It's commonly referred to as regen and it's very important for electric cars.

Ever wonder what regenerative braking is and why it's always found on electric cars? Wonder no more as we dive into the details of regen braking.

In very simple terms, regenerative braking is a system that recovers some of the energy that would otherwise be lost as heat. This regenerative braking process occurs when slowing or stopping a car. 

Cars with regenerative braking (electric, plug-in hybrid and conventional hybrids) feature electric motors that turn the wheels. Battery power is used to turn the motor, but upon braking an unusual thing occurs. These electric motors can be reversed and thus act as generators to slow the car. In the process, the motor recharges the battery. 

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Regenerative braking cannot gain back all of the energy (or even close to all) that was used to propel the car forward, but it does help to extend the range of electric cars. This is basic physics. There are claims that regenerative braking can extend an EVs range by some 30% on average, though this varies by car/terrain/temperature and several other variables.

Furthermore, various EVs/PHEVs have different levels of regen. Some even have selectable regen so that more or less regenerative braking is applied. This is usually user preference oriented, meaning that if you like the car to basically stop on its own when lifting off the accelerator pedal (with no brake pedal application) than a high level of regen is selected. If you prefer a more coasting type of usage after lifting off the accelerator, then a low level of regen would better suit you.

Even though plug-in vehicles have regenerative braking, these cars still all come with conventional brakes too. The conventional brake is used to bring the car to a full stop, to perform emergency braking, for hard stops and even at other times like when the battery charge is too high to allow for regen to send more energy back into the battery.

In many cases, a proper regenerative braking system is very transparent in its operation. This means that the driver often does not know which system (regen or conventional) is doing the work at any given time. This is important because braking is connected to safety and the feel/action must be natural or at least predictable and expected.