If that's an automotive light you refer to, it will need 12vdc to drive it, and probably a high Amp-hr rating at that. This translates to higher cost and weight. A NiMH or Li battery might cost $80-120 depending on A-hr rating. Alternately you could get a small SLA (lead acid, like car) for $20 but it would weigh 3-7 lbs. Also. you'd need a charger, and each of these battery types require it's own special charger.
Halogen MR-16 lamps used to be popular for DIY bike lights. Most folks moved on to LED a year or more ago due to better light for the same battery requirement (i.e. cost and weight).
There are many bike lights available which you could buy (which typically include light, mount, battery and charger), that would probably work as well as the fog light you're describing. I recommend a fairly new one that uses Cree or Seoul Semiconductor (SSC) led. These are high-output, high efficiency leds, and they require 3-4vdc to operate, so the battery is less costly and lighter.
Go over to Electronics,L&G forum and read. Also for more sophisticated discussion of this topic try
I have been thinking about putting on a fog light with led fog light bulb in it and putting on a led head light kit on my touring bike.Wal-Mart has a small driving light kit for twenty bucks cheap all chrome witch I could redo with olive paint after sanding it a little.I have all kinds of tools for this taking chrome off.Has any here ever use a fog light on there bike for touring.Around here we have a lot of fog where I live in the early mornings some time I start to ride at 4am early I know but its so much fun to be out on the road before a lot of people are driving to work and you get to see the sun come up too.My wife and I are thinking about moving to Bangor Maine or maybe to Cedar Hill Mo.Adventure Cycling Association has lot of maps I could use if we move to Mo.For on is the transamerica trail for start.So I think a fog light my be in order.What do we all think on this???Any input is welcome.
Forget the automotive fog light, fog lights are simply lights that are mounted close to the ground, so the light doesn't reflect back in your face. Generally on a bicycle, with a bar mounted light, this should not be a huge issue anyway, however you can always simply take an ordinary bicycle light, mount it as low as you can on the fork, as far forward as you can, if you have a front rack, you can often mount it on the rack, use a long strip of old inner tube to fill the clamp.
I agree that you'd be better off with a bike light adapted to be mounted as low as possible. That would make it suitable for fog and also do a good job of identifying any irregularities in the road surface ahead by creating shadows. Supplement that with a higher mounted light for a better view in the distance and also to show signs and warning reflectors. The combination of the two lights gives you a more versatile system and also provides redundancy if any problem develops in one of the lights.
Lights intended for automotive use are frequently rather inefficient since the car has lots of electrical power available.