I have a Drag Specialties Battery Tender Junior, which I plug in whenever the bike sits at home. The battery is always fully charged when I begin a ride, but upon returning, when I plug in the tender, the red light comes on (less than 80%), after fifteen minutes or so, the light starts blinking green (80% charged) and it's fully charged within thirty minutes.
My question is this: does this indicate my charging system is less than optimal?
Test the voltage on the battery with a volt meter and load tester. That will rule out the battery going bad. Then with the bike running, take a voltage reading from idle to about 2-3k rpm. This will at least let you know for sure.
All four of my battery tenders do that. If mine is green and fully charged and I turn on the bike radio, it will go to red while the radio is on, then stay red a short while after turning the radio off.