I agree with Lee. I just put 2 AGM batteries in my 08 G8 Gt thinking it was going to be better and after some research it seems that older cars don't know how to deal with the AGM format when replaced being that the alternators aren't programmed to deal with AGM batteries.
I started seeing high voltages when starting the car (15.6v) then after driving about 6 miles, the voltages would drop to 14.8v (normal for my car), then after 15 miles it would drop to 13.2v (definitely not normal).
Got both batteries tested and they both show they're bad. I figure the initial charging of 15.6v (which I've never seen in my car before the install of this battery) is killing the unit.
Going back to Autozone to switch back to a regular battery when I get off work.
With that said, let's not push everyone to install AGM batteries in their cars if the car wasn't designed to use it.
The following paragraph from this site helped me draw this conclusion:
"While the installation of the battery may be the same for the two battery styles, some vehicles require an extra step to tell the vehicle that the battery has been replaced. Newer vehicles have a Battery Sensor Module or similar systems. These systems require recalibration with a scan tool if the battery is replaced. If the system is not recalibrated, the alternator might overcharge the new battery and cause the battery to fail soon after replacement."