Hopefully it's not a steady red light for long, it should turn a steady green within a few hours, to indicate a full charge. If your Tender stays red, you've still got a possible battery issue, or, perhaps, the Battery Tender itself is damaged.
I had a Tender go bad once after a year or so, called Deltran up, and they sent me a new one, no red tape, free. Pleasant on the phone, no questions beyond how it was behaving, and they didn't even ask for the dead one back.