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. It’s then supplemented with the friction brake to achieve the desired deceleration.
In parallel regenerative braking, the regenerative brake and friction brake are used over the entire deceleration process (Figure 1). 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. (Image: MDPI energies)
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 braking 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. (Image: HBK)
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. (Image: MDPI vehicles)
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.
Summary
Regenerative braking is initiated when the brake pedal is depressed. It can be an effective way to increase the range of EVs. There are several architectural choices when implementing it, and while it is highly efficient, its effectiveness varies based on operating conditions. KER is a separate system initiated when the driver’s foot comes off the accelerator, but the brake pedal may not be depressed.
References
- Analysis of Kinetic Energy Recovery Systems in Electric Vehicles, MDPI vehicles
- Dynamic Motor Power Measurements Enable In-Vehicle Testing of Electric Vehicles, HBK
- Effect of Regenerative Braking on Battery Life, MDPI energies
- Estimation of the Regenerative Braking Process Efficiency in Electric Vehicles, 6th International Conference LTHU
Images
- Figure 1, MDPI energies, Page 5, composite of Figures 4 & 5
- Figure 2, HBK, Figure 12, three-quarters down the page
- Figure 3, MDPI vehicles, Page 391, Figure 1
Filed Under: Braking, FAQs