house bulbs use a hot filament. during the 120th of a second they're off, the filament keeps lighting because of the stored heat. Led's don't use heat, and unless you have a good capacitor, you can still see the flicker or a dimming effect up to about 2,500-3,000 rpms. Even if you don't see the flicker, it will significantly dim the led's, by up to 50% near idle, compared to using a capacitor and/or full wave rectifier. Half wave is fine, but use a diode and large capacitor to smooth the current out, as this will also reduce any high voltage spikes from the alternator that would otherwise damage the leds.