I carry extra batteries for my blinkies and helmet light. The Niterider something or other USB charged headlight I bought four years ago is fantastic! I top it off every couple of months, or use it by working late then charging it. Compared to my old halogen setup with its need for a heavy 3-4 amp-hour 12 volt battery it is a miracle! No more rolling up to the house as the headlight is fading. I can cary my "phasor"-sized headlamp and auxiliary helmet lamps (plus replacement batteries) in my backpack and be ready to work late whenever the need be. And in the winter I just leave the helmet lights in place. (I transfer my headlight to whichever bike I'm riding).

As far as nomotor posting a reminder...that's cool, and yes, plan ahead so you don't get left in the dark.