fog lights

I'm considering doing a dual headlight mod, or adding auxiliary fog lights. I've read that if lighting has too much draw then the battery will not charge properly. Does anyone know the maximum draw that can be used without impacting battery charging? Is it wattage, amperage or something else...

How much the dual headlight mods improve the light coverage? I'm noticing that the low beam is pretty poor out to the sides and I was considering adding some driving/cornering lights to the bike. Has anyone done this? If I got some small ones it looks like I could mount them on the underside...