Is my AGM battery dead?

I have a 16 month old Big Crank AGM battery. I realized after my last ride that I left one of the lights on and it drained the battery. I tested it with a voltmeter and it was down to 1.6v The only charger I have a trickle Battery Tender Jr. I believe it charges at 750 mah so I know it could take 12-18 hours to charge a dead battery. I hooked up the trickle charger and the light turned solid red...meaning charging. I checked the voltage at 12 hours of charging and it was 4.5v. This morning after almost 24hours in the charger it showed 4.75v...so barely moved. Is that charger just not good enough to bring back that battery or is the battery shot? Seems odd that a AGM battery would be dead after one discharge. The last 2 batteries I've had from the same manufacturer all lasted over 5 years.