This isn't a car battery

Duracell 660 with a sealed lead acid agm battery I found in the army dumpster Viking 15amp tender(charge battery) Car jump start box(trick the tender to charge) Old multimeter(test voltage) GPD win max with 100w charger(power draw) Why... because I wanted to revive the battery from the box and this was the only way I could get the tender to charge the battery reading 6v currently and was 3.5v when I did find it, goal was to not have to buy a new battery because it is expensive. Thoughts comments concerns?