fairlo

Hello, I am very new to biking and have have just bought my first bike a 2008 Yamaha YBR125. Twice now it has let me down with a flat battery giving code 43 and refusing to start via electronic start. Changing the battery over night fixes the issue. So far I have replaced the battery with a new one but the following symptoms still occur and I am hoping someone might be able to help so I don't just keep buying new parts. With the ignition off there is 12.3v across the battery terminals. Running the bike on idle and the voltage starts to drop down to 12.2v then 12.1v and after about 10 mins it's down to 11.8v. revving the bike makes no difference to the voltage at the battery terminals. I am guessing this is why the battery keeps going flat as if I understand correctly the bike while running is using more voltage than its giving to the battery and therefore the battery gets flat and the bike stops running. Does anyone have an experience or ideas where I should go next to try an resolve the issue. Thanks in advance for the help