Brake controllers are electronic devices that can control brakes remotely, deciding when braking begins, ends, and how quickly the brakes need to be applied. In towing situations, for instance, brake controllers can provide a means of coordinating the brakes on a trailer with the brakes on the vehicle doing the towing.
Regenerative braking is implemented in conjunction with anti-lock braking systems (ABS), so the regenerative braking controller is similar to an ABS controller, which monitors the rotational speed of the wheels and the difference in that speed from
one wheel to another. In vehicles that use these kinds of brakes, the brake controller not only monitors the speed of the wheels, but it can calculate how much torque -- rotational force -- is available to generate electricity to be fed back into the batteries. During the braking operation, the brake controller directs the electricity produced by the motor into the batteries or capacitors. It makes sure that an optimal amount of power is received by the batteries, but also ensures that the inflow of electricity isn't more than the batteries can handle.
The most important function of the brake controller, however, may be deciding whether the motor is currently capable of handling the force necessary for stopping the car. If it isn't, the brake controller turns the job over to the friction brakes, averting possible catastrophe. In vehicles that use these types of brakes, as much as any other piece of electronics on board a hybrid or electric car, the brake controller makes the entire regenerative braking process possible.