This article is from the Misc Bicycles FAQs, by various authors.
From: arnie@hp-lsd.COS.HP.COM (Arnie Berger)
> With this setup, one can expect about 2 hours of light. I've had
>drivers dim their brights for me several times, and the field-of-view
>is really wide and deep, making night riding in rural areas or trails
>enjoyable. Keep a spare Cateye for emergencies; this can slide onto
>the existing mounts if necessary. Total cost for me ran about $75,
>including $35 for the 6AH battery.
DigiKey has a good selection of Gel-Cell batteries for sale via mail order.
They would be a good source for such a set-up. I built a similar light using
the bulb and reflector housing from a building emergency light. These are the
lights that go on, like in theatres, when there's a power failure. I used a
6V, 12Watt bulb that lasts for about 45 Minutes with 3 lead-acid "D" cells.
My commute is 20 minutes so I need to charge it daily. Works great. My wife
drove by me once and said it looked like a motorcycle headlight.