Not really. I have a Nashbar 5 LED light. It's a backup to my HID, so I don't use it all the time, but I run it on NiMH rechargables and certainly used it down to pretty cold temps last year (at least 0*F - I rode at -25*F but I couldn't tell you for sure that I used the LED then).
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Work: the 8 hours that separates bike rides.