Looking for an educated guess
The educated don't guess, the educated test and verify.
Got a multi meter? You need one.
Charge the battery at no more than 10% of it's capacity (less than 2 amps) overnight, remove from charger, let it sit for 2 hours and test the voltage. If it is less than 12.7v the battery has issues.
Disconnect the stator from the regulator (the plug with 3 wires).
pull the ignition fuse.
Put the battery back in the bike and hook it up. What's the voltage?
Turn on the bike. What is the voltage
Hold the starter button down. The voltage should stay 10v or above for 10 seconds while the starter turns. (That's a valid "Load test")
put the ignition fuse back in and start the bike.
Test the 3 stator leads for AC voltage, 3 wires/phases need three tests (a&b, b&c, c&a). At idle you need at least 35 v AC, you want over 65-70 v AC at 3000 - 3500 rpm on a modern bike.
If you see those voltages the stator is good.
Plug the regulator back in.
What the voltage across the battery at idle, you NEED 13.5v DC, you want >14v DC
At 3000-3500 you should see >14.2v DC, no more than 15v
If you see those voltages your regulator is good.
Check across the battery for AC voltage.
if you see more than a couple of volts AC, you have a rectifier problem. (A COUPLE of volts... a GOOD rectifier will show 1-1.5 v AC, a ****** rectifier will show 4-5... it's a judgement call. The more AC voltage at the battery the shorter your battery life. Lead acid batteries don't like AC voltage)
These tests take about 15-20 minutes, works on any 12v system