This is for anyone present and future who might find it useful. I've been using a pair of Zefal lights from Walmart. Not expensive, provides a nice bath of light that's adjustable, though I always use them in their widest setting.
These run off 18650 rechargeables - the light comes with a 1200 mAh battery that works okay. I wanted to get something beefier so initially got Ultrafire batteries off Ebay, labeled as 5800 mAh. Clearly that 5800 mAh number is pure fantasy. I haven't tested them to see what they actually are but the lifespan on them isn't any better than the 1200's that came with the bike. Maybe not even as good. Mine say "Made In China" under the rest of it. The ones pictured don't.
Then I got some Nitecore NL189 3400 mAh batteries. Night and day difference in longevity. If you're looking for this kind of battery, these are the ones to get. Somewhat pricey but they're really robust. Also got a Nitecore smart charger.
