Regenerative Braking for Dummies

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Source: autoevolution post

BUCHAREST, Hungary – Regenerative braking has been a common thing for EVs (electric vehicles) ever since the Toyota Prius hit the market some two decades ago. It is a good way to save some of the kinetic energy that would otherwise go to waste every time you slow down, although it will not replace friction braking anytime soon.

To understand how a car can “generate” energy by braking, we should take a look at how an AC induction motor works. Well known for their simplicity, electric motors only feature one moving part, intuitively named a rotor. This sits inside a static frame called a stator, which provides it with torque via electromagnetic induction.

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An electromotive flux (EMF) wave is generated by three or four phases placed at equal distances from one another in the stator. The EMF spins around the rotor, “imprinting” it with movement. Since no motor is 100 percent efficient, the flux wave will always travel faster than the rotor, losing electrical energy in the process, which is supplemented from the vehicle’s batteries.

Generative braking works in reverse. The vehicle is slowed down by decreasing the speed of the flux wave. The rotor will naturally “want” to keep its speed due to inertia, and consequently will now travel faster than the flux wave. In layman’s terms, this will make the rotor “push” on the wave of electrons surrounding it, imprinting extra energy, which is sent toward the battery.

Since the rotor is connected to the wheels of the vehicle, the amount of energy converted will be directly proportional to the kinetic energy generated by the car. The greater the deceleration, the more energy is produced Hitting the brakes hard will see a bigger return to the battery than would coasting. The upper figure for electric energy return is cited at 60 Kw for most models, probably regulated by an electronic limiter. 

Of course, there are myriad of other factors affecting the efficiency of the system, especially considering that it follows a two-way cycle. Energy goes from the battery to the wheel through the inverter (DC to AC), motor, and transmission, and then back again when braking is initiated, with a little bit lost at every stage. In a car with a battery to wheel transfer of 80 percent, like the old Tesla Roadster, the regenerative braking efficiency will be 64 percent, or 80 percent squared.

This means that in ideal conditions, 64 percent of the energy lost through braking will be later available for acceleration. But how does this translate to everyday situations? Will it make a significant impact on the vehicle’s range?

As we have hinted above, it mostly depends on your braking habits, driving environment, and the weight of the car. (You might find useful to know that the kinetic energy stored in a moving vehicle is calculated via the formula mass times speed squared, all divided by two.)

Aggressive drivers, for example, might see more benefit from the system. This doesn’t mean going full-throttle and braking like you’re competing in a rally won’t decrease your range. Cruising at a set speed is still the most effective way to save on batteries. It’s just that sporty driving will cost you less in an EV than it would in a gasoline engine car.

In highway driving, where the speed is kept constant, regenerative braking should barely make a difference, but city routes or going downhill can see you recouping a substantial amount of energy. How much, exactly?

This person reports recapturing some 32 percent with a Tesla Model S through a hilly environment, while most figures cited by people on this Tesla forum gravitate around the 15 percent-20 percent mark.

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