I have a couple of NOCO battery tenders too.
Interestingly enough, mine won't charge a dead battery. I left the door ajar on our Chevy Tracker for 3 days, which killed our fairly new Crappy Tire battery. I hooked up my battery tender and left it on for 24 hours, thinking it would bring the battery back to where I could start the car.
Didn't happen.
I ended up using my little emergency battery pack booster. The Tracker fired right up and I took the car for a long drive on the highway to charge the battery.
When I got back home I hooked the battery tender back up (battery showed 25%) and the next day the battery was back to fully charged.