If the headlamp gives out, it's game over. In autumn and winter, 12 hours of darkness and then some.
A backup light just has to work, and preferably it should weigh nothing. For a primary lamp, there are a couple of hundred models to choose from. Many runners like the Lumonite Compass or the Mila Vega.
But what can you do about the backup light, the one that has to be there but most likely never has to be used?
Strange that it should be so difficult to do the simple thing. Nitecore has managed to get all the critical features into a small NU25 package that weighs 50 grams including a headband with seven hours of light for running! I'm using the NU25 2017 model - it has been revised with a USB-C charging plug.
Rechargeable with micro-USB and can be powered from a powerbank if needed. Four white light modes - low, medium, high and turbo. Two red light modes. Rain and waterproof 1 meter. Lockable so the light won't turn on accidentally. Battery level indicator.
In other words, everything you could wish for. In maximum Turbo mode it pushes out 360 lumens with a battery life of 80 minutes. Medium gives 38 lumens and is great for trail running for up to 7 hours.