I had an interesting experience with a battery yesterday / today that is worth describing. The bike happened to be an F650 Dakar but that wasn't critical or even very important to the story. Yesterday I rode the bike 53 miles into Alpine, made 4 stops in town, and then rode home, dropped some meat off in the freezer and proceeded 25 miles down to Terlingua and delivered some stuff we had picked up in Alpine.
And then - the bike wouldn't start. The dash lights would barely light. After some milling around we used booster cables to start the bike but as soon as we disconnected the cables it quit running. Get trailer. Tow home.
Today we went hiking with relatives/visitors in Big Bend National Park and while we were gone I left the battery on a charger. Then when we got home the charger's volt meter and dash lights said fully (100%) charged. No start. Dim lights.
I pulled the battery out of the bike. Hooked jumper cables from my car to the bike and started it. The voltmeter I had on the car battery (car engine off) said the bike charging system was charging the battery. OK.
So sitting on the workbench the battery showed 12.7 volts. I hooked up a little 12 volt pump I have and the battery wouldn't power it. Huh?
So I hooked up my volt meter. Still 12.7 or 12.8 volts. So with the volt meter still hooked up I hooked up a little test light I have comprised of a little tail light bulb. The battery voltage immediately dropped to 4.8 volts. Some big load test!
My diagnosis is that the battery failed, suddenly and totally. Yet it still showed a standing no load voltage of 12.7 volts. But it had virtually no capacity to power anything. It wouldn't power a tail light bulb, let alone a starter.
The lesson is that you certainly need to test more than the no-load voltage if you think you have a battery problem.