[Secret Squirrel]

Just because the Battery tender has a green light means nothing. It can't detect weak battery cells. I have had batteries last eight or more years on a Deltron Battery tender. Every battery that I have had fail after many years had a bad cell. The light was green on the charger. Batteries don't last forever no matter how good you treat them.

The resting voltage can tell you something but its really not as important as when the battery is under cranking load. Connect the battery to a voltmeter. Read the voltage while you push the starter button. If the reading drops below 10v the battery is bad. It has a bad cell or cells. That's also why it's not taking a full charge. Even if the light is green. The charger doesn't know any better.

 

[Secret Squirrel]

I've had batteries with a bad cell still have a fairly good resting voltage. Over 12.3volts off the tender. But when a load was put on the battery I would notice a slightly slower crank. When I load tested the battery it was dipping down lower than what's acceptable. That's why I don't like going by resting voltage alone. In my case this was in the very early stage of a cell going bad, or a plate shift, or a plate sulfating. When a cell gets real bad and the bike shows bad signs like the OP is reporting it should show up in the resting voltage. You're right about that. In the very early stages it doesn't always show up in the resting voltage.