I would be hard to convince that long-term 13.2 volt floating would be harmful unless temps drop to less than 0C. But I would be the first to agree that underhood battery temperatures in excess of 50C would negatively impact any battery maintained in excess of 13.8 volts. Because of 0% antimony factor the AGM can chemically tolerate a wider temperature and voltage range.
Because of a myriad of customer abuse profiles different manufacturers decide to minimize warranty claims in different ways. For the last ten to twelve years I have been dealing with customers whose batteries have lost capacity because of erroneous charge profiles when their voltage regulator "setting" was on AGM. There is rarely "floating" on a sailboat even though voltage regulators have such a phase.
Do you remember the era when voltage regulators forbade anything other than rote recharge profiles? No increased voltage permitted. And absorbed glass mat manufacturers screamed nothing but DO NOT ATTEMPT TO EQUALIZE THIS BATTERY! ??
Conditioning did not exist. Well, your's truly got on the telephone and fought my way through several layers of battery corporate "Application Engineers" and informed genuine engineers their batteries were losing capacity because of chronic undercharging. One of the odysseys was detailed on this forum with a spiral cell AGM battery manufacturer.
Isn't it a wonder that a dozen years ago I coined the term "conditioning charge" to differentiate controlled overcharge of an AGM battery from equalization of a flooded battery, and now in AGM format we have "conditioning" as a commonly used AGM maintenance term?
That isn't the point. Errors in recharge protocol is. One of the shortcut ways to alleviate chronic undercharging is to increase float voltage by three or four tenths of a volt. When is the last time you saw a pushbutton AGM conditioning charge (a charge profile specific for AGM) on a charger or converter? How old is the charger? Newer chargers may have this feature. Older chargers did not.
This is why properly managing an AGM is NOT A PIECE OF CAKE. When heavily cycled and chronically left undercharged an AGM battery loses capacity and it is not a slow process. Capacity testing is the only valid method to determine if a VRB has been mistreated and has therefore lost capacity.
Subjecting an AGM to 13.6 volts rather than 13.2 volts will not negatively impact battery lifespan, even when left on float charge 24/365. Conversely a battery maintained at 13.2 volts will not negatively impact an AGM battery UNLESS IT HAS BEEN MISTREATED. You fingered the battery when in fact it is a float voltage that has been increased to attempt to compensate for chronic undercharging that is the lynchpin.
So, does this say the LIFELINE would be better off at 13.6 volts than it would be at at 13.2 float? Yes. If chronic undercharging is present. And unfortunately it usually is. Few sailing vessels I have boarded did not have chronically undercharged AGM batteries. Sure the problem would have been lessened had the batteries seen a compensated 13.6 float voltage, but sailing vessels are particularly vulnerable to charge then shut off the engine.
Unless and until I see the lifeline with a lower impedance become negatively impacted by a 13.2 volt float setting, the issue of 13.6 or 13.8 versus 13.2 is a moot point for this battery. You said it yourself, chargers and converters are all 13.6 - 8 these days. Concorde should change their manual to eliminate quibbling.
