Do you really need that much juice?
I've got one of these, and it lights things up very well. Very bright and runs a long time on four AA rechargeable batteries.
http://www.cateye.com/en/product_detail/345 Maybe not mountain bike material, but fine for most city and rural use. Quick releases from mount so also usable as a flashlight.
If rear visibility is a concern, this thing is unbeatable.
http://www.cateye.com/en/product_detail/280
Mounts on rack or bike and works well with racks as the switch is on the side.
If you do need to carry the behemoth, try these folks.
http://www.twofish.biz/bike.html I've used one of their older units on the handlepost of my Birdy with no interference with the fold.