Torque Shop

What is "regenerative braking" in the context of electric and hybrid vehicles? It sounds like a very complex type of braking system and I am told that with EVs especially, the brake pads last much longer than those in petrol or diesel vehicles. Why is this so?

The electric motor used in hybrid and electric cars can also function as an alternator to produce electricity. This feature reverses electrical energy flow from the motor to the battery. Even MRT trains use this to charge the on-board batteries.

For this to happen, it is necessary to provide some mechanical input into the motor for it to turn - just as electrical energy is converted to mechanical energy when the battery supplies power to spin the motor.

When a car is at cruising speed, it possesses kinetic energy. Bringing the car's speed back down to zero requires braking, which in a conventional system involves brake pads applying friction to discs attached to the wheels. The resulting heat is dissipated (and wasted).

In hybrid and electric cars, once the accelerator pedal is released, the vehicle's control system engages the alternator function.

This means the drivetrain now spins the motor to produce electricity. This puts a load on the drivetrain, creating a similar effect as engine braking to slow the car down. The amount of "engine braking" can be varied in many electric vehicles.

When the brake pedal is applied, up to 70 per cent of the retardation can come from this conversion. This means the brakes themselves have less work to do.

This is why brake pads in a hybrid can last at least twice as long as pads in a pure fossil fuel car, while those in an EV can last up to five times as long.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on October 24, 2020, with the headline 'Torque Shop'. Print Edition | Subscribe