advance (e.g. approaching my exit), I'll slowly tip in the accelerator until the car just starts to accelerate, then deactivate the cruise.

 

Coasting is desirable, if not for EV newcomers adapting, for when you gradually want to slow over a distance like when approaching a lower speed limit zone or approaching an exit ramp that is still a way off. The ability to ramp regeneration back to one pedal on the off ramp would become the most beneficial. Hyundai and I'm sure others use the level two (or three) autonomous driving abilities to augment the kind of regeneration dynamically removing the guess work. Until then it's mostly preference and I'm sure you'll find a lot voiced here.

 

It is actually quite a tricky problem. When you are traveling at a constant speed, your power use is your friction losses (rolling + aero) and your accessories load (ie, car/radio/air con/heater etc). If we ignore the accessories, you have the situation:

speed = v
energy = 1/2v^2
Friction = f(X) - ie dependent on speed

You come to a stop when the energy has been used up by friction. If you are coasting, you use no energy to propel the car - but you regenerate none. When you keep going and brake (only regenerative) at the last moment, you will use energy to drive, but get say 75% of your kinetic energy back. If you are going 100kph (27.77m/s) in a car which weighs 1450kg (including driver), then the car has: 559413J (joules) of energy. If you (regen) brake, and get 75% back, you will get 419559J, or 0.116kWh.

At 100kph, the energy required to drive is about 14.5kWh/100km, so you are using 14.5kW propelling the car. You can drive for:

0.116/14.5*3600 = 28.9 seconds before using 0.116kWh. This means if you drive for less than 28.9 seconds before braking, you will use less energy. If you need to drive for longer than 28.9 seconds to cover the same distance as coasting for the same distance, you use more energy. Now in the real world, things may be different. You may only get back 60% of the energy (you are not only slowed by regen, but by friction as well), or efficiency/slope, wind etc changes the parameters, you may use more (or less) than 14.5kW etc., but it gives an idea.

 

The Mini blends the friction brakes and regen when you use the brake pedal to slow down, right? It doesn't just automatically use the friction brakes when you put your foot on the "brake", surely.

Rob

 

Yes but also no.

The brake pedal does not influence the regen but while pressing the brake pedal, the car will be using regen at full strength since the gas pedal isn't pressed (unless you press the gas and brake at the same time for some reason)

In a quick stop the transition typically looks something like this:

1. Release the gas pedal to move your foot to the brake: the car smoothly transitions from power to full regen (feels like around a second or less)
2. Press the brake pedal: friction brakes come on. Depending on how quickly you moved from gas to brake, the regen may already be at full strength or it still may be transitioning to full regen.

With one pedal driving, blended braking isn't necessary since the above method feels so natural. In the majority of your driving you'll never actually have to use the friction brakes to stop. They're really only for emergency stops (and after you have stopped completely to make sure your brake lights turn on)

 

I'm grateful the SE provides such a wide coasting zone not just for efficiency, but also because I hate activating the brake lights unnecessarily. I've read that some other cars like the Chevy Bolt, switch immediately from acceleration to regen braking, making coasting difficult.

 

Wide? I think the coasting zone is pretty small. To me it's like clutchless shifting a manual transmission. Some people can get the skill, but it definitely takes practice. Then there are those of us who just give up.

 

The coasting zone might be wider than you think. My custom interior brake light shows me when I've backed off too far and that has helped me learn how wide the zone really is. Of course, I'm counting on brake-light activation to signal the start of regen braking, which might not be completely accurate.

 

There's a huge difference between green, mid, and sport so it would definitely help to clarify which you are talking about!

 

Sorry for omitting that info. I always drive in the default Mid mode.

I never drive far enough to need a Green mode and I know the SE is just as quick in Mid mode as in Sport mode if I press the accelerator far enough.