Whenever the car is ON the dc-dc converter charges the 12v battery at 14.4v, at room ambient temperatures. Trouble is, as soon as the car turns OFF, its vampire load is significant (I measure 1.25A on my 2013MS85) and pulls down the 12v battery, easily below 12.5vdc. Without any load, the AGM battery would otherwise stabilize around 12.8v. The battery specs say that the ideal float voltage is 13.7v at room ambient. To avoid the car waking up and recharging the 12v battery, I keep a continuous float charger on my 12v battery whenever the car is in the garage. The vampire load used to be ~04A for a few years, but, seemingly after some software updates last year, it now runs a around 1.25A, despite Display Energy Save mode being ON and Always Connected being unchecked.
The old cars don't have a convenient connection for a 12v Tenderizer afaik since the cigar outlets die when car turns off. The cable chargers should have had a built-in Tenderizer from day one considering all the abuse that this poor lead-acid suffers. My 2012 MS currently has this same problem but totally kills the 12v battery.