Originally Posted by
cyccommute
Removing the flashlight from holder isn't a real problem. Removing the back of the flashlight is where things can start to go wrong. It's a small threaded part that is usually black. You now have two parts that you have to keep track of. You shake out the battery. You now have 3 parts that you have to keep track of. You get out your new battery which gives you 4 parts to keep track of. Damn! You fumble the end cap. Now you only have 3 parts to keep track of but one of them is somewhat hypercritical.
Every part of your light setup is "hypercritical" (unless you have a spare light, of course. As you probably should.) The only part you can afford to lose in either situation is your old battery.
I think you've exaggerated the difficulty of changing the battery in a single cell flashlight by a large margin. Certainly, in the real world, it's not difficult at all, even in relative darkness. Maybe it gets a bit worse if you're somewhere cold and are wearing gloves, but you'd have similar problems with your battery pack too.
(It does get a good deal worse if you have one of those lights with a holder inside the light and that holder holds 3xAAA batteries. I'm not a fan of such lights.)
(And if you really are in pitch darkness, carry the flashlight behind your bike so it's illuminated by your taillight and swap the batteries there.)