Interstate Batteries sells a 12v, 5.5ah battery (part # SLA0093) that fits nicely in an old-style aluminum waterbottle cage. I wrapped the battery with an old inner tube to give it some padding and make it a tighter fit, and ran a velcro cinch strap to make sure it stays put. The setup's not waterproof, but I'm not using it for commuting, so getting caught in the rain after dark shouldn't be a problem.

Wired it up to a Malibu model CL507L housing with an MR16 20 watt 12 deg spot bulb (Litetronics L-3817) and took it out to the local MUP for a test ride. Shadowed sections that I'd had to tiptoe through before were no problem. The 12 deg spot bulb still threw enough light to the sides to see a skunk running along about 15-20 feet off of the path (sprint time ). Had to jump on the brakes to avoid a 'possum which ran out in front of me. I probably wouldn't have seen the dumb s.o.b. with my old light. I certainly wouldn't have seen the skunk.

The Malibu housing didn't have adequate support for the bulb, but some Reynolds Wrap folded up to make a shim under the front edge of the bulb's reflector fixed that problem. The housing didn't even get warm on the 20-odd minute ride.

Haven't tested the run time, but it ought to be good for at least a couple of hours. That's plenty for my purposes.

Parts used:
Battery - $26.99
Charger - 12.99
Housing - 12.96 @ Lowes
Bulb - 4.62
Hanger - .69
Strap - 1.50
Wire & connectors - 6.00
Total - $64.75 + tax

Being able to see at night - PRICELESS!!!!