Battery Tender Issues

I recently purchased a Deltran Battery Tender plus and hooked it up over the weekend. I used the ring harness with the quick disconnect. According to the indicator lights on the tender it appeared to be working properly. I went to take the car for a drive this afternoon and I disconnected the quick connect and then when I went to start the car, something weird happened with the electrical. Initially when I turned the key, everything sounded normal and seemed to function properly. But then it all stopped and there was kind of a low hum and the needles on the speedo and tach were kind of jiggling. Anyway, I quickly turned the car off and disconnected the battery tender.

Was this maybe just a loose terminal at the battery? When I hooked the tender up, I was having trouble getting the nut out of the terminal so I connected the ring harness between the terminal and the nut (like where a washer would go). I wasn't exactly sure where the ring harness was supposed to be connected. Should it go in between the the terminal where it squeezes together? Does any of this make sense, lol??? Any advice is appreciated!

 

The quick disconnect cable was still connected to the battery but it was unplugged from the battery tender itself. I'm thinking it was just a loose connection on the battery. I do question if I even need the battery tender. The battery is at least 3 years old so I figured it couldn't hurt. And I try to get the car out as often as I can. I live in Indiana and, during the winter, there's usually at least one sunny, dry day per month where I can take it out for a drive...

 

I didn't measure the volts coming out of the battery (and now I have it all put back together). The next time I'm in there I'll have to check. The tender is a 12 volt tender with no other settings. From all the research I did, it seems to be one of the most popular and reliable ones. It's pretty basic and straightforward.