A motor when physically turned produces power in the same way as an alternator, so adding an alternater just adds %loss through heat and added weight. Also in a gas car, the engine always runs like a generator to turn the shaft in an alternater. EVs already recapture generated electricity through rolling and braking. But there is still %loss through heat, light, computing power, friction. So you drive up a mountain and then coast down, you won't regain all the power, but you do get some.

As time goes on, better wiring, battery and even an addition of a single cylinder engine can be used to create and store power for the batteries to drive the motor. Innovation takes time, but tomorrow will be better than today. Even in recycling of batteries :)

Electric cars don’t have alternators.

In regular internal combustion engine cars, the alternator and the engine work to convert chemical energy from the fuel into potential electric energy for the auxiliary systems. Electric cars already have tons of this potential electric energy just sitting around in their battery arrays.

Regenerative braking basically is doing that.

The probelm is the laws of thermodynamics. To successfully charge the battery using the kinetic energy of the car would make an EV doing that into a perpetual motion machine of the first kind. You’d be converting KE back to PE, to use to convert back to KE, etc ad infinitum.

In reality, you actually could “trickle charge” the battery using an alternator or something similar to draw a small amount of the energy, but due to the thermal efficiency of such a device not being 100%, as well as inefficiencies (energy losses) in the batteries, controller, motor, and wheel bearings, you end up losing energy compared to just using it to drive the car.

Say you feed 500W into this alternator, which is 99.9% efficient (that in itself would be amazing). Your controller is 99.9% efficient. Your battery’s charge acceptance is 99.9% efficient. The battery discharge is also 99.9% efficient. Of that 500W in, your battery is able to put 498W of it back into the drivetrain again — which would be remarkably efficient — but there’s an obvious problem…you’ve lost 2W to inefficiency. You could’ve just not drawn that 500W, and had 500W going through the drivetrain to start with.

Taking the car out of the picture, it’s trying to charge a battery by having it drive a motor to drive a generator to charge the battery powering the system. The end result is less energy coming back i to the battery than if you’d just used the energy in the battery without running it through anything.

Even more simply it’s like trying to charge a battery with itself.

Regenerative braking works to convert the KE to PE at a time when you’d be wasting that KE anyway — brakes work by converting KE to heat, which is lost forever. Compared to that, all the inefficiency of the system in trying to recover some of that KE is still a massive improvement.

An alternator is something that you spin and out comes electricity. It takes effort/work to spin the alternator directly proportional to the amount of electricity you get out. Let's say some theoretical car where everything in the car was perfectly efficient had a motor and an alternator (electric cars don't actually need an alternator). The motor outputs 10kW of power to keep your car cruising at 50MPH (overcoming wind resistance and rolling resistance of the tyres). You switch on the alternator, it draws 1kW of power from the motor, which you then feed back to the battery. So the motor now draws 11kW and you charge the battery at 1kW. That 1kW hasn't gained you anything, the system is equivalent to running the motor at 10kW. In reality because of inefficiencies, this would waste a load of energy.