Don't know the output of the SP90 alternator, but I know that fishslab runs lights on his youth quads without a problem. You shouldn't have a problem with a 30 or 35wt light. But I wouldn't run anymore than that

 

you could also ask on the kids quads and other atv's section and you might get a better response from them

 

I believe it is 12V AC for the Aeon built quads. This would include your Sportsman 90. A DC bulb still works just fine. I run a 35w head light and a 6w tail light on our Eton 90. A real easy test with a volt meter is to hook up a 55w fog light (or something) Hook the volt meter up to the battery. If the voltage drops when the light is on and the atv is at normal running rpm you need a smaller light. The stator should put out around 13v or more to charge the battery. If that voltage starts dropping and goes 12v or below you are sucking juice off the battery and not replacing it quick enough.

 

thanks fishslab! i'll try that.