strange start behavior – 2004 Sportster Custom 1200

Hello together, I wanted to ask if someone here knows the phenomenon or to what extent it is “normal”. Since I don’t have much time to drive, my Sporty is standing most of the time in the garage. If I have not been on the road for a longer time in the summer (more than 2-3 weeks), then I have start difficulties. Not that the starter would turn longer; no, it turns the engine 2-3x and is then “dead”. Meanwhile, in my opinion, the lamp is normally bright (turns it off). unfortunately not). If I put the battery charger on it for an hour, the starter turns completely normal and persistent (one hour on the charger doesn’t make the difference, does it?). With a bridge cable from the car, the motor always starts directly. The battery is an IntAct and now 3 years old. Before that I had the original HD battery, which was just finished after the whole years. If I drive halfway regularly and at least all 2 weeks I don’t have any problems at all. Can the battery be broken again? Would you just connect a maintenance charger permanently? Right next to the stand of the mopped is a socket. Greetings + thanks, Joe.