Hello!
You're in bi-pin territory here, I don't think there are any PR based bulbs that tolerate ~20+V So assuming a bi-pin solution of sorts.... here's some things I had in mind..
here's what I had in mind:
http://www.candlepowerforums.com/vb/showthread.php?t=203785
This particular mod is based on a custom switch I made for myself. It has a copper contact on the base, and a high current Judco reverse "clickie" installed, this may not be an option for everyone as it involves a bit of finesse on the drill press
(read: I really need to get a mill someday, oh- and a lathe, lol)...
120 watts and probably 3000+ torch lumens on fresh but rested cells, probably 15 minutes runtime.
You can also use a 20W bulb instead of a 50W and run around 2.5A and around 50-60W territory for ~45 minutes runtime ~1400 torch lumens.
unfortunately, you won't get 27V to any bulb on 18 eneloops, as soon as the load kicks in, the voltage will drop to around ~1.2V+/- depending on load and such. In my setup there I was actually quite amazed that I was getting over 21V to the pins at 5.7A draw from the cells.
I've found that most "long-life" 12V halogens (some rated 2000, some 4000, some 6000 hours, at these drive levels, it's hard to truly predict how they will behave so best thing is to just experiment with a few bulbs and see what you like)) really start to get pretty around the 20+V territory. They do like to insta-flash though so you kind of have to try a few brands and see how it works out. I've found that resting the 18 eneloops for about 2-3 hours before I pull the trigger has eliminated most of my premature failures on this setup.
Looking at LuxLuthers destructive testing, the closest thing to a "long-life 50W 12V halogen" in his testing is something like that Hikari JC-5043, at 21V, it's pushing a good 4800 bulb lumen. Looks to be very similar to the bulb I'm running based on measured current and such.
The 64458 is another option I think could be considered, with it's massive current draw, i think it would sag a set of eneloops down into that ~1-1.1V per cell area. Would need to rest the cells, but I think you could push ~6000 bulb lumens (+/- a few thousand, lol, hard to say)...
In all great honestly, I believe there are probably a lot of viable options out there, many can be bought at your local hardware store and then abused and replaced conveniently for a couple bucks a bulb.
Eric
PS: I've also been considering the idea of making a 21AA>6D adapter, it would require that the stock spring be cut down and recessed into the tailcap, but with high current cells, I was thinking it might be a viable solution to drive some of those "super-bulbs" like the 24V 250W monsters in a mag host without the need for boring. I might try to put something like that together some day
