This morning I was out riding, and working on my pace. It was going well. Suddenly, my headlight was much dimmer. I must have forgotten to plug it into the charger after my previous ride.
Because I couldn't see as well as I needed to, I not only had to slow down; I felt obliged to cut my ride short for safety's sake. I see people out there when it's pitch dark with no lights; wearing dark clothing; and with minimal or no reflective material. Some are walkers/joggers, but some are on bicycles! They're hard enough to spot with my 500-lumen light going full force. Maybe 1000 lumens would do it!
So the headlight was plugged into the charger all day. If the same thing happens tomorrow morning, I'll know the problem is with the battery. To be on the safe side, I'm charging up the spare battery so I can take it with me when I ride.