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"15.5V for 3 hours is an AGM's equalization parameter. " I don't know what LifeLine quotes but the AGMs I have from other makers will start to cook and boil off electrolyte WAY BEFORE 15.5V. If you can get them up that high, you will definitely harm them by boiling off electrolyte, even if you don't melt anything internal.
Since there are several different technologies and chemistries being used, and an AGM maker can set the blow-off valve to different pressures, I'd second the recommendation to never exceed the manufacturer's ratings.