EV Engineering & Infrastructure

How brake-by-wire is changing regenerative braking control and validation

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Regenerative braking gives an electric vehicle (EV) another way to slow down while returning energy to the battery. In some EVs, regenerative braking already handles most routine braking events, leaving the friction brakes to step in when more deceleration is needed or regeneration is limited.

However, how much regenerative braking is available changes during a drive. That leaves the brake controller with the harder part of the problem, maintaining the deceleration the driver expects when regenerative torque falls away.

Brake-by-wire gives the control system more authority over how regenerative and friction braking are combined during deceleration.

Brake-by-wire is giving engineers more direct control over that process. Instead of relying on a conventional mechanical connection between the pedal and brake actuation, these systems read the braking request electronically and decide how the requested deceleration gets produced.

The pedal hardware is only one part of that shift. More of the braking decision now sits in the control system, which turns the handoff between regenerative and friction braking into something engineers have to calibrate and then prove.

Moving brake blending further into software

Brake blending determines how much of a braking request comes from regeneration and how much comes from the friction brakes. The traction motor produces negative torque during deceleration and returns energy through the inverter to the battery.

The brake controller still has to deliver the requested vehicle deceleration. The amount available from regeneration changes with operating conditions, so friction braking has to make up the difference whenever regeneration falls short.

Brake-by-wire gives engineers more flexibility in managing that allocation because pedal input is measured electronically rather than transmitted through a hydraulic path. Bosch uses an electronic brake pedal to send the driver’s braking request to two independent hydraulic actuators, coordinating electrical and hydraulic braking.

Bosch has also used its iBooster electromechanical brake booster to coordinate hydraulic braking with recuperation in hybrid and electric vehicles.

Bosch’s iBooster uses electromechanical actuation to build hydraulic brake pressure in hybrid and electric vehicles. (Image: Bosch)

Other suppliers are moving further toward electromechanical actuation at the wheels. Brembo’s Sensify system entered series production in May 2026, an architecture the company describes as fluid-free, with braking control distributed at the wheel.

Continental has taken a middle route, a semi-dry architecture that uses electromechanical brakes at the rear axle while retaining hydraulic brakes at the front. That arrangement lets engineers introduce electronic actuation without redesigning the whole braking system at once.

In brake-by-wire systems, pedal feel becomes a calibration problem. The driver still expects braking force to build predictably as pedal input increases, so engineers characterize the relationship between pedal travel and requested deceleration, then hold that response steady as the contribution from regenerative braking rises and falls.

Where regenerative torque runs out

Regenerative braking is limited by how much energy the powertrain can accept at that instant. The battery often sets the practical limit, and its allowable charging power changes with state of charge and temperature.

A battery near its upper state-of-charge limit has less room to accept recovered energy. Cold cells cut the regenerative power available during braking as well, which is why the first few minutes of a winter drive can behave differently from the rest of it.

The strategy cannot assume the same amount of regenerative torque will be available for every similar pedal request. If battery conditions pull the allowable regenerative current down mid-event, motor braking drops while the driver’s requested deceleration holds. The controller then has to raise friction braking to fill the gap.

The vehicle has to decelerate the same way through that handoff, which is what makes blending a calibration problem and not an energy-recovery calculation.

Production vehicles show how often regenerative braking can carry the load. Porsche says roughly 90% of everyday braking in the Taycan is handled electrically, while Mercedes-Benz says the hydraulic braking system is not required in more than 99% of braking events in the GLC EV.

Recuperation reaches 400 kW in the Taycan and 300 kW in the GLC EV. Set against that, Porsche puts the braking capacity required for full braking from top speed in a fully loaded Taycan at more than 2 MW, which the electric powertrain cannot deliver alone. The remaining braking events are where the handoff becomes especially important, because friction braking has to enter without changing the deceleration the driver expects.

Electromechanical brake booster from the Porsche Taycan. The unit converts the driver’s electronic braking request into hydraulic pressure, with the control unit at left and the fluid reservoir at right. (Image: Porsche AG)

The traction motor and inverter also define how much regenerative torque can be produced at a given operating point. Available regenerative torque tapers off at low speed, and in the Taycan the hydraulic brakes take over below about 5 km/h, where the motors produce little braking power. The regenerative limit moves as the vehicle slows and operating conditions change.

Validating the transition from regenerative to friction braking

Ideally, the driver never notices the handoff. The transition between regenerative and friction braking has to be tested across the conditions that change how much regenerative torque is available.

Engineers can reproduce battery conditions in simulation and hardware-in-the-loop environments before moving into vehicle testing, exercising the control strategy while available regenerative power changes.

A typical case runs a steady braking event and clips the allowable regenerative torque partway through, the way a rising state of charge or a cold pack would. Pedal input stays constant while engineers watch measured deceleration through the transition. If friction braking fills the missing torque on time and at the right magnitude, the deceleration trace stays flat across the handoff. A step or a lag in that trace points at the blending calibration.

Physical testing still matters because the driver experiences the combined result, not the individual control commands. A correct torque allocation can produce a poor pedal response if the timing is wrong.

Fault behavior changes too. A system without a direct mechanical pedal connection needs another way to hold braking capability when part of the electronics fails. Bosch addresses this with separate electrical channels and two actuators capable of generating hydraulic pressure. Fully electromechanical architectures require their own redundancy strategy.

Brake-particle emissions add another reason to pay attention to when friction braking enters the blend. Euro 7, the European Union emissions regulation that applies to newly type-approved passenger cars from November 29, 2026, sets brake-particle limits for the first time, and battery-electric passenger cars face a lower PM10 limit than other powertrain categories. North American programs aren’t governed by it, though vehicles built on global platforms are, which gives the friction-blend threshold a type-approval consequence.

The motor still turns vehicle motion back into electrical energy during deceleration. Brake-by-wire gives the controller more authority over that handoff. The engineering challenge is making sure the vehicle responds the same way even when the amount of available regeneration changes.

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