People should look at the battery manufacturer spec sheets as to their recommended charging voltages, and not throw out their broad brush statements, some of which seem to be opposite of actual manufacturer recommendations.,
Here is a link to East Penn/Deka, Bitogs current darling, saying their GEL, and AGMS, should have an absorption voltage of 14.1 to 14.4v, and their flooded batteries 14.4 to 14.7v, so, so much for for the broadbrush declaring AGMS' needing' higher voltage.
Precision of charging voltage is only really important in actual deep cycle duty, and most starter batteries only inadvertently see actual deep cycle duty.
Note that the battery manufacturer recommendations are just that. Some people seem to think if they are not followed exactly the battery fails instantly.
but some people also think their alternator or their vehicle's voltage regulator is some incredibly smart, all powerful, magical battery whisperer that can nearly instantly give the battery an orgasm.
In starter duty only, the battery is likely kept at relatively high states of charge and the voltage allowed by the vehicles voltage regulator, which controls the alternator, is not going to make much difference to lifespan or state of charge. The modern vehicle tendency of voltage manipulation for max MPG throws a wrench to the above sentence.
If deep cycling that same battery, then willy nilly voltage regulation, will have one quickly trying to warranty the battery, calling it a POS, when the recharging was the problem and the ignorance of the owner the ringing alarm bell.
Note in the above link, Deka says to limit charging current to 30% of the amp hour capacity of the battery. 100 amp hours capacity, 30 amps max charging current. The battery will only accept this much current for more than a few seconds, when it is discharged below ~85% State of charge,
As for what any given garage charger's flooded, or AGM setting is programmed to do, the voltages it seeks and for how long it will hold it, this varies wildly. I was hoping more OCD bitogers here would actually check , but most are just happy to read marketing material and see the nearly meaningless green lights.
I get Donald, with new AGMS, wants to treat them right from day one.
I'd recommend he seek who manufactured the batteries for whomever stickered them, and see if they will provide recommended absorption and float voltages, then actually observe what his charging sources actually do.
If one is not going to bother trying to find a charger which meets those battery manufacturer specs, perhaps ignorance is a better strategy.
I'd also recommend that if he intends to long term apply a maintenance charger, that it holds that recommended float voltage, and better yet if it has a battery temp sensor. NOTE that all recommended voltages assume a battery of 77f or 25c, Ideal charging voltage is battery temperature dependent, and extremes of either will have huge effects on longevity.
In my experience both Cycling and just using AGM batteries as starting batteries, they behave weirdly when brand spanking new. When held at their recommended absorption voltage, the amperage does not taper to levels it can be considered full. They do not maintain the high full charge resting voltage either, and they do not seem to turn the engine over nearly as quickly as expected/hoped.
This did not change until I discharged them to around 50% then immediately recharged them with a high amp charging source.
I go well over that 30% recommendation, but most with plug in chargers cannot even come close to it.
After this initial deep cycle and high amp recharge, the amps, at absorption Voltage will taper to 0.5 per 100Ah of capacity, and less, indicating true full charge. After this break in discharge/high amp recharge, the battery does hold its 13.06+ full charge resting voltage, it does spin the starter motor with the expected impressive gusto, and then some, and when deep cycling, it does maintain impressively high voltages as it discharges at 3 to 10 amp rate of discharge, and also during the 150+ amp load of my starter motor.
Even a battery built solely for starting, which most batteries talked about here are, will be able to handle a certain amount of deep cycles. The damaging part of a deep cycle is draining them below 50%, and the cumulative time spent below 80%. A quick discharge to 50% followed immediately by a true full recharge, and charging it at upto that 30% rate, is not going to negatively affect the battery's ultimate longevity, but this break in cycle can be considered mandatory in order to get the maximum potential performance from the battery.
Simply putting on a charger/maintainer to get a 97% charged battery to hopefully 100%, is not going to expand that 'lung capacity'
Voltage is electrical pressure. Yes AGMS and flooded battery manufacturers do vary their pressure recommendations, and who the F knows what charger brand X is going to do when that AGM button is pushed.
Higher pressure on a newish healthy battery is just going to charge it faster.
If the pressure does not reach the manufacturer recommendations, the battery can still be filled at lower pressure, it just takes longer.
Note that older batteries the lower pressure might never be able to fill the battery, no matter how long it is applied. Some will, some wont.
Some batteries once they have been brought to absorption voltage of say 14.4v, will be accepting 20 amps( max output of theoretical charging source). Once 20 amps has achieved 14.4v, then the amuot of amperage required to maintain 14.4v starts declining. the same amperage applied would keep pushing battery voltage higher.
My Deka AGM's, if I lower absorption voltage from 14.4v to 14.1v amps will taper from say 50 to 47. almost a meaningless difference, so I keep the lowered voltage.
My Northstar AGM, if I lower voltage from 14.7 to 14.4, amps will taper from 50 to 33, which is a significant difference, greatly prolonging the time to reach full charge, so I keep the voltage high.
Some charger's agm settings will just hold this higher voltage for X amount of time
Some chargers flooded settings will drop from the 14.4 to 14.1 after a period of time, say 15 minutes.
Who the F knows what any given charger is going to do, and Which is why i always say get an ammeter, and do not just rely on voltage, and especially one voltage reading taken once during the hours long process.
Here is a link to Trojan batteries, one of the most respected makers of Deep cycle lead acid flooded batteries.
They recommend an absorption voltage of 14.82 volts.
While on some boating forums and most all rv forums, there are members who vociferiously believe it is impossible to fully charge theirtrojan batteries fully, at 14.81 volts. These people are obviously to be ignored.
The battery manufacturer recommendations are manipulated by them as needed to reduce the expected warranty returns. They are trying to second guess the ability of the end user's charging equipment to take care of their batteries. 14.82 absorption voltage will charge the deeply cycled battery faster, get it to higher states of charge faster, and more likely be able to get it to 100% charged which in turn reduces the likelyhood of the owner killing the battery within the warranty period.
Yet some will state over and over how 14.4v will never ever fully charge a trojan. It will, it just takes longer.
100% charged, a true 100% state of charge, and kept cool, is Lead acid battery heaven. Anything less is detrimental, to some degree, and even if kept in heaven, it will still degrade with time, it will however degrade much slower.
Those seeking ultimate longevity of their lead acid batteries, insure 100% charged is attained regularly and promptly after any significant level of discharge.
Keeping the battery cool can be a lot of effort, to nearly impossible.
I now have a 420$ group 31 Northstar TPPL AGM in my engine compartment.
I put a K type thermocouple in the area, and on engine shutdown the battery was exposed to underhood air temps as high as 147f, and an hour later it was still 118f in there. I enclosed the battery, leaving 3/4 inches all around it, with a tight fitting reflective radiant barrier and now on engine shutdown the air surrounding the battery only climbs a few degrees above ambient, when not deep cycling and high amp recharging, when the battery heats itself.
Inside insulated battery enclosure on engine shutdown, 83f, immediately on the other side of insulating barrier 147f, ambient, 80f.
I'd obviously not have expended the effort for a 55$ battery, but my batteries deliver and receive many many KWH over their lifespans, and they would achieve delivering a tiny fraction of that if I could not charge them correctly, or just let the stock insane voltage regulator do what it does.
If you were driving from A to B, but did not know where B was, or even A, wouldn't you want a compass and a speedometer and a map?
Yet lost people with their so called 'smart' chargers just say 'tell me when i get there.'
How high a pressure does your charger go to, on its specific setting?
How long does it hold the battery at that pressure?
how much amperage does it actually deliver in its quest to achieve its maximum ( absorption) voltage?
When it quits and throws the green light and holds a Float/maintenance voltage, what is that voltage?
Or does it just stop applying any current until battery voltage falls to 12.69v, as does the Noco genius 1 on its AGM setting
There's is no reason to guess or hope, or spew falsehoods passed down from grandpa.
An ammeter is a wonderful tool.
See what your charger/charging source is actually doing, how much amperage your battery is accepting at the electrical pressure your charger is delivering, or trying to achieve.
Or continue on, in blissful ignorance.
For those that do not enjoy being ignorant, re- spewing grandpa's uninformed opinion:
See what your charging source is actually doing, and then you can also, with experience, estimate its remaining health/ ability/lifespan.