Restoring the vacuum system to your power brakes in an electric car conversion

I am planning to build an electric vehicle and I would like to have regenerative brakes to increase its travel range.

Some people say that a motor and a generator are basically the same thing.

But how come a car alternator doesn't turn on its own even when it's connected to a car battery?

I plan to power the electric motor with four 12volt batteries connected in series to make a total of 48volts.

What should I do to be able to instantly convert the motor into a generator (to convert kinetic into electrical energy and charge
the batteries) as the vehicle is in motion?

is there a need for some complicated circuit boards to do this?

if I were able to switch the positive and negative connections to the motor as the vehicle is moving then would it charge the batteries?