Electric Vehicles and Regenerative Braking: Conservation of energy in motion.
By- Avi Aggarwal
What Is Regenerative Braking and How Does it Work?
Regenerative braking is a mechanism found on most hybrid and full-electric vehicles. It captures the kinetic energy from braking and converts it into the electrical power that charges the vehicle’s high voltage battery. Regenerative braking also slows the car down, which assists the use of traditional brakes.
In a conventional braking system, a car slows down due to friction between the brake pads and rotors. But this system is highly inefficient when it comes to conserving energy. Nearly all of the kinetic energy propelling your car forward is lost as heat when you apply the brakes. That’s a lot of wasted energy!
Regenerative braking solves this problem by recapturing upwards of 70% of the kinetic energy that would otherwise be lost during braking. The amount of energy recovered depends on your car model and driving behavior.
Traditional brakes use friction to bring a car to a halt. This friction is generated when the brake discs come into contact with the wheels as soon as you hit the brakes. Unfortunately, much of the heat energy that is generated gets wasted. Engineers have now borrowed a concept long used in electric trains to recycle some of this energy and route it back to the EV’s battery.
Here’s how it works: regenerative brakes use an electric motor instead of discs (or drums) to slow down the car. The motors capture the energy generated by the forward motion of the vehicle even as it decelerates. This energy is fed back into the battery and used to power the acceleration phase.
Regenerative braking is a mechanism found on most hybrid and fully electric vehicles. It captures the kinetic energy from braking and converts it into the electrical power that charges the vehicle’s high voltage battery. Regenerative braking also slows the car down, which assists the use of traditional brakes. Electric motors, when used in reverse, function as generators and will then convert mechanical energy into electrical energy. Vehicles propelled by electric motors use them as generators when using regenerative braking, by transferring mechanical energy from the wheels to an electrical load.
In a conventional braking system, a car slows down due to friction between the brake pads and rotors. But this system is highly inefficient when it comes to conserving energy. Nearly all of the kinetic energy propelling your car forward is lost as heat when you apply the brakes. That’s a lot of wasted energy!
With regenerative braking, the energy from your spinning wheels is used to reverse the direction of electricity – from the electric motors to the battery. All you have to do is remove your foot from the accelerator or, in some cases, press the brake pedal to activate regenerative braking. The electric motor not only acts as an electric generator, but it also helps slow your car down because energy is consumed by the wheels as they rotate the shaft in the electric motor.
Regenerative braking solves this problem of energy wastage by recapturing upwards of 70% of the kinetic energy that would otherwise be lost during braking. The amount of energy recovered depends on your car model and driving behavior.
Principle of operation
To understand how the regenerative braking system works, it is necessary to remember that every moving body has kinetic energy. When braking a car with an internal combustion engine, this energy is consumed during the contact of the brake pads and brake discs, erasing them, i.e. just “nowhere.” Electric vehicles are taking a more thoughtful approach to energy use. The recovery process is presented here as follows:
When braking starts, the electric motor changes its operating mode: instead of being powered by a battery, it starts to work as a generator, generating energy. At this moment, opposite currents appear in the rotor and stator windings.
The decrease in the speed of the vehicle occurs because a brake torque appears on the shaft of the electric motor.
The kinetic energy available before the start of braking is transformed into electrical and thermal energy.
The emerging additional electricity flows into the battery, thereby increasing its charge.
In a situation where the range of an electric vehicle is limited by the battery charge, any source other than the charging station that can generate additional energy is important. Therefore, regenerative braking DPT is a good and promising way to increase mileage. And 70% of the saved energy is a good indicator, given that just 10-15 years ago such losses were not paid attention to at all.
How Does Regenerative Braking Provide Electricity?
Regenerative braking turns kinetic energy into electricity by reversing the process that drives the car forward. In electric cars, the drivetrain is powered by a battery pack that powers a motor (or motors), creating torque–rotational force–on the wheels. In other words, electrical energy from the battery becomes mechanical energy that spins the wheels.
With regenerative braking, the energy from your spinning wheels is used to reverse the direction of electricity - from the electric motor(s) to the battery. All you have to do is remove your foot from the accelerator or, in some cases, press the brake pedal to activate regenerative braking. The electric motor not only acts as an electric generator, but it also helps slow your car down because energy is consumed by the wheels as they rotate the shaft in the electric motor.
Advantages and Disadvantages of Regenerative Braking
As you can imagine, capturing and reusing more energy from braking has real benefits for the efficiency of your vehicle. Plus, it means less wear and tear on your brakes. Here are the biggest advantages of regenerative braking:
Brake Pads & Rotors May Last Longer
Even though regenerative braking provides a lot of stopping force on its own, EVs and hybrids also come with conventional hydraulic brakes. However, since regenerative braking does much of the work while slowing the vehicle, the brake pads and rotors are used much less frequently.
As a result, they typically last much longer between service, which can help drivers save on maintenance costs. It’s still important to have your brakes inspected regularly, and routine checks may be required as part of your manufacturer's suggested maintenance schedule. Just bring your hybrid or electric vehicle into Tires Plus for a quick and convention inspection.
As an EV owner, you would naturally want to get as much mileage out of your battery as you possibly can. After all, while charging stations are being set up across the country, they are not as widespread as regular fuel pumps. In a typical urban setting, you can expect to brake several times a day at traffic signals or while driving over speed breakers. And the energy generated at such times (which would otherwise be wasted by conventional brakes) can be better used.
It improves the fuel economy of the vehicle. The amount of fuel consumed can be dramatically reduced with this type of braking system due to the regeneration of energy.
It allows traditional friction-based brakes. A friction braking system is included with a regenerative system to ensure a vehicle can stop in time.
It prolongs the charge of the battery. Once the energy is captured by the regenerative brakes, the energy is used to recharge the batteries of the vehicle. Because this energy would normally be lost, it allows each vehicle to experience a prolonged charge while driving.
It reduces the wear and tear on the braking system. Because an electric drivetrain is part of this system, the greater efficiency given to the braking allows for a reduced level of wear on the brakes of the vehicle. With standard friction brakes, there is no way to accomplish this benefit.
It offers a sliding scale of benefits. The effects of regenerative braking decrease with the speed a vehicle is traveling. At low speeds, friction brakes are required to bring most vehicles to a complete stop. That means there is still energy being lost.
It offers a different feeling to the driver. Regenerative braking systems feel different to drivers who are used to traditional systems. The brake pedal on the vehicle often feels soft, described as “mushy” by many drivers. Until you get used to the new system, some may have a lack of confidence in the capabilities of their brakes.
Regenerative braking helps conserve up to 70% of the energy generated when stopping your EV, allowing you to go much further on a single charge. In fact, today’s EVs average around 250 miles or 402 km per charge, depending on factors like vehicle size, traffic conditions, and the terrain you are on.
While the positives of regenerative braking outweigh the negatives, no technology is perfect. Here are a few instances where regenerative braking falls short:
Extended Range Possibilities for EVs
Capturing braking energy and sending it right back to your EV’s battery pack can extend your driving range. Estimations show that regenerative braking can potentially add hundreds of miles of electric driving range throughout the year. That means less time spent charging and more time getting where you need to go.
When charging stations are still far and few between in many areas, every mile counts. Plus, when you plug into the electric grid less often, you help reduce emissions from coal and gas-powered electricity suppliers.
Better Fuel Efficiency for Hybrids
While hybrids still have internal combustion engines under the hood, they’re designed to use their electric motor as much as possible. Regenerative braking helps keep the battery pack charged, so drivers don’t have to rely on their engines as often, helping them reduce fuel consumption and save money.
May Be Less Effective at Lower Speeds
Traveling at slower speeds means your vehicle has less kinetic energy and requires less braking force. As a result, the regenerative braking system is fed less energy and does not supply the battery pack with much charge. Some vehicle manufacturers also feel that coasting may outweigh the benefits of regenerative braking in some situations.
Multi-mode regenerative braking?
Now, what if you could control the amount of power diverted to the brakes even before you started your car? Your daily commute would be smoother, and you wouldn’t have to glance at the battery indicator half as often! Say hello to multi-mode regenerative braking.
The sporty MG ZS 2021 makes this a whole lot more possible. For starters, the latest ZS edition packs a brand new 44.5Kwh power pack, which provides a mileage of 419* km – a 23% increase over the 340km offered by the baseline variant unveiled last year. What’s equally impressive is that it also lets you pre-set the transmission to three different regenerative braking levels- light, moderate and heavy.
Light: As the name suggests, this level minimizes the braking power while increasing the range that the car travels before coming to a halt.
Moderate: This level scales the braking power up a notch from the first level, making it ideal for regular driving conditions.
Heavy: This level brings to bear the full power capacity, ideal for shorter distances and more frequent braking.
Each level corresponds to three driving modes- Eco, Normal, and Sport- that can be selected via rocker switches just below the ‘center stack’.
As you might imagine, the Eco mode is the one to use on city roads. Depending on how much you drive, it can allow you to get by for a few days without recharging your EV’s battery. Normal mode is best when you need both performance and economy, while the Sport mode ramps up power for those winding mountain roads on the way to your monsoon getaway.
With better efficiency, higher mileage and greener solutions, there’s no reason why an EV shouldn’t be your next best way to
How are the efficiency and benefits of regenerative braking measured in EVs?
Regenerative braking is only one of the two ways to capture kinetic energy from a moving EV. Another alternative is kinetic energy recovery (KER). Regenerative braking and KER are suitable for different use cases. In addition, there are multiple ways to implement regenerative braking, including series and parallel architectures.
This FAQ begins by comparing several regenerative braking architectures and their use cases. It then digs into how KER works and when it’s appropriate and closes by looking at the difference between regenerative braking efficiency versus effectiveness.
The braking systems in EVs include friction and regenerative braking systems. Friction braking systems are the same as the brakes on internal combustion engine (ICE) vehicles. Regenerative braking uses the traction motor as a generator to convert kinetic energy into electricity that recharges the battery. In series regenerative braking, the brake is used until its maximum capacity has been reached.
The braking controller determines the type of regenerative braking to implement. For example, regenerative braking can be initiated for low deceleration rates, like under 0.1 g, and the series braking can be used.
Figure 1. Examples of parallel (left) and series (right) regenerative implementations. (
Third option
Depending on the system design and circumstances, friction braking can be initiated first, followed by regenerative braking. Figure 2 shows some powertrain operating parameters related to this scenario.
At the beginning of the period, the vehicle is moving forward with no braking being initiated. As break starts, the three-phase motor currents (red waveform on top) decrease to almost zero. At that point, regenerative braking is initiated, and the current rises. The blue waveform shows the voltages that remain relatively stable until the end of the braking period when the vehicle comes to a stop.
The lines on the bottom measure apparent power (orange), reactive power (purple) and real power (black). At the point when the current begins to rise, the real power goes negative, indicating that regenerative braking is sending power back into the battery.
Figure 2. Measurements during deceleration to a stop with a transition to regenerative braking.
What’s KER, and how efficient is it?
KER involves recovering kinetic energy when the brake pedal is not engaged and occurs when the driver’s foot comes off the accelerator pedal. It allows the vehicle to decelerate slowly, like when entering a zone with a lower speed limit. It also occurs when a vehicle enters a descending segment of travel. In the first instance, a limited amount of energy is recovered. In the second instance, a much more significant amount of energy can be recovered, depending on the steepness and length of the descent.
The efficiency of KER varies with the use case:
When decelerating slowly on relatively flat ground, KER has an efficiency of about 48%.
When descending on a travel segment, KER can have an efficiency of over 85%.
Those efficiencies compare with a typical efficiency of 60 to 70% for regenerative braking.
Figure 3. KER can be a separate system from regenerative braking.
Regenerative efficiency versus effectiveness
Regenerative braking systems are generally quite efficient, returning 60 to 70% of the kinetic energy recaptured during braking back to the battery. Effectiveness is a more important measurement and is more complex. Effectiveness combines the efficiency of the regenerative braking system with the available kinetic energy and the battery’s state of charge (SoC). If the battery has a high SoC, it will have a limited ability to accept the captured energy, reducing its effectiveness. Other factors include:
Vehicle size – Heavier vehicles, like cars, have more kinetic energy available for the regenerative braking system compared with lightweight electric scooters.
Driving conditions – Driving in an urban environment with frequent stops will generate more regenerative energy than highway driving.
Terrain – Driving a route with a net elevation loss will generate more regenerative energy than a route with a net gain in elevation.
The effectiveness of regenerative energy capture typically varies between 15 and 30%. Under highly unfavorable driving conditions, it can drop to 10% or less but rise to nearly 50% under optimal driving conditions.