Moving vehicles have a lot of kinetic energy, and when brakes are applied to slow a vehicle, all of that kinetic energy has to go somewhere. Back in the Neanderthal days of internal combustion engine cars, brakes were solely friction based and converted the kinetic energy of the vehicle into wasted heat in order to decelerate a car. All of that energy was simply lost to the environment.
Fortunately, we have evolved as a species and developed a better way. Regenerative braking uses an electric vehicle’s motor as a generator to convert much of the kinetic energy lost when decelerating back into stored energy in the vehicle’s battery. Then, the next time the car accelerates, it uses much of the energy previously stored from regenerative braking instead of tapping in further to its own energy reserves.
How well does regenerative braking work?
To evaluate regenerative braking, we really need to look at two different parameters, efficiency and effectiveness. Despite sounding similar, the two are quite different. Efficiency refers to how well regenerative braking captures ‘lost’ energy from braking. Does it waste a lot of energy as heat, or does it turn all of that kinetic energy back into stored energy? Effectiveness, on the other hand, refers to how large of an impact regenerative braking really makes. Does it measurably increase your range, or will you not notice much of a difference?
Efficiency
No machine can be 100% efficient (without breaking the laws of physics), as any transfer of energy will inevitably incur some loss as heat, light, noise, etc. Efficiency of the regenerative braking process varies across many vehicles, motors, batteries and controllers, but is often somewhere in the neighbourhood of 60-70% efficient. Regen usually loses around 10-20% of the energy being captured, and then the car loses another 10-20% or so when converting that energy back into acceleration, according to Tesla. This is fairly standard across most electric vehicles including cars, trucks, electric bicycles, electric scooters, etc.
Keep in mind that this 70% does not mean that regenerative braking will give an 70% range increase. This isn’t going to bump your range from 100 miles to 170 miles. This simply means that 70% of the kinetic energy lost during the act of braking can be turned back into acceleration later.
This is why only reporting the efficiency of the system doesn’t really mean much. Someone could be very efficient when they work, but if they only work an hour a day, they probably aren’t accomplishing a lot. What should interest us more is the effectiveness of regenerative braking.
Effectiveness
This is where things get really interesting. The effectiveness of regenerative braking is a measure of how much it can increase your range. Does it make your theoretical range 5% further? 50% further? Even more?
As you’ve probably already guessed, the effectiveness of regenerative braking varies significantly based on factors including driving conditions, terrain and vehicle size.
Driving conditions have a large impact. You’ll see much better effectiveness for regenerative braking in stop-and-go city traffic than in highway commuting. This should make sense, as if you’re repeatedly braking, you’ll recapture a lot more energy than if you simply drive for hours without touching the brake pedal. Terrain also plays a large role here too, as uphill driving doesn’t give you much chance for braking, but downhill driving will regenerate a much larger amount of energy due to the long braking periods. On long downhills, regenerative braking can be used nearly constantly to regulate speed while continuously charging the battery.
Vehicle size may be the largest factor in the effectiveness of regenerative braking for the simple reason that heavier vehicles have much more momentum and kinetic energy. Just like a big flywheel is more effective than a small flywheel, a four-wheel electric car has a lot more kinetic energy when in motion than an electric bicycle or scooter.
Data for comparison can be somewhat hard to come by. Tesla vehicles show you the regenerative braking power, such as 60 kW during hard braking, but that doesn’t answer the more interesting question. We want to know how much energy we are recapturing over a trip, not how strong our brakes are each time we mash the pedal.
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