Yamaha Cygnus 2008 battery/electrical issue

Hi all, im try to diagnose a the following issue.

About a month ago, I started my scooter to ride home from work a 20min ride all fine apart v low on fuel.
Practically home my scooter stutter and jerked and seem to lose power for about 5 seconds but did not cut out, I continued to ride home no problems.
Once home I turned the scooter off and tried to start it again out of curiosity, but it wouldn't turn over properly as if the battery was flat. Strange as started it fine 20 odd mins ago.

So charged the battery and next was fine and has been until two days ago.
Used the bike day before no problems, next morning battery flat again!
So I charged it last night and put it on the bike this am and fired up fine.

Okay some info:
the first time it happend and I charged the battery end voltage was 14. something.
This time end charge voltage was only 12.8.

When connected to scooter and the scooter is running the voltage 13.81 ish, but when revving the voltage does no increase.

So I think some power must be going to the battery to increase the voltage from 12.8. to 13.81 when scooter is running?

So whats your thought?

Thanks in advance

 

Turn the lights on and off with it running and see how much the voltage drops.
Also see if the battery is dropping charge on its own. Measure the voltage in the evening then see if it has dropped the next day.

 

Hi, yes I have been checking voltage. Yesterday the battery read 12.64v and today 12.61v