No lead acid batteries are happy when they are not regularly reaching full charge, but AGM'S are a bit more finicky when they 'never' reach true full charge, even if they remain at a high average state of charge.
People seem to think that the battery is fully charged any time the vehicle is driven, but this is far far from true.
One can discharge a 100% battery to 80% charged in 10 minutes of blasting the stereo engine off, but no vehicle/no charging source on the planet can recharge a lead acid battery from 80% to 100% charged in less than 3.5 hours. Simple fact. not open to debate, sorry.
Those 3.5 hours assume a charger not only capable of producing enough amperage to bring the battery to the mid 14v range, but holding it there the entire 3.5 hours while amps taper to low levels.
No vehicle does this and the charge rate in the mid to high 13's takes 3x as long, and usually much longer, especially with a less than healthy battery.
The voltage one sees after starting their engine is not what it will always be, it will likely drop to the mid to high 13's in short order, from the mid 14's seen right after starting.
The battery accepts as much amperage as it wants, at the voltage reaching the battery terminals, up to the amperage limits of the charging source.
The deeper any lead acid battery is discharged the more important the recharge regimen becomes and the more important it becomes to reach a true 100% charge, if one is seeking good or better longevity from it.
Since AGMS batteries have been marketed as superior, and people are sheep, cheap sheep, manufacturers have responded with cheap as they can make them AGMS, yet the expectations of a super battery immune to abuse, have not diminished in the slightest.
Batteries that last 8 years are not profitable, batteries that last 1 month past warranty, are. We live in times of max profit, at the expense of everything else.
THE ONLY way to determine when an AGM battery is fully charged, is when the amperage it accepts when held at absorption voltage(~14.5v) tapers to 0.5% of the capacity of the battery, ie, when a 100Ah AGM tapers to 0.5 amps at 14.5v at 77f, only then can it be considered fully charged. a flooded battery with caps can have the specific gravity of its cells checked, but amperage at absorption can also be used thereafter as a general guide to when it is fully charged or nearly so, but the hydrometer is the flooded battery polygraph.
Voltage is not a reliable indicator of state of charge, unless the battery has not seen any charging sources or discharging loads for 12+ hours, and also has previous observations to compare it to, at the same temperature. AGMs can vary from 12.75v to 13.23v as to their full charge resting voltage, so blanket statements as to 12.xx volts = xx.x% state of charge should be ignored, no matter how much authority it is declared with.
When AGMS are deeply discharged, it is important to not slow charge them. Deeply discharged AGMS are tickled to death with too little charging current. "Trickle charge it overnight' is almost a death knell for deeply discharged AGMs. They need higher amps for force the electrolyte to migrate through the glass matting. All the top AGM makers say to throw all the available amperage one can produce at them, just limit voltage to ~14.5 and no higher, the lower $ AGMS say no more than 30 amps per 100Ah of capacity, some say 27%, some say 33%. The issue is heat, and higher charge rates will heat the battery more as will charging in high ambient temps. Lower $ AGMS have higher resistance than higher $ AGMS and will heat more. Batteries above 85% charged will only accept high rates for a few seconds before voltage rises to the maximum at which point amps are lowered to keep voltage from climbing any higher..
The Deka intimidator AGM battery did poorly in capacity retainment testing in deep cycle duty by some marine magazine several years ago, and this battery is likely the most rebranded relabelled AGM battery sold in the USA.
If one does not have the ability to insure an AGM does indeed get fully charged( cant do out without an Ammeter, and charging source holding the battery at 14.4ish volts), then one should not be surprised that they do not get the longevity they hoped for when they paid extra for a perceived superior battery.
Reviving a sulfated AGM is generally not recommended by using techniques that are OK on flooded batteries.
An Equalization is a forced overcharge, holding it at 15.5v to 16.2v until specific gravity stops rising on all cells. EQ charges should be performed only when needed and for no longer than absolutely necessary.
Lifeline AGM, who make high $$ Mil spec AGM batteries have a 'conditioning' procedure, where after a normal full charge, holding it at 14.42v until amps taper to 0.5% of capacity, then bringing the battery to and hold it at 15.5v for 8 hours.
check page 21:http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf
Odyssey AGM is a thin plate pure lead, TPPL AGM. Usually 'thin plate' and deep cycle are not uttered in the same sentence, but both Odyssey and NOrthstar TPPL AGMS, retain capacity extremely well in deep cycle duty, as long as they are recharged promptly and to truly full, often. These also have extremely high CCA figures and retain voltage during high loads much better than other AGMS, but they are 2x as expensive as the cheapest AGMS that the american consumer flocks to, and assumes is some magical super battery because of the higher price than traditional flooded batteries.
The high $$ AGMS also have much less self discharge than cheaper AGMS and flooded batteries. The lower$$ AGMS are not all that much better than flooded batteries in the self discharge department, and not all that much, or perhaps any better, than a flooded starting battery in the CCA department. These cheaper AGM's chief advantage over flooded batteries is the nearly complete lack of terminal corrosion, and maintenance required, and if discharged to the 30% or under range, are more likely to still be able to start the engine.
People not having issues with terminal corrosion and who do not regularly allow their parasitic loads to drain the battery to 30% or less over 3+ weeks of non usage of the vehicle, are not likely to see any benefit from putting an AGM where a flooded battery once resided. If the AGM is allowed to slowly drain to 30%, this is a very deep cycle, and AGMS need high amps when cycled this low, and really want to reach 100% true full charge when discharged this low. If they don't get it, they get petulant.
The Odyssey AGM reconditioning procedure of a chronically undercharged/ sulfated battery is different than Lifeline, they want a relatively large load to drag the battery relatively quickly down to 10.0 volts under that load, then immediately apply no less than 40 amps per 100Ah of battery capacity until 14.7v is reached, then hold 14.7 until amps stop tapering or taper to near Zero. Lather rinse repeat.
Got a 40 amp charging source available? Didn't think so. Most garage chargers available today are limited to 25 amps in this litigious day and age, and even if one could get a 40% charge rate, they could not trick the 'smart' charger into holding the 14.5v+ for the time required to truly fully charge the battery.https://www.odysseybattery.com/documents/ODYSSEY_Battery_Reconditioning_Charge_Procedure.pdf
Regular smart chargers do a very poor job at actually fully charging either flooded batteries or AGMS, they are primarily designed to never Overcharge. Those who believe and spout the marketing mumbo jumbo of smart chargers, should generally be ignored.
Smart chargers are incapable of performing either Lifeline's or Odyssey's' restorative' procedure on a sulfated/weak battery.
The 'pulse' desulfation chargers have not outperformed a regular charger holding the sulfated battery at the same voltage for the same amount of time by those with the tools and experience to do capacity testing before and after with either charger.
A vehicle's voltage regulation is also quite poor at returning a battery to true full charge, especially an older battery which needs higher voltages held for longer to reach anywhere near full charge, and many more modern vehicles intentionally keep the battery at a lower state of charge in a foolish attempt to eek out a few more hundreths of a fraction of an MPG.
No lead acid battery is immune to living life chronically undercharged, but not knowing the average state of charge of the battery, nor the average temperature, one cannot claim battery X is awesome and battery B is a piece of junk, based on user report, also the battery made 5 years ago is not made by the same place to the same standard today as the rebranders are always seeking maximum profit from whoever will offer them the lowest price.
I could also post links about what certain AGMS manufacturers recommend as to charging amperage and voltages, and these will almost be in lock step with flooded batteries, yet the myth that an AGM battery needs hugely different charging needs than flooded is untrue, especially when the battery is just a starter battery, never deeply cycled. The more deeply the battery is discharged, the more important the recharge regimen becomes, and most every vehicle is going to be far from ideal, even if it is driven the minimum of 3.5 hours that it requires to get any lead acid battery from 80% charged to 100% charged.
Any charging source applied to an undercharged battery is better than no charging source applied, but all lead acid batteries ideally want to live their lives out truly fully charged, and achieving true full charge is time consuming, and true charge is not determined by a green light of a smart charger.
The green light on a smart charger only indicates that the charger has decided to stop holding the battery at a higher voltage. This could be based on a simple egg timer algorithm, a 'one size fits all' approach, when the same battery needs a different approach if it is being charged from 83% compared to 61% state of charge. One size fits all approach means it falls well short of ideal 99.8% of the time.
Just remember the automatic smart charger is not designed to fully charge, it is designed to never allow the battery to be overcharged. No matter how much faith one puts into any one brand of charger, the primary goal is to NOT overcharge, and as such, they undercharge, sometimes badly so.
When one can take a battery warm off a smart charger, but stuff 10 more Amp hours into it over 3 more hours when holding it at 14.5v, then it is incredibly obvious just how short the smart charger fell in its task. Its also obvious to an observer with an ammeter and voltmeter just how much more voltage the battery maintains cranking the engine or when powering lighter loads for longer, when it has been truly fully charged, compared to even 98% charged.
Smart chargers refusal to actually fully charge has me using an adjustable voltage power supply instead. The ammeter reading amps into the battery( not also powering DC loads) tells me how long to hold the battery at my chosen absorption voltages, and this time has been increasing for the last several years as the battery degrades with time heat and accumulated cycles.
I am closing in on 6 years with a Northstar AGM, that has about 1300 truly deep cycles in the 50% charged range, several hundred well below 50%,, thousands of shallow cycles, and many thousands of engine starts. It is nearing the end of its useful life, as the voltage it can maintain while cranking the engine keeps dropping lower and lower when fully charged, but I have gotten more life from this battery than any other.
It is not just the quality of Northstar AGM, but the fact that I can indeed truly fully charge it, often. The true full charge, often, is the key to battery longevity, of ANY brand of lead acid battery, with average temperature the secondary influencer of battery life.
Defects aside, The best lead acid battery chronically undercharged will not last as long as the worst battery kept fully charged.
Reports of longevity of a lead acid battery, good or bad, without knowing the average state of charge and average temperature are not worth the pixels used to type them or brain wattage used to read them.
Relying on the vehicle to truly fully charge the battery when cycled, shallow or deep, or keep it nearly fully charged, is unwise in the extreme.
Obviously some vehicles are better than others in keeping the battery at a higher average state of charge, but all it takes is an avid engine off stereo listener to halve the battery life, or the vehicle to sit unused for 2 weeks compared to the previous battery when it sat no more than one week between uses to have one battery last 5 years and the next 2. And Aging vehicle and its connections will impart more resistance, lowering the voltage the battery receives, slowing down its recharging rate. The OEM battery might only be lasting longer for this reason.
While lead acid batteries are said to have no memory, it is often observed that they perform better after testing their capacity. A true capacity test means holding a fully charged 100 amp hour battery under a precise 5 amp load while the voltage falls from ~12.8 to 10.5v. a 10.5v battery is considered 100% discharged. If it can maintain 5 amps for 20 hours then it still has 100 Ah of capacity available. Usually after the 100% discharge then a true full recharge, the aged battery will perform better on the next capacity test.
My personal observations on my northstar AGM when it was new, was that it would not hold its true full charge resting voltage(13.06v) until I drained 45 of its 90amp hour capacity, then promptly recharged it at 25 amps until 14.4v was reached then held for ~3.5 to 4.5 more hours.
When its performance ( voltage held under load while discharging) was starting to lag in regular deep cycling, the cure( up until about 10 months ago) was always a high amp recharge to full, from a well depleted state, and the higher the amps the better. I combine chargers for 65 amps into a 90Ah battery, and it can accept 65 amps for no less than 22 minutes before voltage reaches 14.7v, at which point less and less amperage is needed to maintain that constant voltage.
Lazy poor performing AGMS might respond well to a full 100% discharge followed by an immediate higher amp recharge to full. I'd say no less than 30 amps per 100 Ah of capacity on a lesser $ AGM, and no less than 40 amps on a TPPL AGM, or a Lifeline/ Concorde AGM mil spec battery. Make sure voltage does not exceed 15 and temperature does not exceed 120F. note that once it starts rising past 95f it will likely rise exponentially faster thereafter. Heating while charging can help desulfate, reverting sulfate back into electrolyte is easier at higher temps than colder ones.
Lifeline AGM says no less than 20% charge rate, 20 amps per 100Ah of capacity when deeply cycled, and if this cannot be achieved then at the end of the charge cycle, constant current the battery at 2 amps per 100Ah of capacity for 2 more hours to slow the capacity decline. Voltage will likley climb into the 16's constant currenting a battery at such a rate, and it is usually recommended to not allow 12vDC items prolonged exposure to 16+ volts.
Again, disregard the 'trickle charge it overnight' recommendations when it comes to AGMS. They enjoy high amperage charge rates, especially when high amperage rates bring them to, then hold them at high enough voltage, for long enough, to actually reach fully charge.
Reaching 98% charged is certainly good enough, for starting the engine, but 100% charged is twice as good when it comes to long term capacity and cranking amp retainment.
Most smart chargers will stop charging a lead acid battery in the 92 to 95% charged range, and this can be proved time and again by anybody with an additional charging source which can hold the battery at high enough voltage and an ammeter to see how much amperage the battery is accepting at absorption voltages.
Yet 99% believe the green light on a smart charger is gospel. I often use my 'smart' charger until it gives the green light, remove it, then use my power supply to hold it until the amps taper to prescribed levels or amps stop tapering, and never has this taken less than 2 more additional hours held in the mid 14's and my amp hour counters show anywhere from 6 to 22 more amp hours forced into the battery to reach full charge.
But any charging source applied to a less than fully charge battery, is better than no charging source applied.
Sulfated, underperforming AGMS that have not been sitting fully discharged for weeks/months, will likely respond to some degree, to a full 100% discharge quickly followed by no less than a 20% charge rate until 14.5v is achieved at battery terminals, then 14.5 held until amps either taper to 0.5% of capacity or stop tapering.
Simply slapping any smart charger on a sulfated AGM already at 89% state of charge, is not going to do much if anything. it is likely the charger will shut off in under an hour, yet 6 hours more at 14.5v might be required.
There are very inexpensive (sub20$) 30 amp adjustable voltage LED power supplies online that one can use to hold the battery at high enough voltage for as long as they desire, but these cheapo power supplies might not have' constant current limiting' on overload, and a well discharged battery will be able to suck up more than 30 amps and thus overload the power supply. My initial cheapo 22$ 30 amp power supply lasted 17 minutes outputting 38 amp before it released its magic smoke, and this was with better ventilation and heatsinking. I got tired of manipulating voltage to keep amperage just under 36, Above 36 the power supply was making clicking noises of discomfort. When it did smoke I ordered a meanwell power supply capable rated at 500 watts but will do 600 at any voltage between 13.12 and 19.23, all day long, with better ventialtion and heatsinking.
One should always know how many amps the battery is accepting at the voltage at the battery terminals. There are simple mostly accurate inexpensive devices one can easily splice inline on their charging leads.https://www.amazon.com/GT-Power-Analyzer-Consumption-Performance/dp/B00C1BZSYO
This inexpensive device and the multitude of clones available, are not perfectly accurate, but it will count amp hours and watt hours and display peak wattage and peak amperage and minimum voltage as well as displaying voltage amperage and wattage in real time.
I have a hall effect ring sensor ammeter displaying on my dashboard, how many amps the battery is accepting or delivering. The more amps delivering the less charged the battery, or the lower the voltage I have commanded my adjustable voltage regulator to seek.
Charging a battery and not knowing how much amperage it is accepting is like driving from A to B, but not knowing how far apart A and B are, nor how fast one is driving between them.
Some are happy enough to believe marketing literature and a green light to tell them when they have arrived, and ignorance is almost always blissful, at least until premature failure, then it's finger pointing time.
People expecting good to great longevity from their lead acid batteries, AGM, flooded, or GEL, should also expect to need to regularly apply a grid powered charging source to get the battery to as near a true 100% state of charge as possible.
Anytime the battery is known to have been discharged significantly, then immediate recharging to true full is ideal, and the deeper the discharge the more important it is to promptly reach true full, or as close as possible to this ideal as their plug charging sources allow, and AGMS are more susceptible to degradation than flooded when they are not allowed to reach true full charge after such a discharge.
Often smart chargers can be tricked into achieving higher states of charge by restarting the charger, but one needs to bleed off the surface charge by loading the battery with headlights until voltage falls below ~12.7, then restart charger on the next lowest amperage setting, then turn off the headlights. Complete disconnecting of the smart charger from the grid and the battery might be required before reattaching and restarting it.
The AGM setting on most smart chargers will almost always do a superior job on any lead acids battery, than the 'regular' setting as they will limit voltage to 14.7ish but hold it for longer, and when they decide the battery is full ( prematurely) they will 'float it at 13.6v, opposed to 13.2v for the flooded setting. Floating a nearly fully charged at 13.6v stands a much better chance of approaching a true 100% full charge than will 13.2v in whatever additional time the charger is plugged in and attached to the battery.
Batteries are just rented, replace as needed, but the length of that rental contract is directly related to their average state of charge, and average temperature. Average temperature might not be able to be influenced, but average state of charge easily can, if one makes the effort to plug any charging source into the grid for a period of time.
Ideal/maximum lead acid battery longevity requires ideal 100% recharges and as high an average state of charge as possible at all times.
Is it worth your effort/ time and money to acquire the equipment required, and expend the effort to approach this ideal?
Likely not, but one should at least know what is ideal, and that 'automatic' smart chargers or the vehicle's own charging system is likely far from ideal.
Its Your money, do what you want, but don't expect a chronically undercharged lead acid battery to live an ideal lifespan, and don;t believe that well marketed smart chargers are capable of delivering a true 100% charge to your battery and do not for a second believe your vehicle cares one whit about achieving and then holding a battery at an ideal 100% state of charge. Not even if you have a recently polished chrome alternator