One of the tricks up the sleeve of hybrid and electric cars, is the ability to slow down by charging their batteries. Interested in finding out how? Let’s delve in.
Table of Contents
What is Regenerative Braking
Regenerative braking, often referred to as ‘Regen’ braking, is a braking system fitted to hybrid and electric vehicles which allows for some of the braking energy to be captured and reused to move the car again.
When your car is moving along, it has a type of moving energy called kinetic energy. Because car’s are heavy, they require lots of energy to accelerate and also lots of energy to stop.
Have you ever tried pushing a car? The hardest part is to get it started. Once its rolling, you’re laughing, until you want to stop it! A portion of the energy invested in starting it changes into kinetic energy.
Law of Conservation of Energy
Traditional braking systems exchange kinetic energy, for heat which is then lost to the outside environment. This racing car’s brakes have got so hot they are visibly glowing:
As you can see, converting kinetic energy into heat is impractical / impossible to capture for reuse! However, converting kinetic energy into electrical energy is something that a battery for example can store and reuse (unlike heat).
Energy cannot be created or destroyed
First law of thermodynamics
Fundamentally, regen braking is a way of converting kinetic energy into electrical energy so it can be converted back into kinetic energy.
The regen process is not 100% efficient, so don’t expect all kinetic energy to be recovered and reused. You’re still better off at a steady speed for maximum efficiency – avoiding braking and accelerating will improve range / reduce fuel consumption.
In some mild hybrid systems, where there is no big battery and the regen effect is very mild. It doesn’t really do much braking but it will run the smart alternator to charge the 12v auxiliary battery. Carry on reading to find out how it works and what some of the disadvantages are.
How Does Regenerative Braking Work
Despite studying automotive engineering for years, I never fully got to grips with how a regenerative brake system works. So I took the time to conduct a whole lot of research for this section.
Are Regen Brake Systems Safe
Safety is always the first priority when designing cars. Regenerative brake systems should be just as safe as conventional brakes. Engineers would never design a system that was more fuel efficient at the cost of safety. You can’t sell fuel efficient cars if the brakes occasionally don’t work and kill people.
Because the safety of this system is so important it feeds heavily into the design of how the system is implemented.
Having said all that, my research into this topic has revealed that the regen systems fitted to various hybrids is different and some fail safer than others! Older Prius systems, when in failure mode would only brake the wheels on the front axle with no brake assist (basically, very very heavy hard to push brake pedal). Conversely the system fitted to my Outlander PHEV has a conventional brake booster fitted, so the driver would not feel anything was wrong, (the dashboard lights would warn them).
Do Regen Brake Systems Use ‘Fly by Wire’
Under normal operating mode, when everything is working correctly, a regen brake system must be able to operate in a ‘brake by wire’ configuration.
When you push the brake pedal on a car fitted with conventional brakes, the force of your foot on the brake pedal generates pressure in the brake fluid. This brake pressure is transferred to the brake calipers at the wheels.
Regenerative Braking System Operation
For regen to work, it’s no good if the brake caliper is allowed to operate, otherwise the car would slow down using the friction brakes. So regen brake systems must have a method to decouple the brake pedal from the physical brake calipers. This is always done with mechanical valves and computer control.
For regen to work the driver will press the brake pedal, this will tell the computer to request braking torque (rather than acceleration torque) from the electric motors. The computer software, typically in the ABS module, will also ensure that the friction brakes are not allowed to operate. I made a video to explain it in more detail:
The ABS module has been able to prevent wheel lock ups / skids for decades by diverting brake pressure away from the brake calipers. Regen takes this idea to the next level!
To summarize the explainer video above:
- Brake pedal force goes to a brake feel simulator (essentially a squashy dead end)
- Brake fluid in the friction brakes remains unpressurized
- A sensor in the master brake cylinder measures brake pedal travel
- The brake control unit converts the brake pedal distance travelled in mm into a braking request in newton meters (Nm)
- Brake request is sent from the brake control unit and received by the drive control unit
- Brake request is typically negative torque (whereas accelerator pedal will be asking for positive torque)
- Drive control unit will operate the motors as generators. By inducing current in the windings of the motor generator the battery is charged. Like a dynamo, it slows the car down. The level of current charging the battery is proportional to the amount of brake torque. This is all controlled by drive control unit
- When high braking request is made (more travel on the pedal) a pressurized actuator bleeds pressure into the brake system connected to the friction brakes
- The system is designed to be fail safe
- The master cylinder, which usually routes pressure to the brake feel simulator can be rerouted to send pressure directly to the friction brakes, via a fail safe valve.
Do Hybrids Have Friction Brakes
Yep. Friction brakes are not going anywhere EVER! Friction brakes (discs and pads) are here to stay and here’s why.
They..
- Add redundancy, see above about safety
- Provide phenomenal stopping power in an emergency (far greater than regen could ever offer)
- Can hold a vehicle stationary indefinitely
- Don’t rely on computers, electricity and are very simple
Do Hybrids Have Hydraulic Brakes
Absolutely! Hydraulic brakes are still present on all cars, even hybrids and electric cars. In future the method of operation for a friction brake could change. Busses and lorries use air actuation for example. Some cars now have electric handbrakes, where a worm gear replaces the ‘old fashioned’ cable.
In my opinion hydraulic fluid is likely to be superseded one day. It’s too clumsy for the future. I wonder which car will first launch without hydraulic fluid?
Air actuation like on big heavy vehicles may creep into passenger vehicles. It could be that motors (in conjunction with heavy springs) actuate the brakes, who knows? Watch this space I guess.
Do Cars with Regen Brakes also have Normal Brake Pads
Yes, ‘normal’ friction brakes are still required on all vehicles. Typically cars fitted with regen brake systems wear through brake pads at half the rate of an equivalent non regen system. This is due to much less use of the brake discs and pads.
Advantages of Regenerative Braking
- Saves fuel
- Brake pads typically last twice as long (50,000 miles for a conventional car vs upto 100,000 miles for a car with a regen braking system)
- It paves the way for integrating more safety systems into vehicles, such as emergency autonomous braking
Interested in saving fuel. have a read of some of my other posts, Why is a Hybrid Car More Efficient than a ‘Normal’ Car or Does a Plug-In Hybrid Recharge While Driving
Disadvantages of Regenerative Braking
- ‘Brake feel’ can seem vague, if you watch my explainer video you’d understand why – its simulated!
- Blending between friction brakes and regen braking can feel odd and depending on the state of charge of the battery the brakes may behave slightly differently
- Brake calipers have a tendancy to seize
- Brake maintenance & servicing are now far more important, to prevent seizing!
What Limits Regenerative Braking
A number of factors affect the level of regenerative braking.
- Battery state of charge. Clearly if the battery is full, you cannot use regenerative braking as there is no where for the charging current to go.
- The power from the motors is typically much less than what we expect from brakes. A 60kw motor can’t absorb more than 60kw in braking power. Put simply, a car will always stop quicker from 100mph than it can get to 100mph.
- Regen system is about 69% efficient, (kinetic to battery to kinetic) All the associated components along the way will waste some of your energy:
- Motor (80-95% efficient)
- Tyres (85-95% efficient)
- Inverter (96-98% efficient)
- Differential (95% efficient)
- Battery chemistry (99% efficient)
- Batteries are not able to charge as quickly as they can discharge. This limit on charging will impose a limit on how much regen your car will do
What Cars Have Regenerative Braking
Electric cars all have regen brakes:
The following car manufacturers offer an electric car (basically everyone now in 2024 ):
Audi, BMW, Cupra, Citroen, DS Automobiles, Fiat, Ford, Honda, Hyundai, Jaguar, Kia, Lexus, Nissan, Peugeot, Polestar, Porsche, Renault, Seat, SKODA, Smart, SsangYong, Subaru/Toyota (the Subaru Solterra is the same as the Toyota bz4x), Tesla, Vauxhall Volkswagen & Volvo.
Do Mild Hybrids Have Regenerative Brakes
Not really. Mild hybrids have very weak electric motors and consequently very weak regenerative brakes. A mild hybrid system often has a starter generator to facilitate seamless stop-start.
A starter generator, as the name suggests, replaces the starter motor and the traditional alternator. This system, whilst being slightly more fuel efficient is not very effective at slowing the car.
Having said that, the 12v battery may be charged during a braking event through the generator function, this will in effect cause some braking to occur. The system would then stop charging the battery when the driver wants to move off again and may provide some very light assistance to the internal combustion engine also.
Types of Regenerative Braking System
All passenger cars on the market use electric motors and batteries. The electric motor provides negative torque to the wheels, effectively slowing the car down. The current created during these regenerative braking events is stored as chemical energy in a battery. The current can then flow back out of the battery and into the motor, this time creating positive torque to move the car.
It is possible to use a flywheel in a regenerative brake system to capture kinetic energy. Formula 1 previously had a Kinetic Energy Recovery Systems (KERS) which basically converted the kinetic energy of the car into kinetic energy in a flywheel, a spinning mass. During braking, the flywheel was coupled to the wheels and the act of accelerating the flywheel converted the kinetic energy of the car into kinetic energy of the flywheel.
sources: https://chargedevs.com/features/regenerative-braking-a-closer-look-at-the-methods-and-limits-of-regen/; Audi-media-regen animation
