Regenerative energy capture, or regenerative braking, also called a kinetic energy recovery system (KERS), in an EV can be implemented in several ways. And it works differently on different classes of vehicles.
This FAQ looks at some options for implementing regenerative energy capture and how its efficacy varies for different classes of vehicles and under different driving conditions.
The most basic form of regenerative braking is called potential energy loss (PEL) recovery. It occurs when the driver takes their foot off the accelerator, and the traction motor is used in reverse mode to capture kinetic energy. It can be controlled in several ways, including the driver setting some basic limits on the amount of regenerative braking and varying the braking based on battery conditions (see “What control strategies can be used for regenerative braking in EVs” for more details). This type of braking will slow the vehicle but not bring it to a complete stop. For stopping, regenerative braking must be combined with conventional hydraulic braking.
The PEL recovery form of regenerative braking is primarily used in two use cases: slowing the vehicle when entering a slower speed zone on the highway (or slowing to increase the distance from the vehicle ahead) and descending a hill. Typically, descending a hill produces more recovered energy. It can be implemented by the driver taking their foot off the accelerator or automatically by the cruise control system.
Basic regenerative braking requires that the motor run in reverse and act as a generator. In more expensive EVs, an additional KERS is added to the rear wheels (Figure 1). The second regenerative braking system can significantly improve energy capture and increase driving range under the right conditions.
Figure 1. Location of the rear KERS in a BMWi3 electric vehicle (Image: MDPI Vehicles).
Conditions matter
Conditions like the driving environment, terrain, and vehicle weight can significantly impact the effectiveness of regenerative braking. For example, regenerative braking does not produce significant energy during highway driving at mostly constant speeds. On the other hand, it can be very effective in stop-and-go traffic and during city driving.
Regenerative braking is less effective at slower speeds where less kinetic energy is available to capture. The amount of kinetic energy available is related to the square of the velocity. A vehicle moving at 1/2 the speed has only 1/4 of the energy available to be captured; at 1/5 the speed, only 1/25 is available, and so on. In addition, the generator used to capture the energy generally produces less voltage when rotating at lower speeds, compounding the challenge of energy capture under low-speed driving conditions.
Driving downhill will produce more recovered energy compared with driving uphill. The temperature also matters. For example, if the batteries are not being operated in the optimal temperature range between about 15° and 35° C, regenerative braking can be less effective since the batteries cannot be recharged as quickly. Under optimal operating conditions, regenerative braking can increase vehicle range by up to 30%.
Vehicle weight is a major determinant in the amount of available kinetic energy and is a significant factor in determining the effectiveness of regenerative braking systems. Regenerative braking is more effective on heavier vehicles. In a typical car, regenerative braking can save about 10% of the energy. That increases to 17% for an electric train and drops to only 2% for an electric bicycle or scooter (Figure 2).
Figure 2. Vehicle weight is a major determinant of the percentage of energy that can be saved using regenerative braking (Image: IOP Conference Series: Materials Science and Engineering).
Summary
Regenerative braking is an important system in all types of EVs. Its primary purpose is to increase vehicle range by recapturing kinetic energy when the vehicle is slowed. The performance of regenerative braking systems varies widely. It’s affected by the system design and control scheme, environment, driving conditions, and vehicle weight.
References
- Analysis of Kinetic Energy Recovery Systems in Electric Vehicles, MDPI Vehicles
- Regenerative braking on electric vehicles: working principles and benefits of application, IOP Conference Series:
- Materials Science and Engineering
- Regenerative braking systems, Bosch
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