Regenerative braking does not convert thermal energy into electricity.
Whilst driving a vehicle with a regenerative braking system, the electric motor draws power from the battery to turn the wheels, creating the kinetic energy it needs to move. When the brakes are applied however, the process switches into reverse mode. Now the kinetic energy that was initially used to propel the vehicle, makes the wheels rotate the electric motor, turning it into a type of generator. Instead of consuming electricity, the motor/generator starts producing it, using the vehicle’s kinetic energy. The electrical energy is then stored in a high voltage battery, where it is used again to help propel the vehicle. Since regenerative braking converts the vehicle’s kinetic energy into electricity, it is also able to slow it too, in the same way hydraulics brake do with friction.