Dear Ask a Mechanic,
I keep coming across the term “regenerative braking” in articles and videos about EV’s and hybrids. I know that in some of them, you can vary this function using paddles and such. I get that it has to do with slowing the vehicle down, but what exactly does the “regenerative” part mean? Why don’t normal cars have it?
Stop Watts
The key difference between conventional cars and electrified models such as full hybrids and electric vehicles (EVs) is, as you’d likely expect, the use, in some capacity, of electric motors for propulsion.
Critically, whether it’s a hybrid or an EV, those one or more electric motors are connected mechanically through some means to the wheels, and there are rechargeable batteries on board to store energy that can be used to drive the vehicle.
Because electric motors can also function as generators, it is possible to use the drive motor (or motors) to produce electricity. And that electricity can add some charge to the on-board batteries.
If you have ever used one of those generator-type bicycle headlights, the kind that was driven off of one of the tires (or even a hand-crank emergency flashlight), you’re familiar with how much effort just one little light bulb requires to illuminate.
It should not be difficult to imagine then that a larger motor/generator being used to produce a fair bit more electricity would require a commensurately larger amount of effort to turn.
In simplified terms, the law of conservation of energy states that energy cannot be created or destroyed, only converted. For that reason, in order for our motor, acting as a generator, to produce electrical power, an equivalent amount of energy must be put into it. The energy in this case is rotational force from the wheels, which itself is converted from the kinetic (motion) energy already present in the vehicle’s momentum.
Because its kinetic energy is being transformed into electricity and consumed, the vehicle’s speed is reduced, which is, in effect, a kind of braking.
As this process is recapturing (or “regenerating”) energy that was already used to get the vehicle moving, it is called regenerative braking.
By comparison, the brakes on a conventional vehicle transform the kinetic energy into heat through friction. That heat energy is then lost to the atmosphere.
How much braking can be accomplished through regenerative means depends on numerous factors, such as available battery storage capacity, the maximum rate of charge of the battery, the generating capability of the motor(s) — permanent magnet motors are better suited for this and more effective than induction motors, for example — and tire traction.
Because of these limitations, and because regenerative braking can’t be performed if the battery is fully charged, all EVs and hybrids still have traditional friction brakes.
Under harder braking, or when sufficient “regen” isn’t available, the vehicle’s systems will apportion braking duties to the conventional brakes as needed.
The ability of the motor(s) to provide braking effort through regeneration drops off steeply as the vehicle’s speed approaches zero, regardless of all other factors, so there’s what’s referred to as a “hand-off” at low speed as the friction brakes are blended in and take over. Some automakers do a better job of making this seamless than others.
As you note, many electrified models have selectable amounts of accelerator pedal-released regen. Some can be driven in “one pedal” mode, needing the brake pedal only for hard braking or to hold position while stopped. (This is much like driving a gasoline vehicle in a low gear.)
Regenerative braking doesn’t provide total energy recapture in any case. There are appreciable frictional and efficiency losses involved, but the end result is that a still sizable portion of the energy that would otherwise be wasted as heat can be used to propel the vehicle later.
This is a major part of what makes hybrids so much more efficient than a comparable nonhybrid in city stop-and-go driving conditions.
It’s also why driving style has a substantial influence on an EV’s range or a hybrid’s overall efficiency gains, something EV owners, in particular, should be aware of and pay attention to.
Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle
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