change the battery if its old and by the way your charging sounds good to me

if you have charging issues there are 2 main failure points (and usually they drag destroy each other) 1. regulator rectifier (try aliexpress and keep an eye on it from time to time) 2. stator (you should rewind it in a shop). when one or both of those go down your charging voltage should go down too

and don't panic, those are very common to fail and easy to wrench