What's the yield of regenerative braking during stop & go traffic?

[voyager]

Can't seem to get a clear answer on this. The ability to win back energy during braking is one of the nicest feature of EVs of course. However, many cars spend a lot ot time getting stuck in stop and go traffic each day. Therefore my question.

See if you can fault my way of reasoning. During stop & go traffic, friction is already proportionately higher than when momentum has been built up during a steady speed (of let's say 50 mph).

Which is actually another way of saying that for instance during 10 miles of stop & go traffic, the amount of coasting meters is practically nothing... compared to having driven the same 10 miles with a steady speed of 50-60 mph.

If the amount of coasting meters is practically nothing, so can't regenerative braking yield much won back energy...

 

[arnis]

EV's can't regenerate juice at very low speeds. And there is not a lot of kinetic energy at those speeds in the first place.

Though I think I didn't understood the question itself.
Friction losses per distance travelled (1 meter) is more-or-less the same at 10km/h as at 50km/h.

Depends on driver actions. Ideally, even in stop-go traffic, driver should try to avoid speed changes.
I, for example, smooth out traffic jam traffic. If I see cars inching forward every few seconds I usually
just try to roll at walking pace. Sometimes distance to vehicle in front gets huge, like 2-5 car lengths,
but it dissolves as soon as vehicle in front of me stops and I slowly crawl closer. People behind me
just crawl with me. This is more efficient than aggressive jerking like 99% of drivers (and EAP) does.
Chill mode might solve that somewhat.

PS: climate and auxiliary draw from battery way surpass driving needs in traffic jams (walking speed).

 

[daniel]

Yeah, as arnis says, there just isn't much kinetic energy at low speeds, so there's not much energy to recover. And regen braking is not terribly efficient anyway. It does help (at faster speeds) but not nearly as much as one might think. On my Roadster, the gauge registers up to 200 kW on the positive side, but only up to 40 kW on the regen side.

 

[BrettS]

In the grand scheme of things, regen is hugely inefficient. From some tests I saw someone do years ago on a Prius forum using regen to stop from X MPH will get you back about 30% of the energy needed to get you back up X MPH in ideal conditions.

Of course even 30% is better than the 0% you get in most cars, but you shouldn’t expect miracles from regen even in the best cases.

And as you stated, stop and go traffic at very low speeds is far from ideal. You don’t get much evergy from regen at all under 5 or 10 mph. And you really don’t get any power back when you’re coasting (if you take your foot off the accelerator with “low regen” enabled then you do get a small amount of regen, but that’s not really coasting.) To get a significant amount of power from regen it needs to be actively slowing the car.

As has been stated above, to get the best efficiency you should try to maintain a constant speed wherever possible. Use regen to stop or slow down when you need to, but don’t expect to recover all or most of the power you used to get up to speed.

 

[macpacheco]

Energy spent to accelerate from 0-10mph (0-15km/h) is tiny.
Most energy wasted by cars is due to high speed cruise and slowing down from high speeds to medium-low speeds (say down to 25mph or 40km/h).
There is very little energy to be saved by aggressively smoothing out sub 10mph traffic, even sub 20mph traffic.
kinetic energy = mv²/2
Lets call 0-10 one relative unit of energy
Then 0-20 has 4 relative units
0-30 has 9 relative units
0-40 has 16 relative units
0-50 has 25 relative units !
Trying to regen from 10-0 or even 15-0 is of little importance. Even 20-0 isn't that much.
A single rolling from 50-15 on a freeway exit (without regen or stopping) will save you far more energy than a dozen 10-0 100% efficient regens. But that 50-15 regen is highly efficient (say 85% of kinetic energy becomes battery charge). Of course having to accelerate again looses some further energy, hence the 70% round trip estimate (regen-charge-acceleration).

Far more important than avoiding breaking / regen is avoiding acceleration to begin with. Judge how far you're going between slow downs and only go fast enough not to be left too far behind. If you do a good job there, most of your deceleration will be rolling rather than regen or breaking.

 

[SageBrush]

Stop&Go is poorly defined. Is it on a clogged freeway, or is it driving between city signals ? If the latter then speeds are typically up to ~ 40 mph and could include 1-2 stopping/slowing instances per minute. Since the KE at 40 mph in a 1500 kg car is
17.89*9.8*1500 = 262,983 Joules. Over an hour of driving, between 15,778,980 and 31,557,960 Joules

At 30% regen efficiency, between 9,467,388 and 4,733,694 Joules recovered per hour
At 60% regen efficiency, twice that amount

All in all, between ~ 1.3 -- 5.2 kWh regen per hour of driving depending on circumstances and driver habits.

----
So, is regen useful ?
Not very much to me, because I coast to stops and signals
Very much so to my wife, since she coasts little but has learned to avoid friction braking.

 

[arnis]

Regen is around 2/3 efficient, not 1/3. 1/3 might be true for hybrid.
Average sized EV (not as big as Model S) will recover around 0.1kWh going from 100km/h down to 10km/h.
This means do that 10 times and that is 1kWh of regenerated power. Which will allow 4-6 accelerations
to that same speed, depending on how aggressively you accelerate from 10 to 100.

 

[David99]

The drive train efficiency during regen is well above 80%. I remember seeing someone talk about the data actually being available on the CAN bus. The battery has a round trip efficiency of well over 90%. While those number look awesome, that is not what you get in real world driving. When you accelerate from 0 and then regen back to 0, you are also driving. In other words, a good part of the energy is used to move the car. That part of the energy is not available any more for regen to capture.

I have an app that is measuring energy usage and regen energy separately during a trip. In typical city driving with lots of lights and stops I get about 30% back. Say I need 12 kWh to drive from A to B through Los Angeles, I usually gained back 3 kWh through regenerative braking bringing down the net energy usage to 9 kWh.On longer freeway driving, the ratio is much lower of course. On a freeway (without much traffic) you hardly slow down, so there is no opportunity for regen to get anything back.

I have done a test / video about how much regen can be captured.

 

[David99]

In the grand scheme of things, regen is hugely inefficient. From some tests I saw someone do years ago on a Prius forum using regen to stop from X MPH will get you back about 30% of the energy needed to get you back up X MPH in ideal conditions.

The 30% is true, but it's not because it is 'hugely inefficient'. The motor/inverter and battery are very efficient getting energy back. The 'problem' is that you are driving. You are using most of the energy to move the car. If you subtract the energy you used to drive any given distance, and see how much is left for regen you see that the energy conversion back into the battery is actually pretty efficient.

 

[Carl]

There was this Russian Tesla owner who tested regen while being towed by a semi, no? IIRC regen was about 50% (which isn't bad at all of course).

 

[Fiddler]

There was this Russian Tesla owner who tested regen while being towed by a semi, no? IIRC regen was about 50% (which isn't bad at all of course).

All the EV subsystems have very high efficiency. The battery, charger, inverter, gears, motors, tires, rolling resistance. All of these are above say 95% efficiency. The overall efficiency of the system is the product of the subsystems. So maybe (0.95) to the six power. That's about 73%.

Sounds good, HOWEVER, energy has to go through (the battery pack, electronic, motors/generators, tires, gears, ...) Twice to get back into the battery. So 73% times 73% is 53%. And that's about what I measured at speeds around 35mph and drafting.

At higher speeds most of the energy goes into air resistance (TWICE). I say something well below 50% is about the best we can do if the vehicle is not drafting.

Engineering Explained

Says 60% one way, so that would be 36% (0.6 x 0.6= 0.36) round trip from battery back into the battery again. High speed air resistance is the biggest loss by far.

 

[daniel]

[...]
At higher speeds most of the energy goes into air resistance (TWICE). [...]

Why twice? I'd have said the energy goes into air resistance continuously while the speed is maintained. At any given speed that loss is proportional to the time the speed is maintained. On, say, a one-hour drive on the freeway, the energy recovered through regen slowing down for the exit is negligible compared to the energy expended on the trip.

I would imagine that regen saves more money on brake pad replacement than it does on charging cost.

 

[macpacheco]

Why twice? I'd have said the energy goes into air resistance continuously while the speed is maintained. At any given speed that loss is proportional to the time the speed is maintained. On, say, a one-hour drive on the freeway, the energy recovered through regen slowing down for the exit is negligible compared to the energy expended on the trip.

I would imagine that regen saves more money on brake pad replacement than it does on charging cost.

Air drag force is proportional to speed squared.
Air drag power is proportional to speed cubed.
If you go twice the speed, air drag power increases 8 fold, but time for the same distance halves, so 4x as much energy is spent.
If you drive a mere 10 miles at a constant highway speed, say 60mph, the energy lost due to drag should be over 10x as much as what you would waste if you had ZERO regen.
That's why going slow is key to getting the best range.
Regen only makes a big difference when you're frequently slowing down.
Regen helps big time if going down hill, since using neutral would substantially increase your speed, throwing much of potential energy into air drag. Regen while going down at the same speed you would use on a level segment is much better.

That's why I have a problem with people that want a huge battery pack to go faster. Going 41% faster doubles your power consumption for the same distance. A mere 23% faster already increases power consumption by 50%.

Want more range, slow down.

 

[daniel]

Why twice? I'd have said the energy goes into air resistance continuously while the speed is maintained. At any given speed that loss is proportional to the time the speed is maintained. On, say, a one-hour drive on the freeway, the energy recovered through regen slowing down for the exit is negligible compared to the energy expended on the trip.

I would imagine that regen saves more money on brake pad replacement than it does on charging cost.

Air drag force is proportional to speed squared.
Air drag power is proportional to speed cubed.
If you go twice the speed, air drag power increases 8 fold, but time for the same distance halves, so 4x as much energy is spent.
If you drive a mere 10 miles at a constant highway speed, say 60mph, the energy lost due to drag should be over 10x as much as what you would waste if you had ZERO regen.
That's why going slow is key to getting the best range.
Regen only makes a big difference when you're frequently slowing down.
Regen helps big time if going down hill, since using neutral would substantially increase your speed, throwing much of potential energy into air drag. Regen while going down at the same speed you would use on a level segment is much better.

That's why I have a problem with people that want a huge battery pack to go faster. Going 41% faster doubles your power consumption for the same distance. A mere 23% faster already increases power consumption by 50%.

Want more range, slow down.

There is no dispute that driving faster consumes more energy. (Though one person's "waste" is another person's "discretionary expenditure" for value received. And someone who complains about people who drive 80 mph would probably rebel at a law requiring him to slow down to 35 mph. Each of us decides how much we're willing to spend in return for more time at our destination.)

But I don't think you explained why you said "TWICE" with reference to air drag. At any given speed air drag is proportional to the time spent at that speed.

I think you and I agree on the fact that at freeway speed over any substantial distance, air resistance consumes far more energy than regen recovers. In city driving, regen certainly helps conserve a little bit of energy, but I still think that regen's biggest benefit is saving wear on the brake pads. I'd be willing to bet that at average electric rates, the typical EV, compared to a hypothetical EV without regen, saves more on brake pad replacements than on at-home charging costs.

I'd love to see an actual measurement of kWh put back into the battery by regen, and kWh consumed charging, for a typical EV over the course of a year. I can see the gauge on my Roadster going negative when I slow down. But as noted above, the gauge only goes to negative 40 kW, whereas it goes to positive 200 kW. And the car is expending energy all the time it's accelerating or moving at constant speed, and regenerating only when it's slowing down.

 

[SageBrush]

And yet the common experience of ICE city drivers is that fuel economy is better on the highway despite increased aero losses per distance. Since this is not the EV experience it implies that regen (minus idling losses) in the city is more than the marginal aero losses on the highway. That is impressive.

 

[mspohr]

I frequently drive to Mt. Rose Meadow which is 3,000 feet above my 6,200 house and 17 miles distance.
On the return trip, I typically arrive home with the same number of miles range as I have at the top of the pass. I typically use about 40 miles of range to go from my house to the top and about zero miles of range to return home.
This potential energy of about 20 MJ (2100 kg x 1 km x 9.8 m/s2) = 5.7 kWh which is enough to travel about 20 miles at 300 w/mile which is about the distance I travel home. (This trip consists of about 10 miles at constant elevation of 6200 ft then 7 miles gaining 3000 ft elevation.)
This would seem to indicate a very high regen rate since the return trip travels down 3000 feet over 7 miles (showing up as about 2 kWh or 6 mile increase in charge at the bottom of the hill). So, I travel 7 miles and gain an extra 2 kWh of charge out of the 20 MJ (5.7 kWh) of potential energy. If you assume the air/mechanical friction to travel 7 miles is another 2 kWh then the regen rate is about 70% since we have recaptured 4/5.7 of the potential energy.
These are rough calculations but seem to confirm the more precise calculations of others.

 

[daniel]

And yet the common experience of ICE city drivers is that fuel economy is better on the highway despite increased aero losses per distance. Since this is not the EV experience it implies that regen (minus idling losses) in the city is more than the marginal aero losses on the highway. That is impressive.

Or else it means that the efficiency curve for an EV is entirely different than for an ICE. ICE cars are so inefficient that anything other than the steady highway driving they are tuned for is less efficient. E.g. most ICE cars run their engine while stopped, and are very inefficient while accelerating. A electric motor draws no power while stopped and does not lose efficiency while accelerating, at least not to the degree that an ICE does. My Zap Xebra had no regen, yet was still about as efficient as any other EV.

I don't think we can draw any conclusions about regen from comparisons with stinkers. There are too many other factors, especially the fundamental differences between motor types.

Somebody would have to actually measure kW regenerated and kW from the charger on a standard city route over a long enough distance.

 

[jerry33]

Can't seem to get a clear answer on this. The ability to win back energy during braking is one of the nicest feature of EVs of course. However, many cars spend a lot ot time getting stuck in stop and go traffic each day. Therefore my question.

One of the reasons there is no clear answer is that there are different types of stop-and-go driving. In some cases, the stop/slow and go is from a relatively high speed and quite a bit of regen can be had. (It helps if during the "go" part you can limit your acceleration so that you don't just spend all of the regen right away.) In other cases, it's steady enough that a relatively steady speed can be maintained, not much regen but low energy use. The worst case is where you stop and start from low speeds because you are continually accelerating the mass of the vehicle. In every case, regen never gets back all the energy you spent to do the acceleration. In addition, terrain, tires, tire pressure, and alignment make a material difference in the results achieved.

 

[Sertis]

Or else it means that the efficiency curve for an EV is entirely different than for an ICE. ICE cars are so inefficient that anything other than the steady highway driving they are tuned for is less efficient. E.g. most ICE cars run their engine while stopped, and are very inefficient while accelerating. A electric motor draws no power while stopped and does not lose efficiency while accelerating, at least not to the degree that an ICE does. My Zap Xebra had no regen, yet was still about as efficient as any other EV.

I don't think we can draw any conclusions about regen from comparisons with stinkers. There are too many other factors, especially the fundamental differences between motor types.

Somebody would have to actually measure kW regenerated and kW from the charger on a standard city route over a long enough distance.

That and engine control systems are usually using lookup tables that work well in steady state part throttle conditions, but perform poorly during acceleration because the A/F measurements lag the actual conditions in the cylinder, and the load is variable, so the proper stoichiocastic ratio is not maintained precisely. No major issues in electric motors, though the transmission friction loss adds up. I would prefer to use regen more aggressively if it's reasonably efficient, since if everyone coasted to a stop, we'd be reducing road capacity significantly during heavy traffic and that's hardly green.

 

[voyager]

EV's can't regenerate juice at very low speeds. And there is not a lot of kinetic energy at those speeds in the first place.

Though I think I didn't understood the question itself.
Friction losses per distance travelled (1 meter) is more-or-less the same at 10km/h as at 50km/h.

Depends on driver actions. Ideally, even in stop-go traffic, driver should try to avoid speed changes.
I, for example, smooth out traffic jam traffic. If I see cars inching forward every few seconds I usually
just try to roll at walking pace. Sometimes distance to vehicle in front gets huge, like 2-5 car lengths,
but it dissolves as soon as vehicle in front of me stops and I slowly crawl closer. People behind me
just crawl with me. This is more efficient than aggressive jerking like 99% of drivers (and EAP) does.
Chill mode might solve that somewhat.

PS: climate and auxiliary draw from battery way surpass driving needs in traffic jams (walking speed).

Driving conditions have a large impact. You’ll see much better effectiveness for regenerative braking in stop-and-go city traffic than in highway commuting. This should make sense, as if you’re repeatedly braking, you’ll recapture a lot more energy than if you simply drive for hours without touching the brake pedal.

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