My battery was dead a week ago so I charged it and installed a battery tender jr. The battery tender showed that the battery was charged when I unplugged it today and tried to start it. When I turned the key it turned over very slow and almost acted like it wasn't going to start when all of a sudden it fired up. I shut if off and checked the voltage at the battery with a multi-meter it was 13v. When I started it again it did the same thing. Any ideas what might be causing it to crank so slow?