What wattage headlight to use?

Does anyone have any thoughts on what minimum wattage bulb would be appropriate to allow reasonable safety on a MB while going 20 MPH on a dark street?

I’m building a 12v electrical system for my MB and I’m trying to figure out what size bulb (in Watts) to use for the headlight. This seems to be the hardest part to work out ‘cause there’s lots of trade offs with respect to brightness, weight of battery, life of battery, the reflector used, etc.

I'll probably end up building the headlight since I haven't found anything commercially made that will meet my needs. I’ve got most other things figured out. For example, I want to use a 2 A-hr gel cell battery max to save weight. I only need about an hour of ride time, which puts me in the 10 to 20 Watt range for the bulb, but I have no idea what can I get away with.

I'd appreciate hearing from anyone who has been down this road before.

Thanks

Thanks for all the feedback. For experimenting with different wattage bulbs, I’ve built a headlight out of a 4” dia., Schedule 20 PVC cap, some scrap aluminum, and a 12VDC, 11 Watt landscaping flood light (photos attached). I like the look of a larger headlight – I think other motorists will see it better as well.

The flood light I’m experimenting with now makes a rectangular beam that is about 20 deg. wide by 3 deg. high (see photos). I don’t know if its brightness is adequate for night driving because it’s raining here and I haven’t been able to go for a ride to test it. But, I like the pattern of light it projects.

As far as batteries go, I’ve just about given up on using a lead-acid gel cell and will probably end up using nickel metal hydride batteries. This is because of the following:

Lead-Acid Batteries:
1. Relatively inexpensive, but heavy (so am I, so I want to keep the weight down).
2. Their discharge curve is such that the voltage keeps dropping with time, so the headlight will dim as well. This is annoying.
3. They don’t self-discharge as readily as other types of batteries, so chances are the battery will not be dead when I need to use it. I don’t plan to ride at night very often.
4. Discharging lead-acid batteries below about 10.5 volts is bad for them – it decreases their service life.

Nickel Metal Hydride Batteries:
1. They’re relatively expensive, but much lighter than lead-acid. Also, if properly maintained, they may be less expensive than lead-acid over the long run.
2. Like nickel cadmium batteries, their discharge curve is relatively flat until just moments before they die. In other words, their voltage is nearly constant until they die quickly with little notice.
3. I can have a battery pack made to just about any voltage in steps of 1.2 volts. If I what to “overdrive” the bulb for additional brightness, but shorter life, I may be able to do it as long as I can make or buy a proper charger for the battery.

Right now, I’m thinking of carrying two nickel metal hydride battery packs of about 4 A-hr each. That way, if one dies, I’ll be able to swap in a fresh battery pack. This way the “die with little warning syndrome” won’t be as much of a problem.

Anyway, that’s where I am today. I may change my mind again after I tinker some more with my new headlight.