The bigger the battery the heavier, bulkier and more expensive your light will be, but the longer you can run it at a higher output. Obviously, if you’re planning to ride all night long on technical trails then you’ll need some serious cells to keep you going. Just thrashing around the local woods for an hour with a quick limp home if the warning light comes on, then an hour at full gas might be enough. Remember that battery life reduces with age though and cold nights really kill run times so buy a bit more than minimum just to be safe. Again be cynical about claimed run times and read proper user reviews like ours, not just the packaging.