If you have digital multimeter, you can disconnect the stator from the rectifier/regulator and measure each coil resistance, plus each coils to ground.
Each coil should measure around .2 to .5 ohms, and open (infinite resistance) from each coil to ground.
Next you can measure the leakage current from the battery to ground.
It shouldn't be over .1 mA.
If it does show a value over this, then you have a short somewhere.
If that's ok, then the next item is the rectifier/regulator.
Start the bike and measure the charging voltage. It should be 13.5V to 15V.
It should never be over 15V and if under 13V, then it's not charging the battery properly.
A fully charged battery should be around 13V, and needs charging if it's below 12V.
If the battery is at or less than 11V it's essentially dead and needs charging.
Every battery has a life span, and a battery can get to a state where it won't take or hold a charge.
I've had batteries that discharged internally, and would be stone cold dead the next morning. Or can develop a bad cell, so while the rest of the cells are fully charged, the overall voltage would still only be 10 or 11V.