Regenerative braking is the feature that makes hybrids efficient in traffic. What it recovers, and what it cannot, follows directly from where a moving car's energy goes when it slows.
Where the energy goes in a conventional car
A moving vehicle holds kinetic energy proportional to its mass and to the square of its speed. Friction brakes convert that energy into heat at the discs.
That heat disperses into the air and is gone. Every stop therefore discards energy that fuel was burned to create.
In stop-start driving this happens constantly, which is why urban fuel consumption is so much worse than steady cruising.
How the motor reverses its role
An electric motor and a generator are the same machine operated in opposite directions. Supply electricity and it produces torque; turn it mechanically and it produces electricity.
When a hybrid slows, the drive motor is switched into generating mode, and the resistance it develops decelerates the car.
The electricity produced flows back into the battery, so energy that would have become brake heat is stored for reuse.
Why friction brakes remain essential
The braking force available from regeneration is limited by how much current the motor can generate and the battery can accept.
Heavy braking demands far more force than that, so friction brakes handle whatever regeneration cannot, blending the two so the driver feels one consistent pedal.
Regeneration also fades at very low speed, where the motor is turning too slowly to generate usefully, and friction takes over for the final stop.
What limits how much comes back
A full battery cannot absorb charge, so regeneration is reduced or disabled, which is noticeable on a long descent after a full charge.
Very cold cells accept current poorly, so regeneration is limited until the battery warms, and the car relies more on friction braking in the meantime.
Conversion losses in the motor, inverter and battery mean that even under ideal conditions only part of the energy taken out returns as usable drive.
Why brake wear falls sharply
Because most routine slowing is handled electrically, friction material on a hybrid can last far longer than on a conventional car.
That introduces a different problem, since discs used rarely can corrode and pads can seize in their carriers through lack of use.
Many systems periodically apply the friction brakes deliberately to clean the discs, which is maintenance carried out by software rather than by a technician.