What is Regenerative Braking?

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Regenerative braking is used on automobiles to recoup some of the energy that is lost while the vehicle is stopping. This technology is used on hybrid vehicles that use both gas and electricity as sources of power. The energy that is recouped during braking is saved in a storage battery and used later to power the motor whenever the vehicle is using its electric power source.

Conventional Braking Systems

In braking systems on conventional vehicles, friction is used to counteract the forward momentum of a moving vehicle. As the brake pads rub against the wheels or a disc that is connected to the axles, excessive heat energy is created. This heat energy dissipates into the air, wasting as much as 30 percent of the vehicle's generated power. Over time, this cycle of friction and wasted heat energy reduces the vehicle's fuel efficiency. More energy from the engine is required to replace the energy that was lost by braking.

Regenerative Braking Systems

Hybrid gas/electric automobiles use a completely different method of braking at slower speeds. Hybrid vehicles still use conventional brake pads at highway speeds, but electric motors help the vehicle brake during stop-and-go driving at slower speeds. As the driver applies the brakes by pressing down on a conventional brake pedal, the electric motors reverse direction. The torque created by this reversal counteracts the forward momentum and eventually stops the car.

Generates Electricity

Regenerative braking does more than simply stop the car, however. Electric motors and electric generators — such as a car's alternator — are essentially two sides of the same technology; both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems take advantage of this duality. Whenever the electric motor of a hybrid car begins to reverse direction, it becomes an electric generator. This generated electricity is fed into a chemical storage battery and used later to power the vehicle at city speeds.

Some Loss of Energy

The technology employed during regenerative braking takes the energy that is normally wasted during braking and turns it into usable energy. A hybrid vehicle is not, however, a type of perpetual motion machine. Energy is still lost through friction with the road surface and other drains on the system. The energy collected during braking does not restore all of the energy that is lost during driving. It does improve energy efficiency, however, and it assists the main alternator.


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Post 64

In regenerative braking of an induction motor, how is energy controlled for the rotor back, emf excitation and stator power outing at a same time by controller? If we control the excitation, how can we get power from stator? Won't their frequency match?

Post 61

Some aircraft from many years ago had starter generators all in one unit. Perhaps that technology is adaptable to regenerative braking in vehicles. I guess the added variable is the wide range of speeds encountered in every day driving. Some aircraft also have constant speed drives to turn generators but I think in automotive applications, that expense would be prohibitive.

Post 60

what was the conclusion from the article. Whether chargers are required or not to charge the battery if regenerative braking is present?

Post 59

how do you modify the conventional braking system to a hydraulic regenerative braking system?

Post 55

can you tell me the operating principle of regenerative braking systems?

Post 54

I'm an undergraduate student and I need some information, regarding the history of the regenerative braking system.

If possible, can anyone suggest me the different types of RBS, it's advantages and disadvantages, the cost of installation, etc. Please help me soon. Thank you.

Post 53

If the toyota car company use this technique, then why did it end in failure?

Post 52

Not sure who wrote this explanation, but its is not very accurate. should have a better knowledge before posting.

Post 51

is it possible to use regenerative braking systems in ordinary vehicles with ic engines?

Post 50

Regenerative braking can introduce in EOT crane. Otherwise what are the other types of brakes may be proposed.

Post 49

How can this technique can be used in an electric bike?

Post 48

Is it possible to use regenerative braking in a washing machine?

Post 47

As I understand it, you really need three things to make this work best in your electric vehicle: 1. you need enough space in order to place multiple batteries, for example, 8x10 lead-acid batteries would be nice!! (for a total of 80 batteries wired in series) 2. you need a large mass. (think of a transit bus) 3. you need to start and stop often. (think city bus)

Post 46

is this regenerative braking system

the latest braking technology?

tell me something about other modern braking technologies.

Post 45

can a golf cart with regen breaking malfunction? If so what can malfunction and what effects can it have on the cart's speed?

Post 44

could anyone tell me the cost of a regenerative breaking unit? i want to use it in a four-seater fabricated non-street legal car made as a team work........please reply asap

Post 43

Is there any examples that use regenerative braking??? If so please list for me

Post 42

I want to replace the diesel engine in my 25' sailboat with an electric drive - about a 3 to 5 hp DC motor. Thinking about using the motor and controller and hopefully batteries from an old golf cart.

I'd like to use regeneration so that the prop spinning while we are under sail will recharge the batteries. Question is, will any DC motor work? I know some electric golf carts use regenerative braking and others do not.

What sort of controller do I need?

Post 41

The Electric motor *does not reverse*. The current flow reverses direction, instead of going into the motor to produce drive, the motor produces current which is returned to the battery pack. You can't use regen braking on regular cars because (A)the single 12v battery is not large enough nor depleted enough to be much of a load for braking (B) there needs to be an electric motor attached to the wheel to provide the braking effect, which standard cars don't have (C) If you did attach a motor simply to use it for regen braking, it would apply drag to the motion of the car at all times, thus lowering the fuel economy.

Post 40

i'm a student of 12 standard and want to make a project for ncse, for that this topic looks really good and interesting. can anybody help me for this by telling how can i work on this on a small level?

Post 39

How is regenerative braking done in railways?

Is there is separate traction motor & generator?

Post 38

Hydraulic launch assist is a very good idea that will help increase fuel mileage. Another useful thing is the regenerative braking and how it uses forward momentum of the car to create more energy.

I wonder why the engineers can't just put on 2 or more electric generators onto the wheels. If the easiest way to create energy is to spin a shaft why can't the engineers use the turning wheels to put energy right back into the batteries of the cars. That along with regenerative braking or Hydraulic launch assist could really cut back on fuel consumption.

Does this sound legit or is there a reason why this wouldn't work?

Post 37

You can't use regenerative breaking for a conventional vehicle because there is really nothing that's worth regenerating. For one, your vehicle runs on gas not electricity. Secondly the 12v battery in your vehicle is already being charged by the alternator (a generator that takes the mechanical energy from the vehicle to charge your 12v battery).

Since the 12v battery is the only electrical source in a conventional vehicle and it is already being regulated by the alternator, there is nothing that's really viable in implementing a regenerative braking system.

Post 36

Regenerative braking unfortunately does not produce petrol!!

Post 35

Anon 1969.. you can't put the brakes on and then get petrol.

Post 34

it can be used in commercial cars we at KJ Somaiya are making it possible

Post 33

I'm a final year electrical engineering student in search of a project. This topic seemed to be very interesting. Can anybody give me an ideas as to how to implement regenerative braking?

Post 32

Why use this complicated breaking system? Why not just use a motor (generator) that pushes on the wheel whenever brakes are applied instead of using it to stop the car? Doing this you would not be using energy from the motor to slow the car, you would be able to use it when braking at high speeds and it is much simpler to build. Just make it move in against the wheel whenever the brakes push against them. Just my thoughts on this I could be wrong but from everything I have studied this seems like the best and most basic idea. Seems everyone has to over-complicate things.

Post 31

My name is Jon. Posting at 9:22 AM EST on 6/13/08.

Just trying to make sure I understand variable regenerative braking. Here is what I gather from this conversation:

The spinning rotor will induce emf in the stator coils. If the ends of the coil wires are not connected, no current will flow through them. There will be emf, but not current, therefore no reaction torque (drag) and no power produced. If the ends of the coil are connected there is no resistance (ideal), meaning max current meaning max reaction torque meaning the coils will probably melt. Vary the resistance between the ends of the coil to vary the current which will vary the reaction torque. Emf is proportional to

the angular velocity. Current through the stator coil is proportional to the emf and resistance between the ends of the coil. Reaction torque is proportional to the current through the stator coil. Power generated is proportional to the current through the resistive circuit connecting the ends of the stator coil and the emf between the ends of the coil.

Am I right? Thanks to all who contributed. Jon

Post 30

Hi, I'm doing a physics A level coursework project on regenerative braking, and was wondering if someone could tell me how the systems BMW use turn the alternator to charge the battery from the rotating axles of the car, or is it a separate generator? Any help would be appreciated.

Post 29

in addition can you suggest me and drive test softwares which are in use today......i m using a software named advisor 2004....its a matlab based software.

Post 28

sorry b jones...i dint know my first comment was posted anonymous......i m just into engineering and the concept of regenerative braking seems very interesting...can you tell me where to start and how to proceed upon the topic. as i said i want to do a study oriented project on it and any help regarding this is appreciated :)

Post 27

Hi, rahulbmv, what is your question? It’s kind of hard to answer a non-existent question.

Moderator's reply: i believe rahulbmv's question is posted under anon13671...thanks for your help!

Post 26

pls respond people.........i don have much time to impress my instructor regarding this wonderful topic :(((

Post 25

sorry to post this anonymous......

i m doing a project on the efficiency of regenerative braking......and different forms of storing energy tat i can conserve in an automobile.......i get a feeling regenerative braking has a gr8 potential given da emission norms being introduced in many cities..... can any bdy pls guide me in this regard....

Post 24

Well in a ideal world if the ends of the coil are free and not a circuit you would have no drag. You will have a voltage though. If you use that voltage to produce current you will then have drag i.e. you will be breaking.

Also, don't just switch your leads on the motor to try and charge your batt. you will either lock up the motor or fry it. To do reg. breaking use a half bridge or full bridge circuit with PWM. You will need to break the motor with both low side switches turned on for a small% duty cycle, this will produce current in the motor(i.e. inductor for now) now open one of

the low side switches. This will now force the current that was flowing in the motor to flow through a the Freewheeling diodes in the bridge circuit in to the batt. you half to be careful not to over voltage your batt. or your switches. if you look at a BOOST type circuit you will get the basics. Do not try reg. breaking with out researching every thing involved with current sensing, voltage sensing, Motor drive design and batt. charge design. You would be wise to start small
Post 23


I was wondering if you had a vehicle and powered the rear wheels using an electric motor and regenerative braking - BUT on the front wheels there is an insulated wire coiled around the wheel shaft and permanent magnets on the back inside of the wheel rim. Therefore you would be generating (through induction) electricity through any spinning of the front wheels. The wheels effectively spin the magnets around the coil. How much electromagnetic drag would you get from the magnets on the front wheels? aaron

Post 22

Welcome David. Thanks for your contribution. (I’m not quite sure why I’m playing moderator, but I am.)

Post 21

Not meaning to post anonymously on purpose, but here goes.

The only way to change the braking torque on a motor used for braking is to use a motor with stator windings instead of a permanent magnet. Typically a brake pedal only has an on/off type button arrangement and so you would need to add a torque sensor. As a motor torque is proportional to the current applied to the motor, you would be able to work out a control system for working out how much torque to apply using the motor but applying current to the stator windings.

The next problem I see is, the output voltage created by the motor is proportional to the rotational velocity

of the motor. I think the prius has a 500v motor however has an array of batteries that makes about 220v. However the motor could applying a large range of different voltages. You would need a boost/buck converter to control the charging voltage on your battery. Ie. I think a lithium cell likes to be charged at about 4.0v for a 3.7v cell obviously raising the voltage a little more as the battery is more charges as a typical 3 phase battery charger does. The duty cycle of a buck converter which change the output voltage. Lastly converting ~220v to 500v to charge the motor.

I'd be very keen to know if anyone else has more information on this topic, I'm currently developing a system for regenerative braking in dc control robotics for my thesis project and there is not a lot of material out there.


Post 20

3/20/2008 7:06 PM EST

Perhaps more. It would all depend on how efficiently you can turn the kinetic energy into potential energy, such as mechanical energy into electrical energy.

Post 19

Since it mention in the above article that, 'this heat energy dissipates into the air, wasting up to 30% of the car's generated power.'

so can we assume that the regenerative braking can restore 1% from the 30% loss generated power?

Post 18

Not a clue. Anyone else?

Post 17

Is that possible for me to know how much the energy will be generated through the regenerative braking? Is there any formula? Thanks.

Post 16

"Anonymous" is partially correct, but the problem is that the wheels are turning in the same direction. Typically, a motor and generator can be the exact same thing (polarity and all) if the shaft input is reversed from the motor-driven direction. You can picture this as physically turning a water pump backward to move the water back into the tank from where it was pumped to - the reverse turning of the shaft induces a current in the coil that puts the charge back into the battery.

As stated, though, the problem with regenerative braking is that the wheels turn in the same direction for driven and braking operations. So instead of putting charge back into the battery, the wheels want

to drive the charge in the direction it was already going. But since the charge is driven by the wheels now, and not the battery, the torque required to drive it inside the coil must put a reaction-torque on the shaft (known as "electromagnetic drag"), which slows down the drive wheels. This is very similar to leaving your clutch engaged when not accelerating - the friction in the motor slows the car down. As a solution, a clutch or torque converter is implemented to effectively disconnect the motor from the wheels, preventing such drag.

Now, as BJones stated - the solution to the generator question is simply to switch the polarity on the motor while braking. Obviously, the electric clutch must be re-engaged, as well. As stated, the charge will still flow in the same direction in the motor coil, but since the polarity is switched, it goes to the opposite pole on the battery, recharging it.

The problem with variable regenerative braking is that you must figure out a way to vary the electromagnetic drag. The easy way to do this is to meter it with another external motor or other power source, but this negatively affects the efficiency since some energy is spent in order to get some back (sound like economics?) - this goes against your purpose. The other way to do it is by variable coiling, which significantly raises the complexity of the design and the amount of work to achieve it - raising your cost of entry.

Welcome to engineering. :-)

Post 15

01/20/07 - This is a semi-active thread. I seem to be the only active “regular” and while I have a pretty strong idea on how regenerative braking works, I don’t profess to be an expert. Anonymous, the project you describe sounds to be quite the undertaking. Good luck! One piece of advice that comes to mind is, make your system high voltage. The higher your voltage, the less amperage you need for a given amount of power. One area of knowledge I do profess to have an above average knowledge in is UPSs. I have participated in all of APC’s Data Center University’s classes. But enough blowing my own horn, back to business. A white paper that might

be a smidge of stretch for this topic, but I personally think it would help is Use of 208V vs 120V for Servers in case the link doesn’t work, it can be found on APCs site. It talks about how 208V is overall a better choice in the data center over 120V. If formulas are more your style, P = I × V also proves my point.

Let me know if I can be of further help.

Post 14

01/13/08 - I'm thinking about trying to add hybrid electric to my 1982 Toyota pickup truck and am interested in this thread. A few preliminary questions: I can't see any post dates from my view of this conversation. How can you tell if the conversation has ended or very old? If this is posted as anonymous it is because the login procedure doesn't seem to allow me to do it any other way.

To BJones: My thoughts exactly, I want to try and recover some of that energy that I have paid for to go up hill, on my way back down. And I've been noticing that it doesn't take much throttle to maintain a speed on a level surface

, so it shouldn't require a huge electric motor to be able to use it then too. If energy recovery works good, I would later try increasing the battery pack size, and experiment with it being a plugin hybrid, much like people are currently doing to convert Prius's.
Post 13

Sorry for taking so long to respond. Anonymous, what type of truck are you designing? When I wrote my first response, I thought you were talking about a pickup truck, like a Dodge Ram. However, your last response makes me think you’re talking about a semi, like a Peterbilt 387. Mind clarifying?

Post 12

Bjones, thank you. Do you know the braking force created from a truck with a 40ft trailer on full load? or know of anywhere i can find such information? I need this to establish how much can actually be saved. Id agree with a thin motor to stretch across the bed.



Post 11

Sweet, a chance to influence the future of automotive design! I would suggest placing the battery beneath the bed of the truck. Have it take up the whole area of the bed, but thin enough not to make the bed too shallow. High voltage, so that amperage can be relatively low. I would think in the truck realm, a high torque motor to get the truck started from a stop would be the way to go. It would not need to go fast on battery, but be powerful. I actually drive a truck and one thing that I hate is braking. I spend money to get going and then “pay” to break (break pads). Another thing that I hate is driving a truck in stop and go traffic. If my truck worked like a Prius and practically ran silent in that situation, it’d be a major plus for me. I’d love to help you any way I can.

Post 10

hi there, i was wondering if any of you guys would be able to help.I am designing a truck for my final year project at uni and plan on using regenerative braking to capture some of the energy lost. What i am struggling with is how to work out how large to make the storage unit, and also how to work out how long the energy stored will last once in use. Any advice or guidance would me greatly appreciated,


Post 9

I agree with BJones. I'm in college, and just learned about Regenerative braking. The polarity changes inside the motor. This makes it a generator.

Post 8

BJones - the way the motor behaves depends on how it's connected. If you just take your foot off the accelerator, the motor will spin freely because it's effectively disconnected. To get the generator effect, you need to reconnect the motor to an electrical load. In regenerative braking that load is charging the batteries.

Post 7

So what you’re saying is that the polarity is never switched, but rather when one takes their foot off the accelerator, the once motor turns into a generator because the wheels are still spinning?

Post 6

BJONES you are terribly wrong bro.. it doesn't matter if you change the polarity.. that only makes the motor run the other way.. (reverse).. a motor creates rotational power when electricity is added.. so it "spends" the electricity.. to transform a motor into a generator you just do the opposite you apply rotational motion and you'll get electricity.. its that easy.. if you don't believe me grab a small motor stick the two wires to your tongue and rapidly turn the axle.. you can feel the small current..


Post 5

Because everyone in this discussion is Anonymous except me, it makes it hard to reference your comments. So we’ll do it this way, Anonymous #2, it’s not that they are putting the motor in reverse, but rather simply switching polarity, in turn, making it a generator. The motors in a hybrid are AC powered, but for the sake of simplicity, I’ll make it DC in this explanation. A motor has a + and a – terminal, when it’s a motor. When you want to make it a generator, you simply make the + the – and the – the +. This in turn makes it put energy into the battery, rather than taking it out. So, when you LIGHTLY brake, you tell the

vehicle to regeneratively brake by switching the polarity of the motor.

The reason they have chosen to only allow minor regenerative braking is because if they allowed full regenerative braking, it would lock the wheels. Think of it this way, the “motor” can be just as powerful a generator as it is a motor. It would brake with all the force it has when it’s a motor. (Entering a spot of uncertainty) I guess it’s hard to allow for variable regenerative braking, one where the further on the brake you press the more load put on the generator and more it’ll slow you down. So they just went with a moderately set level of regeneration. (Entering certainty again.) So when you press harder on the brakes, the vehicle employs its traditional brakes. (For the sake of ability of the regenerative technology right now, and to put those people’s minds at ease that don’t trust not having “actual brakes”.) Someday, your vehicle probably won’t have “actual brakes”, just dynamic regenerative brakes that can brake lightly (like now for the most part.) up to all of the braking ability of the system with the help of ABS (2.0?)

One last thought. To give you an idea of the power I’m talking about, a DeWalt Heavy-Duty 3/8" (10mm) 12V Cordless Compact Drill produces 230 watts of power (and on a full charge, I can’t stop it from spinning.) the Electric Motor of a 2007 Prius is 50 kW, or 217.4 times the power of the drill. So respect the motor/generator.

Post 4

BMW have implemented regenerative braking on a range of their cars. Instead of using a separate capacitor/larger battery, i.e the prius and civic hybrid, the car's main battery is charged through regenerative braking and hence reduces load on the alternator.

Post 3

I find myself surprised by the claim ( in the original article ) that, in order to achieve regenerative of dynamic braking, the direction of rotation of the motor is reversed. This sounds very extreme! I think the claim perhaps needs checking out. I think you may well find that all that is involved is some reconnection or change in control of the windings of the motor so that its normal ' back emf ' in motor mode is increased to above battery voltage. In this way the direction of both current and power flow are reversed, the motor continuing to turn in the same direction, but now acting as a generator. This is the way it USED to be done - I'll be very happy to be proved wrong, but, as I have said, the sudden reversal of direction of a rotary machine does sound extreme, probably unnecessary and thus somewhat unlikely.

Post 2

The simple answer to your question is “Because conventional cars do not have the parts necessary, and if they did, they’d be a hybrid by definition.” Longer answer, in order to take advantage of regenerative braking (my personal reason for purchasing a hybrid.), a vehicle would first need a generator. (motor in reverse as the article phrases it.) Second, it would need a place to put this newly generated energy; a battery seems to be the method of choice right now. Third, how to uses this newly stored energy? Electrical motors seem to be the way to go. So, we now have a generator/electrical motor and a battery; add that to the other parts of a conventional car and short of a few pieces I omitted for simplicity, you’d have a hybrid. How’s that for an answer?

Post 1

why this regenerative braking can't be used in conventional cars?

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