All gasoline vehicles have a generator (the alternator) that recharges the starter battery. The thinking must be that electric cars must be able to do the same. The problem is that a generator attached to the wheels slows the car down, converting the vehicle's inertia into electricity. That's how Regenerative Braking works. The electric motor is engaged with the wheels in a mode that uses the motor as a generator, diverting some of the inertia from the vehicle, slowing the vehicle down, while at the same time generating some electricity that's funneled into the battery pack.
Enough energy is converted through regenerative braking that, for some cars, the car can be stopped solely with regen, and not require the mechanical brakes. In the right conditions, one can start at the top of a mountain, drive 20 miles downhill and gain 4 miles of range.
There's nothing magical about this, because it works within the parameters developed by Physics researchers over the last few hundred years. The Law of Conservation of Energy applies to electric vehicles. A generator attached to the wheels diverts the inertia energy, causing a decrease in inertia, resulting in lower speed, while converting inertia into electricity. A little wind turbine attached to the car degrades its aerodynamics making the car less efficient, and also diverts inertia to generate electricity with the effects already stated.
If your energy conversion device were 100% efficient you could divert the entire inertia to generate electricity, and in the meantime the car would stop moving. That result isn't exactly what's desired, and the energy gained that way is only enough to get the vehicle back to speed, not to move it for a significant distance.
Another description of this problem is :- Perpetual Motion. According to Physics it's impossible to create a Perpetual Motion device - that is, a device that's active indefinitely.
But what about energy conversion that doesn't interfere with inertia?
What about solar panels? The problem is getting enough surface area on a car to give a significant amount of power, much less to drive it at highway speed. It's difficult or nigh-on-impossible to ensure the panels are optimally oriented to the sun while parking, much less driving, to capture energy. Even if there were enough surface area, solar panels interferes with the aerodynamics causing the vehicle to require more power to do the same thing. In other words, solar panels are highly unlikely to give benefit unless you purpose-design the vehicle for the solar panels.
Most solar-electric cars are special-purpose vehicles for solar races, and completely wrong for daily driving. Balancing between utility as a regular passenger vehicle while incorporating a large flat area for solar panels while still having decent aerodynamics is not impossible. A student team in the Netherlands came up with such a vehicle that isn't too bad. The constraints mean you'll be charging the car while it's parked, and unlikely to charge while it's moving. How many parking lots let you park in the correct orientation to capture sunlight? What if you need to park indoors?