How Does Regenerative Braking Work?

Learn How Hybrids & All-Electric Cars Create Their Own Electricity

Prius pancake motor/generators
Toyota Prius pancake motor/generator (M/G) set.


Hybrids and all-electric vehicles create their own power for battery recharging through a process known as regenerative braking (regen mode). We've explained what regenerative braking is and how the process works in general terms, but many folks are interested in the deeper nuts and bolts of electricity generation. They understand that in a hybrid or all-electric vehicle the word "regenerative," in terms of regenerative braking, means capturing the vehicle's momentum (kinetic energy) and turning it into electricity that recharges (regenerates) the onboard battery as the vehicle is slowing down and/or stopping. It is this charged battery that in turn powers the vehicle's electric traction motor. In an all-electric vehicle, this motor is the sole source of locomotion. In a hybrid, the motor works in partnership with an internal combustion engine. But that motor is not just a source of propulsion, it's also a generator.

Any permanent magnet motor can operate as either a motor or generator. In all-electrics and hybrids, they are more precisely called a motor/generator (M/G). But the technologically curious want to know more, and they'll often ask "How, and by what mechanism or process, is the electricity created?" It's a good question, so before we get started explaining how M/Gs and regenerative braking work in hybrids and electric vehicles, it is important to have basic knowledge about how electricity is generated and how a motor/generator functions. 

So How Does a Motor/Generator Work in an Electric or Hybrid Vehicle?

No matter the vehicle design, there must be a mechanical connection between the M/G and the drivetrain. In an all-electric vehicle, there could be an individual M/G at each wheel or a central M/G connected to the drivetrain through a gearbox. In a hybrid, the motor/generator could be an individual component that is driven by an accessory belt from the engine (much like an alternator on a conventional vehicle--this is how the GM BAS system works), it could be a pancake M/G that is bolted between the engine and transmission (this is the most common setup--the Prius, for example), or it could be multiple M/Gs mounted inside the transmission (this is how the two-modes work). In any case, the M/G has to be able to propel the vehicle as well as be driven by the vehicle in regen mode.

Propelling the Vehicle with the M/G

Most, if not all, hybrids and electrics use an electronic throttle control system. When the throttle pedal is pushed, a signal is sent to the onboard computer, which further activates a relay in the controller that will send battery current through an inverter/converter to the M/G causing the vehicle to move. The harder the pedal is pushed, the more current flows under the direction of a variable resistance controller and the faster the vehicle goes. In a hybrid, depending upon load, battery state-of-charge and the design of the hybrid drivetrain, a heavy throttle will also activate the internal combustion engine (ICE) for more power. Conversely, lifting slightly on the throttle will decrease current flow to the motor and the vehicle will slow down. Lifting further or completely off the throttle will cause the current to switch direction — moving the M/G from motor mode to generator mode — and begin the regenerative braking process.

Regenerative Braking: Slowing the Vehicle and Generating Electricity

This is really what the regen mode is all about. With the electronic throttle closed and the vehicle still moving, all of its kinetic energy can be captured to both slow the vehicle and recharge its battery. As the onboard computer signals the battery to stop sending electricity (via the controller relay) and start receiving it (through a charge controller), the M/G simultaneously stops receiving electricity for powering the vehicle and starts sending current back to the battery for charging.

Remember from our discussion on electromagnetism and motor/generator action: when an M/G is supplied with electricity it makes mechanical power, when it's supplied with mechanical power, it makes electricity. But how does generating electricity slow the vehicle? Friction. It's the enemy of motion. The armature of the M/G is slowed by the force of inducing current in the windings as it passes over the opposing poles of the magnets in the stator (it's constantly battling the push/pull of the opposing polarities). It is this magnetic friction that slowly saps the vehicle's kinetic energy and helps scrub off speed.