Regenerative braking uses a vehicle’s motor to slow it down, allowing the kinetic energy to be converted into electricity and stored in the battery. This contrasts conventional friction brakes, which convert kinetic energy to heat, which is dissipated into the environment.
Using regenerative brakes reduces the external energy required to charge the EV’s battery, so there are lower running costs and an increased range between battery charges.
How regenerative brakes work
Electric motors contain fixed magnets and rotating magnets. When their opposite poles are aligned, they repel each other, creating a torque that causes the motor to rotate. One of the magnets must reverse polarity for this rotation to continue for more than 180 degrees. This is achieved by using an electromagnet and reversing the direction of the current.
In dc motors, the electromagnets change polarity using a simple mechanical switch to reverse the current, essentially acting as an inverter. EV motors have a three-phase ac supply that produces a more consistent torque and higher efficiency.
The flow of electrons through a wire is similar to water flowing through a pipe. The voltage serves as the pressure, causing it to flow faster. A magnetic field is generated around an electrical current, which is somewhat like the mass of water, which requires no energy when it is in motion. However, a magnetic field resists a change in current by absorbing and releasing energy when the current changes.
This is a two-way street, like the force and acceleration relationship. If the current changes, energy is stored in the magnetic flux, and if the magnetic flux changes, it releases energy by causing an electrical current to flow.
When the motor is rotating, a current is flowing. When the battery allows the current to flow “downhill” into the motor, the motor’s torque will drive it in the direction it’s rotating — adding energy to the vehicle’s motion. When the motor exerts a torque that acts against its direction of rotation, it causes the current to flow “uphill” to charge the battery.
In an ac induction motor, the inverter controls the ac supply’s frequency and phase to control the motor’s speed and torque. In regenerative braking mode, the inverter can control the motor’s torque, acting as a brake. It then converts the ac generated into dc, which is the voltage required to charge the battery.
Improving energy efficiency and range
With a battery-to-wheel efficiency of 80% and a wheel-to-battery efficiency of also 80%, the net efficiency of recovering kinetic energy is 64%.
It’s more efficient to avoid accelerating and braking as much as possible except as needed to navigate bends and flow with traffic. Recovering 64% of this energy can be significant.
It’s important to note that this does not mean a 64% increase in range. Energy will still be dissipated by air resistance and the rolling resistance of the tires, and regenerative braking does nothing to reduce these energy losses. For someone driving smoothly and efficiently, which offers the best range, regenerative braking might increase the range by about 10%. A driver braking hard into every bend and then accelerating quickly will greatly reduce the vehicle’s range. But regenerative braking could still make an impact of about 40%.
Filed Under: Braking, FAQs