Right, well, if you're going to overvolt *that* much then, yes, you can greatly increase the lumens/watt efficiency (which is what I assume you're saying in talking about increasing the "watts of light" of a 20W bulb).

Even with drastic overvolting, though, I doubt you could really get 50 lumens per watt out of a halogen light. If you overvolt to 133%, which is what you propose, I don't think you would get a *real* increase of more than 266% in light output, which is what you would need to do to match the HID light efficiency. (I admit, though, that isn't certain given the information I have available to me. See the link below. But, even if we suppose that efficiency of overvolting continues undiminished beyond overvolting of 120%, you'd still only be in the ballpark of total lumens-watt compared to an HID lamp, not "blowing it away". See the link and see what I mean.)

But most manufacturer's specs for overvolting halogen give lumen-watt increases of only about 20% or so. In fact, I don't know how much more efficient the lights get beyond 20% overvoltage, exactly. According to the GE specs for overvolting within about 20% of the rated voltage, though, moving from 12v to 16v should more than double the light output, but cut the bulb life by over 97% (!)

See this (p. 8) (You need Acrobat reader):

http://web.media.mit.edu/~ladyada/ad...logenstech.pdf

There's something else to say about overvolting, though. It doesn't really give much advantage over HID, if it does, in total cost of use for a given light output. See what I mean.

Now, you say the overvolted 12v bulb will last about 20-30 hours, which is roughly consistent with these figures in the link applied to a 2500 hour bulb, though perhaps life dives down even more than the figures in the link indicate. I'm not sure, but we'll use your figures derived from real life.

If you overvolt to the extent you do, the cost of bulbs for your light system brings the price of using the halogen system much closer to HID. Sometimes it will still be significantly cheaper, sometimes more expensive, depending on which bulbs you get and how much they cost.

Now, maybe you can get 12v halogen bulbs for $3 a piece, but some of them are about $6 or so retail. Let' s just use $6 for now.

You said you can put together a bright halogen bike light system, with charger and all the rest, for about $200. We'll compare that to the commercial L&M Metal Hydride ARC at, say, $360.

Let's just assume the battery voltages are adjusted so that brightness and run times are equal. We're not comparing battery drain here.

Now, L&M say their HID lamp will last 700 hours before replacement. You may say you don't believe this, but as I mentioned on another thread I've had my HID lamp for ~250 hours of use, and it's doing fine. But anyway, let's just be very conservative and say that the L&M lamp will give out at 400 hours, not 700 hours.

If you get 20 hours out of each overvolted halogen bulb, and they're $6 a pop, then, you've added $120 dollars to the price of your halogen system for 400 hours of use. That makes the halogen system $320 compared to $360 for the HID system for 400 hours of use.

If we consider the price of 700 hours of use of the halogen system, which is how long L&M say their HID lamps last, then that's an additional $210 dollars for the halogen system, which would make it *more* expensive than the HID system over the life of the HID lamp: $410 vs. $360.

If you get the bulbs for $3 each, though, and if you can get 30 hours out of them, instead of 20, then things work out better for the halogen. The cost of 400 hours of use would be $42, which would save you a little more than $100 compared to the HID system. 700 hours would be $72, which still makes the whole set up cheaper than the HID light.

Still, I haven't figured the cost of labor in putting together these halogen lights, which is unfair since one *could* make an HID light, too. I've also assumed that the price of halogen bulbs stays the same during what would normally be years of use. Then there's the hassle of replacing the halogen bulbs every few weeks.

More importantly, though, after working through this one would have to ask why not just get (or make) a more heavily powered halogen system with bulbs not so drastically overvolted. Yes, you'd add battery weight compared to the HID system and to the overvolted halogen system to keep the same brightness and run time. However, you'd save *money* on the bulbs for the same light output with the more soberly volted halogen system compared with the overvolted halogen system. Extra battery capacity is cheaper than buying a bunch of bulbs, after all. But now you're talking about the logic that leads to something like the high end halogen Jet Lites, as an alternative to HID. And much of what I was saying above applies to this kind of comparison.