if you don't need to see where you are going and only need a "to be seen light" then consider rechargeable AAs in something like a Planet Bike headlight

http://www.campingworld.com/shopping...FYHc4Aod22lPyw

they also make a rear light which is also important that takes AAAs

I like generic style batteries like AA and AAA because they are easy to find and you can carry a spare set of non rechargeables in your bag just in case.