"I'm curious ... is your particular AGM battery type specified to be floated at around 13.5-13.8 volts (at 77 degrees F)?"
Phil, here are the specs for the AGM I got. I use a different charger that will do 14.6v or so if it needs recharging. I float it on the 7445 at 13.7ish.
http://www.hespv.ca/hesproductspecs/HES/Stark_Battery_TD_ENG.pdfThe battery is made in China. It seems to be the same as the one sold by UPG in Texas. (121 is the AH at the 100hr rate which seems to be where its name came from)
https://www.amazon.com/Universal-UB121000-45978-100AH-Cycle-Battery/dp/B00S1RT58C/ref=pd_lpo_263_bs_img_2?ie=UTF8&psc=1&refRID=789CYR3NBFDNM49NW7AG