If a long battery life is your aim why not get a Shimano hub dynamo built into your front wheel and get a Busch-Müller IQ-X light. It's among the most powerful one you can get that runs off dynamo, no battery fuss. Also works as a parking light for around 10 minutes or more (uses internal capacitor I guess). I really love mine since I don't like messing with batteries (bad for the nature too!).