My headlamp went out on me on my last night run, despite "new" (?) Batteries. Fortunately, I had a smaller handheld light with me, but it was much more difficult to run fast on those rocky trails. The right lighting really makes all the difference there. After something like 2 hours, that small light went a bit darker, too, as I had considered it just an extra, not an essential ( big mistake), and had not checked its batteries before I had left. I feared that I would be left out there without a light, but as John had advised, I still had a tiny emergency light with me, which weighs less than an ounce, and which would have been just enough light to allow me to walk out, or to change the batteries, if I only had packed any. Lessons learned! I'd always pack two sets of spare batteries for at least the headlamp for a long run, just because you never know how "new" they really are.