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Hi - using this to charge while parked is an excellent idea. Using it while driving is an inefficient one. There are power losses every time power changes form so driving to generate wind to generate electricity will waste more power than charging from your vehicle's battery system. You are right, in your later comment, that a smaller set of blades will cause less drag but, as a direct consequence, they will also generate less electricity.
LED as light source for an LCD display - yes, it will work.Straight LED display (e.g. organic LED, old 8 segment calculator LED display or the other technologies Betty P3 listed) it will not work because those displays do not use polarized light.