Voltage regulator strikes again. Looks like you are getting intermittent charging so the battery voltage is gradually diminishing.
This is very common, I think most owners seem to have had this problem at some time or another. Although since MMC repositioned the rectifier to the chassis there seem to have been less failures.
Swapping the Cycle Electric rectifier for the Harley one is the way to go. A lot more reliable but I have had one of those fail as well.
