Source: Rick Popely wrote the following post for cars.com on regenerative braking (regen).
CHICAGO – Regenerative braking is technology used on hybrid and electric vehicles to recharge the battery that powers the vehicles and to assist the hydraulic braking system to stop them.
When the driver removes their foot from the accelerator pedal and coasts or applies the brakes, the electric motor (or motors) spins in reverse and acts as a generator to recharge the hybrid or EV battery (or batteries). That energy also is applied as resistance on the wheels to slow the vehicle without using the conventional brakes (the pads and rotors at each wheel).
Regenerative Braking Better with Each EV Generation
When the driver of a hybrid vehicle or EV applies the brakes, the kinetic energy (the energy of an object in motion) provides all or most of the initial stopping power until the driver fully applies the brake pedal; that’s when the hydraulic system activates and squeezes the brake pads against the rotors.
That’s why pads and rotors can last longer on hybrids and EVs than on vehicles powered solely by internal combustion engines. Regenerative braking does most of the early work to stop the vehicle, and the hydraulic brakes don’t get heavily involved until closer to the finish line. The hydraulic system will engage sooner in panic stops or hard stops from high speeds.
Vehicles with regular brakes also develop kinetic energy, but the energy converts into heat created by the friction of the pads against the rotors during braking. That energy simply dissipates into the air and is wasted.
With regenerative braking, as much as 70% of the energy is reportedly captured and used to recharge the batteries or slow the vehicle. The amount that’s captured depends on how a car is driven, much like fuel economy depends on driving style.
On hybrid vehicles that aren’t the plug-in type, regenerative braking recharges the hybrid battery pack, which can store only enough electricity to power the vehicle for short distances — a couple of blocks, max — or for a burst of passing power. The frequent slowing and stopping of city driving keeps hybrid batteries charged, allowing hybrid vehicles to operate in their electric-only mode more often. This is why hybrid vehicles tend to have higher fuel economy estimates for city driving than for highway driving, where the electric motor seldom gets used.
On EVs and PHEVs, regen braking can recharge the batteries only by small amounts; the vehicles still need to be plugged into charging stations to fully recharge the batteries.