A battery can be strong enough to start a healthy motor without being fully charged. It may be getting weak and can't take a full charge.
When I first got a BT, maybe 4 or so years ago, I remember reading to first plug into the bike then to plug into the wall outlet. And I have always done that. Glad to know I actually did something right for a change.To make the tender work right it has to be plugged into the bike first then the wall..
It does not matter been using tenders now over 20 yrs and always have unplugged the vehicles leaving the tenders plugged into 120v once return plug tenders back on vehicles. A few of our tenders are near 20 yrs old.To make the tender work right it has to be plugged into the bike first then the wall..