An electric car slows noticeably when the accelerator is released, and the energy meter shows power flowing back into the battery. The mechanism is the drive motor running backwards as a generator.
A motor and a generator are the same machine
Feed electricity into a motor and it produces torque. Turn the same machine mechanically and it produces electricity, resisting the rotation that drives it.
In an electric car the wheels are turning the motor whenever the vehicle is coasting, so the inverter simply changes which way energy flows through it.
That resistance is felt at the wheels as braking force, and the energy that would have become heat in a brake disc becomes charge in the battery instead.
The battery sets a hard limit on how much can be recovered
A pack can only absorb charge at a certain rate, and that rate collapses when the battery is already nearly full or when it is very cold.
This is why an electric car driven downhill from a fully charged start often has noticeably weaker regeneration, and the car must make up the difference elsewhere.
Some vehicles warn the driver of reduced regeneration in those conditions, because the change in pedal behaviour is otherwise surprising at exactly the wrong moment.
Traction limits it further
Regeneration acts only on the driven axle. On a front-wheel-drive car that means the front wheels carry all of the retardation, and on a rear-driven car the rear ones do.
Braking a single axle hard on a slippery surface risks losing grip, so stability systems cut regeneration back sharply when the road is wet, icy or loose.
Friction brakes act on all four wheels and can distribute force more safely, which is one reason they can never be removed entirely.
Blending is the difficult engineering
When the driver presses the brake pedal, the car must decide how much retardation comes from the motor and how much from the pads, and change that split continuously.
The transition has to be invisible. A driver who feels the handover as a step in pedal pressure will describe the brakes as inconsistent, which is a serious complaint.
Getting this right is a large part of why braking feel varies so much between electric models that are otherwise mechanically similar.
What it means for maintenance
Brake pads and discs last far longer on an electric car, because most ordinary slowing never reaches the friction brakes at all.
The unexpected consequence is corrosion. Discs that are rarely used accumulate rust, and some cars periodically apply the pads lightly to keep the surfaces clean.
Inspection therefore matters more than wear measurement on these vehicles, since a disc can be well within thickness tolerance and still be in poor condition.