AGM self discharge less, but when they are discharged deeply, require that they be returned to a full 100% state of charge, to get good longevity, and the deeper they are discharged, the more amperage they should be allowed to feed upon. Deeply discharged AGMs can, and are, tickled to death by low amperage charging sources.
They should get no less than 20 amps per 100 Amp hours of capacity when dischrged to 50%. In general the largest 12v battery rwhich might be found in some trucks are a group 31 and these are usually in the 100 amp hour range, meaining it can support a 5 amp load for 20 hours before is voltage falls to 10.5v, which is considered fully discharged. The group 65AGM battery is about 85 amp hours, group 24 similar, and group 27 can be in the 100 range too, these 4 group sizes are generally the largest 12v batteries one will find underhood on bigger engined vehicles.
They should be brought to 14.4 to 14.7v, assuming a battery temperature around 77F. colder temps require higher voltages, hotter battery temps should have lower voltages allowed. 15v is not some threshold which instantly destroys the AGM battery, and the cold AGm battery should be brough to and exceed 15v when charging.
They ( the Lead acid battery discharged to 50%) will take no less than 3.5 hours after a 20% charge rate gets them to these voltages ,easured at theh battery terminals, before they will be near fully charged.
Full charge on an AGM can ONLY be determined when the amperage required to maintain these higher absoprtion voltages tapers to 0.5% of the capacity, meaning 0.5 amps for a 100Ah group 31 battery. Smart chaagers guess, and throw the green light, drop to float voltages, and mock the human who believes it. Haw hawwwww.
Some are better than others. Reversion to a float voltage and the soothing green light, does not mean the battery is full, nor does charging stop at float voltage, it just slows down a very significant degree and while a healthy battery will eventually reach full charge at float voltage, an unhealthy battery will not.
The lesser the depth of discharge, the less important it is to approach/achieve this Ideal recharge regimen.
Do not listen to those that say to trickle charge deeply discharged AGMS. They respond incredibly well to higher amp recharges, and Odyssey's reconditioning procedure says to discharge to 10v under a fairly high load, then immediately apply no less than 40 amps per 100AH of capacity until 14.7v is reached, and hold until amperage tapers to near Zero. lather rinse repeat if required.
Concorde AGM, who malke Mil spec AGm batteries, one of their brands being Lifeline, say to normally fully charge( meaning hold them at 14.2 rto 14.6v until amperage tapers to 0.5% of capacity( 20 hr rate), and then bring them to and hold them at 15.5 volts for 8 hours as their recommended reconditioning procedure .
If AGMS are deeply discharged, and not recharged at a fairly high rate, to a true full state of charge, they will not have a good lifespan. This is pretty much a guarantee, but it is pretty amazing how much CCA they can still provide when seriously capacity compromised, and near fully charged to near that remining potential
While many alternators have internal Voltage regulattion, others do not.
Most alternators will have NO issue achieving a 40% charge rate on a deeply discharged battery, IF and only IF the voltage regulator not only seeks mid to high 14 volts, but also holds that higher voltage for a good period of time and ideally, until the battery is full, or gets too hot, which is a possibility when high amp recharging.
Getting any healthy lead acid battery that is 80% charged, from 80% to 100% takes no less than 3.5 hours. This is a simple fact, and higher voltages, over 14.7ish volts, cannot safely take any significant time off this fact. If the battery is less than healthy 80% to 100% takes even longer. If these higher voltages (14.4 to 14.7) are not held for longer, the unhealthy well discharged battery starts looking for the nearest cliff to dive over.
Almost NO Vehicle, even if driven for these 3.5 hours, will hold mid to high 14 volts, and 1/2 to 1/3 the amps flow at 13.7 compared to 14.7v. Many people assume the voltage they see after starting the engine is the voltage the vehicle always holds. This assumption is unwise in the extreme.
All lead acid batteries wish to always be fully charged, and kept cool.
Anything less than this ideal, compromises their potential longevity, to some degree. The lower their average state of charge, the more they are negatively affected the higher their average temperature the more negatively they are effected. Cold temperatures slow down self discharge, and will not damage a fully charged battery, but cold temperatures reduce CCA and capacity and will reveal a compromised battery. Heat is the cumulative killer, but cold is the revealer.
Since the vast majority of people, even very qualified mechanics, seem to assume the alternator is some magical battery charger that can defy those 3.5 hours from 80% to 100%, most batteries intentionaly or unintentionally discharged a fair degree, remain in a constant state of undercharge.
This seems to affect AGM batteries more than wet/sloshy/flooded lead acid batteries, and one gets to pay more for the privilage of a battery that is less tolerant of chronic undercharging abuse.
But if one can indeed recharge promptly to full 100% state of charge, and to full charge at a high rate when deeply discharged, then one can achieve almost ridiculous lifespans from a quality AGM, or a deep cycle flooded battery. Starting flooded batteries are never going to be happy for long in deeper cycle duty even if recharger ideally and promptly to full. AGMs blur that line between deep cycle and starting, and flooded marine batteries have little in common with flooded deep cycle batteries, despite the deep cycle claims on the sticker.
I am on 5.5 years on a Northstar AGM battery. That by itself is no great accomplishment, until one factors in the fact that it has no less than 1100 deep cycles on it, many hundred of those cycles down to 35% charged several dozen to the 20% charged range.
If I cound not promptly and truly fully charge this battery, at a high rate when deeply discharged. It would have been recycled long ago and I could join the legions who blame the battery for my ignorance in its proper care and feeding.
Blame the vehicle's or charger's voltage regulation.
Blame ignorance of the 3.5 hour rule from 80% to 100% state of charge rule.
Most vehicles do not, will not have ideal battery charging voltages for good battery longevity.
Ideally ones vehicle would bring system voltage upto mid 14's until the battery was indeed truly full, then settle in the mid 13's once full.
Almost No vehicle does this.
If the battery is never discharged much at all, then the battery does not need 'ideal' to live well beyond the warranty.
If the Lead acid battery is discharged regularly, well below 100%, and lives perpetually in this undercharged state, it will be lucky to last the warranty period.
Maintenance minded Bitogers seeking good battery longevity should regularly charge the battery overnight on a charger capable of bringing the battery to the mid 14's for a few hours, but any charger is better than no charger. Low rate' trickle' chrgers might not have enough time overnight to get the battery to the mid 14's, much less hold them there for a few hours.
If one inadvertently drains their battery to the point it needs a jump start, the battery should be put on a charger immediately once one gets home, but surface charge from the alternator might trick many smart chargers and they will all too quickly revert to float voltage. Knock off the surfacee charge voltage with the headlamps for a few minutes( drop voltage below 12.5 or so), then hook up and start the charger, then turn off the headlamps.
Ther person who believes the drive home after a jumpstart was required, even if it is 4 hour drive, was enough to fully charge the battery, is seriously deluded.
If the voltage regulator is seeking and holding voltages in the mid 14's( which is quite rare), the jumpstarted battrey can be returned to 80% state of charge fairly quickly perhaps as little as 30 minutes, but That depends on the ability of the alternator to not only power the vehicles electrical needs, but have plenty of excess amperage producing capability to feed the depleted battery at a high amperage rate. It also depends on teh health of the battery. an unhealthy battery can be brought up to higher voltage quicker with less amperage required, where a healthy battery's voltage will rise slower with higher amperage applied.
And there is no getting aroud that 3.5 hours minimum( often much longer) to get from 80% to 100% state of charge, not even if you have a 300 amp chromed alternator you just polished while telling it how pretty it is.
Don't fear AGMs in older vehicles or vehicles that did not come with them, although you might not get any advantage from them. their charging voltages ar elargely the same as wet/flooded batteries despite what one reads parrotted over and over on the internet that says the opposite
Dont fear high rate recharging, unless ambient temperatures are quite high, which underhood at slower speeds, they could certainly be.
An AGM is not killed instantly if the vents should open. It takes a lot to actually pop the vents, and would require high amperages applied from a low state of charge, at high ambient temperatures for this to occur, and it would have to occur often and regularly for it to actually 'dry out' an AGM.
If the battery is nearly fully charged, it cannot accept high amperages, and high voltages are extremely likely to pop the vets and 'dry out' the AGM.
But I have to keep in mind 'AGM' is the new battery marketing buzzword, and they are getting cheaper and cheaper. These mass marketed lower priced AGMS are likely cutting a lot of corners for maximum profit. They should be fine in most instances unless they are regularly deeply discharged and they bake underhood while the voltage regulation is seeking high voltages and the dutiful and capable alternator has no issues providing high amperage to the battery, over and above what the vehicle needs to run its electrics, for a good period of time.
