Anything other than a RED light on the back of your bike is a violation of the law in most (if not all) states. Remember that you are a VEHICLE, and must use the same rules.

Planet Bike Superflash uses AAA batteries. Used to be the gold standard. I prefer the USB rechargeables myself- I have a Cygolite Metro headlight, but haven't replaced the SuperFlash yet for the rear. I had one on each bike, but one had the lens pop off, so I only have one left. Not sure what to replace it with yet, but Cygo makes nice stuff.