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I'll see your puny cells and raise you the 18650 cell.
Hope District+ has two of those, I think. It lasts so long you forget the last time you charged it and goes from sensible to unsafely bright. Also has an interchangeable battery pack with some of their front lights (e.g. the now-discontinued Vision 2) which you can use with a splitter cable to power both from the same pack.
rhowe-
Wow, that's a hell of a tail light.
http://www.chainreactioncycles.com/hope-district-rear-light/rp-prod88101
The customer reviews are all fairly gushing, and the magazine reviews only balk at the price and lack of battery indicator (but yet it lasts over 11 hours on full burn, a month on low burn).
The choice of front lights nowadays is great. I still love my Lumicycle and it's 3,500 lumen high burn, with a 1,100 lumen standard burn.
But... tail light choice is shitty. The companies that are great on front lights tend to use splitters to power high quality casing but low quality rear lights. Outside of them are all the cheap and ineffective lights, the USB lights, etc.
It's good to see a serious tail light. I've always thought that dim London winter streets with a high number of distracting lights means that there is a need for brighter tail lights to be sure you're seen.
Velocio
I'll see your puny cells and raise you the 18650 cell.