The ones I linked to are only 10 watts, and based on the output of the larger 20 watt ones I bought, I think one should be enough, two would be more then plenty for a small (slow speed) atv. If I go 2 up front then a LED taillight would be the way to go out back. The plastic is gone off my sportsman so I could mount one front and center or one to each side. Also want to do a Zuki LT 80, there it looks like on the handlebar mount out front would be where to mount the light. The Zuk would maintain 14.6 volts reved up with the 20 watt light.
'With a faster rider a 20w would be ok, but for kids putting 1/2 the time a 10w would allow a cushion of surplus watts and still have plenty of light imo.
I agree on a good battery, short wiring via relay and power conservation are the name of the game here though. before led there was simply too few watts.
Now to find the time..
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