So there I was in a gas station parking lot with darkness descending, and the bike would not start. It first just seemed like it would turn over without firing, but it very quickly became a spent battery. But how? I had just ridden about 15 miles and stopped to check air pressure. It had started fine and ran well ! Regardless, the machine died, and fortunately a police officer rolled in for a coffee break, and ran back to the station for his jump pack. The machine fired up right away, and I returned home without stopping.
I thought that the battery was not that old, but it was a BMW battery and may have been original. It had been on a trickle charger most of the time. Was that a faulty trickle charger? A bad battery? Why was it not charging while riding? In the end after sticking a meter on the battery while running, I decided that the battery was at fault. Bad trickle charger, plus bad alternator was unlikely, and a healthy 14v was present when tested.
The solution was a Shorai battery. It was more costly than a regular AGM solution, but lighter and supposedly better at resisting discharge. It matched the dimensions perfectly.