What Is Regenerative Braking In Electric Cars?

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Electric Cars: The Basics

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For those of you new to zero-emission electric driving, we recommend a read of the following articles:

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The BMWi3 Slowed Down Before I Could Even Get My Foot On the Brake Pedal!

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I still vividly recall my first experience of regenerative braking.  I had just started driving an all-electric BMWi3 and within a few metres had to stop.  However, before I could get my foot on the brake pedal, the EV had already started to slowdown noticeably, as I released my foot from the accelerator pedal.  

My immediate reaction was ‘there is a problem with the car’, but was quickly assured by the owner of the EV, that it was the ‘regen’ kicking in and no need for a mechanic call out! 

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What Is Regenerative Braking? And What Are The Advantages?

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Regenerative braking has been around for a long time.   In fact, conventional hybrid models like the Toyota Prius HEV have been using regen for the past 20 years.  Today all plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs) use regenerative braking to increase EV efficiency.     

Quite simply, regenerative braking, sometimes referred to as brake recuperation, is the process that captures and converts the kinetic energy from the motion of a vehicle into chemical energy for storage in an onboard vehicle battery.  The chemical energy stored is reused for acceleration and driving.

In traditional internal combustion engines (ICE), like petrol and diesel cars, the kinetic energy is lost at the time of braking i.e. displaced as heat, making driving ICE motor vehicle less efficient.  In particular, in urban or congested environments that require frequent braking.  

In general terms, the kinetic energy in a moving vehicle is a product of the mass and speed. A heavier and faster electric car (all else being equal) will have more kinetic energy.  The regen profile varies from one EV model to another.  For example, the Audi e-tron regen profile is less ‘aggressive’ than the BMWi3 EV.   When I drove the Audi e-tron, the regen profile was quite subtle.   The automotive manufacturer is keen for the default regenerative setting in the e-tron to be as familiar as the driving profile of a conventional vehicle.  However, one can change the regen profile of the e-tron, such that the regen braking is more pronounced.  

In any case, the regenerative braking commences the moment you take your foot of the accelerator. In most cases the regen continues all the way down to 0 mph, however, some models may have a profile where there is some ‘coasting’.   

It is ‘regenerative’ because the kinetic energy can be reused at a later stage.  Lets say your Tesla Model 3 is approaching a traffic light. You release the accelerator pedal and apply the brakes.  The regenerative braking process comes into immediate effect and captures and converts the kinetic energy from the forward motion of your electric car.  

When the traffic lights turn green and the accelerator is pressed, the energy captured a few moments ago is first reused to propel the vehicle forward.  This way, the vehicle uses less of the stored energy in the EV battery, hence more efficient and increases the electric vehicle range.  

The ‘efficiency’ of regenerative braking varies depending on a number of factors, to include, the type of plug-in electric vehicle, the driving mode, the driving profile, the road etc.  

However, in most cases regen can harness between 60% to 80% of the kinetic energy displaced during braking.  In regards to range increase, it must be noted that regenerative braking will not result in significant increase in the EV range.  It certainly helps getting the most efficient range profile, but at marginal levels.  What regen does instead, is reduce the inefficiencies of constant braking.     

In most EVs, the driver will have the choice of the regen profile to choose while driving. The more aggressive the profile, the greater the gain in efficiency.  For example in the all-electric Nissan Leaf, the e-Pedal driving mode is a ‘one-pedal’ system i.e. at lower speeds the force of regenerative braking does not require the use of a brake pedal.   

For avoidance of any doubt, regen braking does not replace mechanical frictional brakes. Regen braking is not confined only to electric cars.  The concept has also been deployed in other forms of transportation that use electric energy.  For example, e-bikes, e-scooters etc.  

Back to the BMWi3, it did not take me much time to get used to or appreciate the regenerative braking profile in the electric car.  In fact, I got used to it much faster than my expectations.  So don’t fear the change, it is a positive step towards more efficient and lower emission motoring.    

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