I wouldn't be in a hurry to sling a battery away before getting a meter on it and checking the voltage readings.
BTW the 13.6-14.2V referred to above is with the engine running at a decent lick, not at idle. The 12.5V is more like what you should see with the bikre idling.
Those white connectors to the solenoid are often the culprit. Orthe solenoid itself. If you clean up the connector, you can bridge across the two big HT terminals with a screwdriver or summat. If that starts the bike, then the solenoid needs replacing. They're cheaper than batteries.
All that's not to say that the battery isn't the most likely cause, though.
Or the rectifier....
