Batteries

Published on October 24th, 2015 | by Kyle Field

71

The Hidden Benefits of EVs – Regeneration

October 24th, 2015 by  

We spend a lot of time here on CleanTechnica discussing the many benefits of EVs but there’s one specific benefit that really surprised me – regeneration. At its core, regeneration is utilizing an onboard generator (usually the primary drive motor) to slow the car and convert that power back to electricity… but in practice, it’s much more than that and can have a large impact on the range of your EV if you know how to use it most effectively.

Chevy Bolt at Santa Monica Alt Car Expo

A 2014 study noted:

Regenerative braking is an efficient approach to extend the driving range of the EV without any additional cost; at the same time, it plays an important role in energy saving. Recently, many efforts have been focused on developing models of the regenerative braking system and improving brake performance.

Studies across the board cite regeneration as providing 10–15% of the expended energy back, which, with the range of today’s EVs, can mean the difference between making it to your destination or having to stop and recharge.

How Does Regen Work?

You have expended battery power to get the car moving (building up kinetic energy) and you need to stop, so you ease onto the brakes. Behind the scenes, as you step on the brake pedal (maybe we should rename it the “decelerator”?) the car engages the motor backwards to slow the car down and, at the same time, generates power.

Regeneration Simplified

Image Credit: Michael Goodman, uOregon.edu

A Practical Study

It seems simple and really, it is but it can have a huge impact on your range and also reveals many hidden inefficiencies of gasmobiles that were not previously as visible.  One of my favorite examples of how regeneration works was on a recent hike I went on with my family in my wife’s Mercedes B-Class Electric Drive.

The start of the hike is ~39 miles from my house – not bad considering our car has a range of 87 miles, BUT the route includes 4000’ of vertical gain heading up a curvy highway back behind Ojai, California. Both gasmobiles and EVs expend more energy going uphill when compared to flatland – we just haven’t paid attention to it. The switch to range-limited EVs puts our estimated range front and center, making inefficiencies like this much more apparent. Driving up, the elevation gain took a toll on our range as if we had a leak in the battery. By the time we arrived, we were down to just 29 miles of projected range left, leaving me wondering just how this experiment was going to work out, nervously offering reassurance to my wife that it would work out.

After our hike, we returned to the car and hopped in. As the kids buckled up, I nervously glanced at the range, hoping it had miraculously recharged while we were out on the trail, but alas, it was not to be. We charged out and made our way back down the mountain. In a gasmobile, I would have been idling the whole way down hill, using friction brakes to slow down and the gas for the occasional acceleration – essentially paying to fight gravity on the way down the hill. In the EV however, we could immediately see the benefits of regeneration. We were essentially just rolling down the hill, using little to no battery power at all to propel the car. On top of that, when I put the brakes on, our momentum is converted back to electricity and stored.

In real life, as we coasted down the hill, our range stayed mostly static – even gaining a mile or two every so often. The gamble paid off, and as the miles flew by, I was confident we would make it home and in love with this regeneration thing. In a gas car, the best we could have hoped for would be to idle down the hill, using a fraction of the gas we had used on the way up but still a net negative, not to mention the wear on the brakes. The end result for us? We pulled into the driveway at home having only consuming 4 miles of our precious 29 miles of range across the 39 miles we had travelled (25 miles of range left). It sounds extreme – and it is!

Leaf Infinite Mileage

This example is definitely on the extreme end but does a great job of illustrating how regeneration allows EVs to benefit from the energy expended getting the car up hills or just up to speed on flat ground. Even if you don’t live on a hill, drive up and down hills on the way to work, or even have hills in your state (I’m looking at you Kansas), regeneration will still help you make the most of the moving and braking energy in your EV. In my experience, the 10–15% increase in range is a fair estimate of what folks can expect. It’s also worth noting that you can increase or decrease this based on aggressive or conservative driving, just as you would expect to see in a gasmobile. This also helps EVs excel in low-speed, stop-and-go traffic, which makes rush hour that much more bearable.

Net – regeneration is a unique advantage of EVs that I simply had not realized would be such a significant impact that also allows users to extend the range of their EVs with little to no effort. Chalk up another advantage for team EV!

EV charging Santa Monica


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About the Author

I'm a tech geek passionately in search of actionable ways to reduce the negative impact my life has on the planet, save money and reduce stress. Live intentionally, make conscious decisions, love more, act responsibly, play. The more you know, the less you need. TSLA investor. Tesla referral link: http://ts.la/kyle623



  • Ex-PCP

    Hello Kyle:

    First, thank you for this very enlightening post. I’m interested in finding out two key performance stats for EVs like the Leaf or Mercedes:

    • kw-hrs used per, say, 1,000 feet of climb on a mountain road (going slowly enough such that wind resistance is a trivial consideration); and

    • kw-hrs recovered per 1,000 feet of descent on a steep grade (say around 5 – 7%) using maximum regenerative braking (so-called “B-mode” on the Leaf; I don’t know if the Mercedes has something equivalent, but I’d bet it does).

    Using your data, I’m trying to back out an estimate for these parameters (and I emphasize the word “estimate” or within about 10% or reality). But, I’m not having much success, perhaps because a key fact or two is missing. I’m wondering if you could provide them.

    We can use some VERY simple physics to try to get a rough idea (but as you’ll see the simple physics clearly an oversimplification in trying to derive these performance numbers). If the car has a mass of about 4,000 pounds (1,800 kg) with passengers on board, then assuming no losses (and obviously there are — that’s what I am trying to determine), the amount of energy consumed going up 4,000 feet (about 1,220 meters) in kw-hrs is:

    E = mgh

    E= 1,800 kg * (9.8 m/sec^2) * 1,220 meters * (1 kw-hr/ 3.6 megajoules) = 6 kw-hrs

    And if regenerative braking is perfect (we all know it isn’t of course), then that 6 kw-hrs should be recovered going back down the steep grade.

    Now let’s look at the data you had from your car. You stated that you drive about 39 miles to your hiking point, and that starting out the car said you had a range of 87 miles. I am going to assume (this could be a bad assumption) that the ‘range’ indicator was based on the onboard computer’s baseline for driving on a flat surface (or reasonably flat) at around 40 mph. Under these conditions, most electric vehicles get about 3.5 miles per kw-hr of juice stored. So, ostensibly the 39 miles on a flat surface would consume about:

    39 miles * 1 kw-hr / 3.5 miles = 11 kw-hrs.

    At the end of 39 miles on a flat road, one would assume the range indicator would be 87 – 39 = 48. But your ‘range’ indicator at the top of the mountain was 29 miles, or about 20 miles less than predicted. That 20 miles represents, again on a flat road, this many kw-hrs:

    20 miles * 1 kw-hr/3.5 miles = 5.7 kw-hrs

    That’s pretty close to the 6 kw-hrs calculated above, but I can’t believe it is correct because there are surely resistive losses (other than air friction which I’ll assume to be negligible if climbing at a slow rate of speed) from huge amounts of current moving from the battery to the motor when propelling the car uphill.

    So, what would REALLY be helpful is know the actual “State of Charge” of the battery at the start of your trip and at the top of the 4,000 rise. Does the Mercedes report that info?

    Now thinking about regenerative braking efficiency in your Mercedes, let’s consider the data from going back downhill to see if we get any further insights. You traveled 39 miles back home (which theoretically would have consumed 11 kw-hrs) but actually consumed only 4 miles or range in kw-hr equivalents or about 1 kw-hr. So, assuming all else is equal, you gained 10 kw-hrs back. Obviously this can’t be right either.

    One explanation for the too-good-to-believe results above is that the range estimator in the onboard computer adjusts continuously based on most recent (whatever ‘most recent’ actually means) driving conditions. Going downhill gives inflated range estimates (compared to flat road driving) and uphill the opposite.

    So, to summarize my questions:

    – Does the Mercedes give an actual State of Charge (SOC) reading and if so, what were those readings when you started out, when you got to the top of the hill, when you got to the bottom of the hill and when you finally got home?

    – Alternatively, have you figured out how many kw-hrs of battery juice you used in the actual hill climb and how many kw-hrs you recovered coming back down before hitting the flats for the drive home.

    Perhaps an enterprising engineering type can provide these key numbers of energy consumption and recovery per 1,000 feet of steep grade (understanding of course that the mass of EVs vary but are roughly the same). I’ve asked a Nissan sales representative these questions but he was unable to answer them.

    Thanks!

    • neroden

      All I can tell you is my experience in my Tesla in a *very* hilly area.

      I get *exactly* the EPA mileage rating.

      I get very slightly better mileage if I take a long drive on the flat. So I guess I am losing a *little* by going downhill and uphill… but it’s not significant. The efficiency is just about the same on an up-and-down trip as it is on the flat.

  • *LOVE* This Article. 😀

  • What it look like

    If GM takes this seriously, it could transform transportation. The question is will it take it seriously?

  • DSNI

    What other vehicles let you “Re-Fuel” on the way back down the hill?

  • Steven F

    One thing to keep in mind is that the effectiveness of regen depends on how fast the motor spins. At high RPMs the motor is a very efficient generator. At low RPM it isn’t. So in low speed driving you wil need breaks. if your electric car has a transmission. you will find regen to be more powerfull in low gear than it is in drive. This is because the gears in low allow the motor to spin faster than the wheels to increase wheel torque.

    I took advantage of that on a recent trip to Yosemite in my 2015 volt. When I left the valley I had about 11 miles on the battery. so I had the engine on while on 41. I used low and regen to control my speed for the entire drive on the curvy road (minimizing dreak use) Before I left the park I felt I had just enough battery range to reach oakhurst where I was planning to spend the night From the park entrance to town is all down hill . When I got to Oakhust I the battery was almost fully charged

    • Randall Mathews

      At low speed, the extra generating torque necessary is gained by switching the rotating magnetic field into reverse and giving it whatver speed it needs.

    • Kyle Field

      Great perspective and a good add as to why EVs need brakes AND regen. Low speeds my friends…low speeds.

      • Omega Centauri

        The one non-smooth thing about the Leaf, is braking. Somewhere at roughly 2mph the friction brake comes one, and you feel a jerk.

        • Chris_in_Raleigh

          Try asking your dealer if NTB14017 is applicable to your Leaf. It fixed the grabby brakes on my 2013.

    • Omega Centauri

      Its good to have the battery charge low enough to absorb the downhill. Once the battery is fully charged, then the cars computer shuts off regen braking. Another reason for having a standard brake as well. It may also shut off regen (or reduce it), if it thinks the battery is overheating.

      • Calamity_Jean

        I was going to ask about that, and now I don’t have to. Thanks.

  • David Porter

    With an internal combustion engine it will use less fuel going downhill by keeping it in gear rather than coasting at idle. It will be safer and not burn out your brakes too.

  • Use B mode and proper regen settings, watch your screens too.

  • Richard Poore

    Excellent article. While the mileage gain on the trip back is a nice point, perhaps a more important facet of the story deserves a little more development.

    The overall trip was 78 miles, with an expected range of 87 miles. So one would expect a remaining range of 9 miles after the trip. Instead the car had 25 miles of range left. This is a very significant increase in range, showing how the right driving style and right conditions can improve your EV.

    All cars mileage varies from the manufacturer’s sticker, it is important to realize that on an EV it can improve!

    Of course, the range shortening effects of cold weather have to be considered as well, if the EV is going to be used in the winter.

    • larry shaw

      Is there any energy regenerated by just using the B mode without touching the brakes? I always assumed that if you shift to the B for even the EC mode that’s some regeneration takes place.

      • Matt

        The B mode in my Prius does regeneration as you remove pressure from the gas pedal. Assume your B mode works the same.

    • Omega Centauri

      I think the net energy consumption is dominated by aerodynamic drive, which per mile varies as roughly speed squared. For your mountainous driving, you probably aren’t traveling very fast, so your range goes up.

      • Kyle Field

        Conversely, as incline increases, the effort required to move that block of mass up the hill increases, driving range back down. So it’s a balance but my net reaction was surprise that driving up a steep hill then back down was actually more efficient than driving the same 79 miles on a flat road.

        • Omega Centauri

          It would likely be less efficient if driven at the same speed. But flat road probably means highway speeds, whereas a winding mountain road is probably more like 20-30 miles per hour.

      • eveee

        Yes. This is what you are looking for.

        http://i.stack.imgur.com/PKkDf.jpg

        • neroden

          Which car is that for? I’ve seen it for the Roadster and a projected one for the Model S. (Model S is bigger so the optimal speed is around 30 instead of 20.)

  • hybridbear

    It would be nice to fix the spelling error in the infographic. It’s “braking”, not “breaking”…

    • Bob_Wallace

      Kind of embarrassing, especially coming out of a university. Not something this site can fix without photoshopping someone else’s image.

    • Kyle Field

      I saw that in advance but this was one of the better infographics I was able to find so accepted it for what it was. I appreciate you calling it out though 🙂

      • hybridbear

        I understand. It’s too bad the university couldn’t fix it for you. I knew that you couldn’t fix it without using something like BlueBeam to edit it.

  • Thanks Kyle for the interesting article. While CT readers know about regenerative breaking, it’s interesting to learn of its impact on a real life example (of traveling 39 miles and only using 4 miles of range due to going mostly downhill). I know that area so I can appreciate the hill. This is the kind of personal and insightful story that I’ve come to appreciate on CT over the years. Thanks, Zach!

    • Randall Mathews

      It is a good article. My Dad used to say, ‘brakes waste petrol’. My Prius lets me hear the gentle frequency sound of the reversed magnetic field applied to the motor stator as I touch the brakes.
      A pity the infographic perpetuates the semi illiterate confusion between braking and breaking.

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  • Michael G

    Many years ago, before hybrid cars (which have been using regen braking since the first hybrid) a mechanical engineering professor gave a lecture on regenerative power on a small bicycle he made to test out the theory of it. He used a mechanical device, I think it was a small gas piston, to store the energy. It then gave the energy back by helping him up hills.

    Flywheels and other non-electric storage devices can serve as well, so regen by itself is not the sole province of batteries.

    • Kyle Field

      Completely agree and I fully expect these other methods for regen to play increasingly larger roles in utility scale energy storage…and we’re seeing some of that already. Exciting times we’re living in for sure!

    • Larmion

      Flywheels for kinetic energy recovery have been used in various motor racing disciplines for a long time. Interestingly, they have never really taken off in ICE cars (though they can sometimes be found in larger vehicles like buses). Volvo released a few flywheel models iirc, but those came and went quietly.

      That would suggest that while energy recovery in non-electric vehicles is perfectly possible, it’s just too expensive to be worthwhile for anything but heavy duty vehicles.

      Regenerative braking in an ICE vehicle adds a whole lot of complexity, whereas it’s just something that’s pretty much built into a standard electric drivetrain.

      • Omega Centauri

        A flywheel is essentially a gyroscope. Changing the axis of rotation requires huge torque. You can have counter-rotating flywheels so the torque required to move one is offset by the other, but there will be huge torque between them. Now maybe for a well known racecircuit, it can be arranged (to not need to change the axis of the flywheel), but for general driving probably not.

      • Bob_Wallace

        Someone has played with a hydraulic system for storing braking energy.

        Best to just move on to EVs rather than trying to kludge a bit more efficiency out of ICEVs.

        • Bert

          Caterpillar has one of those on one on their hybrid excavator. I don’t know the specifics, but it seems interesting. I’m guessing it offers a cheap method to increase efficiency in that application.

        • harisA

          Eaton has a hydraulic system for road trucks. I think not commercialized though.

  • TedKidd

    I think we will find long term health benefits from quitting the brake dust habit.

    • Kyle Field

      Totally agree. Yet another point for EVs. On the more contrarian front, I full expect there to be issues with EVs long term as there always are. Less than gasmobiles, lower impact but still issues. That’s one of the thing the incumbents and detractors love to focus on. There’s always going to be haters 😀

    • RobS

      Get rid of brake dust and particulate exhaust emissions from urban areas and i am very excited about the “cleanliness” of EV adoption particularly in large cities like New York. Even if we accept that emissions will still occur in regions with gas and coal fired power predominating at least the emissions will occur away from population centers which will have huge health and urban environmental benefits.

      • Kyle Field

        The great thing about the emissions is that you’re decoupling them from the car. That allows us to clean up vehicle emissions by cleaning up the grid vs having to clean up every auto maker one by one. So…EV driving emissions get lower over time as the grid cleans up. Also, drivers can opt in to zero emissions driving by putting up solar (as we did), wind or other local renewables which is fantastic!

        • RobS

          I agree they will get cleaner over time, I’m simply adding the point that even if they don’t the simple fact that the emissions are moved away from population centers already has big benefits.

  • Martin

    Yes you use less braking (as in applying the brakes) but at the same time less wear and tear on brake pads (brake pad material is somewhat toxic I think, do they still use asbestos?)
    I talked to a Cab driver, even with a hybrid, brakes last up to 3 times as long!

    • Kyle Field

      I agree, I never had to change the brakes on my Prius in 75k miles vs a normal change frequency of 30k on a gasmobile. What’s interesting is that the single recurring maintenance item on our Leaf is the brake fluid every 15k miles. I thought that was odd considering I’d expect a lot less wear on them in an EV…

      • Oollyoumn

        I am absolutely on board with regenerative braking, and the savings on brake pads, but 30k mi normal life? I’ve drove a standard gas 94 Escort from new to 185,000 mile on the factory brake pads. Strangely the lifetime warrantied replacements had to be replaced before I sold the car at 260,000 miles. While that is an exceptional distance, I’ve owned other cars that made to the 100,000 mile range on their original pads. My last two cars have used regenerative braking, but I’ve not driven either near the miles I used to drive. I imagine there is the possibility that with regenerative braking that pads may actually be good for what it typically considered the life of the car, like mufflers are now (an item that I hope will soon be extinct).

        • Kyle Field

          Ah…I guess I did quote the wrong mileage estimates for pads…not sure what happened there (brain fart?!). Brake pad wear is one of the many benefits of regen…with the primary being the ability to make use of the energy by pulling it back into the battery. Thanks for the completely warranted correction!

          • Bob_Wallace

            30k is on the low end of the range I see on line. 20k to 70k seems to be ‘normal’ life. People who can drive without using their brakes very much can take it much further.

            http://auto.howstuffworks.com/under-the-hood/car-part-longevity/brake-pads-last.htm

          • NRG4All

            As you know so much depends on how you drive and whether you are driving a “stick” or an automatic. A bit of anticipation and proper following distances will do wonders for pad longevity. Many times I’ve point out to the wife the amount of braking some moron needs just because they are following too close.

          • Steven F

            Break pad life is not only dependent on driving style. It is also strongly influenced by decisions made by the car manufacture. If a manufacture decides to use smaller pads, the pads will wear out faster. if the manufactures decides to use larger pads, the pads will last longer. The weight of the car, materials used in the pads, expected driving style, motor size, and how long the manufacture wants the pads to last also play a role. I personally went over 150,000 miles on a set of pads and rotors and I my driving style is average and my milage matched the EPA estimates.

          • Michael B

            Only if your mistake was still under warranty was the correction warranted. 😉 Sorry, I probably should have put the brakes on this comment…

          • Kyle Field

            too soon

        • mike_dyke

          If you’ve got a Tesla with all wheel drive, you’ve got regen braking on all wheels, so do you actually need physical brakes?

          • Omega Centauri

            Normally, the “real” brakes are reserved for hard emergency panic-stops. Also at very low speed, you won’t get much braking force, so the friction brake is called upon. Now Tesla has a more capable battery/engine, so it can probably handle harder braking than say a Leaf or plug-in hybrid, but I’d bet they are still there for you in an emergency.

          • Kyle Field

            It’s worth noting that the average Tesla is over 5000lbs and drives like a race car. That warrants a bit more safety as it relates to braking capability.

          • Bob_Wallace

            People like to emphasize the weight of the Tesla S.

            The Mercedes S550 has a curb weight of 4,630 lbs. ($95,650)

            The Tesla S85 has a curb weight of 4630 lbs. ($85,000)

          • neroden

            The mechanical brakes on the Tesla are used routinely when stopping on a steep downhill slope, so that you don’t keep rolling downhill.

            That’s… basically what I use them for.

        • Matt

          Break pad life varies more the MPG based oh how you drive. The feed back with a region system (seeing the graphic of juice flowing back in battery) can improve driving style, so that you never have to change the pad. Now as for changing break fluid every 15k, that sounds like maybe a left over from ICE days.

      • Guy Hall

        Kyle,

        Good catch about changing the brake fluid. It’s not a well understood item of electric cars. As it was explained to me by some Nissan engineers, in all cars moisture will collect in the brake reservoir. In gas cars, the heat from the engine tends to evaporate off most of that water. However, with electric vehicles there is no heat buildup and the water continues to collect. This can be very damaging to the brake system, and should be checked frequently, or at least whatever the service manual says. This is a piece of trivia that can surprise most electric vehicle owners. What requires more maintenance on the electric vehicle than gas vehicle?

        • eveee

          Once enough EVs are on the road, brake designers and fluid mfrs will catch up. There are ways to deal with water in hydraulics. And there are other kinds of brakes.
          http://m.hydraulicspneumatics.com/200/TechZone/Seals/Article/False/13916/TechZone-Seals

        • neroden

          Oh, so that explains why Tesla changes the brake fluid every 2 years.

        • Joseph Faires

          If you mean the replacing the brake fluid in the Leaf every 1 to 2 years. This requirement is the same for the Nissan Altman and most all of the 2011 on Nissan it has nothing to do with Leaf being a electric car.

    • Ronald Brakels

      Asbestos has been banned in car parts since the start of 2004 in Australia and it’s use had been in decline since well before that.

      Holy cow! Looking it up, I see the US still hasn’t banned its use in car parts and quite a few other uses. Sub-Canada! Y U know ban asbestos? It the asworstos! Don’t you know the Blue Sky Mine song about asbestos? The lyrics, eg. “The candy store paupers lie to the shareholders” make more sense when you know the mine where they produced blue asbestos was owned by a sugar company.

      And a country with a huge asbestos problem is India. Of course, its use does save lives from fires that otherwise would have occurred, but it’s time for a phase out.

      • Matt

        The US does not like to make things hard on our corp leaders. When we banned “use” of DNT in US we did not ban “making” it, so it could be exported, put then put on food that would be imported into the US.

    • Stan Hlegeris

      As a Tesla driver for almost a year now, I can report that I NEVER use the brakes in normal driving. Tesla’s regen is aggressive enough to control the car’s speed in every situation I have encountered. Usually I only touch the brake pedal when I arrive at my destination. The fact that every smidgen of excess momentum goes to regen is one of the many great joys of the car.

      • Frank

        I have a Prius, and the electric motor used in addition to the gasoline engine is much smaller than the electric only and much larger electric motor in the Tesla, so I would fully expect it to be capable of producing much more electricity, and with it, braking force than the Prius. Dreaming of the mod 3.

      • neroden

        I now know you live on the flats. 🙂 I use the friction brakes in normal driving, but my normal driving features 8% grade hills — don’t wanna roll away…

    • Guy Hall

      I have talked to hundreds of electric vehicle owners, and other than the design flaw in the brakes for the old Toyota RAV 4, I have not heard of one eV owner replacing brakes pads yet.

      • Martin

        The cab driver I had talked to had been driving not a full hybrid but a Toyota Camry and he pointed out that the savings in reduced fuel cost
        would offset the slightly higher price to buy the car in the first place.

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