Any lead acid battery that sees regular deep discharges, requires the complete full recharge to 100% state of charge, promptly, after any significant level of discharge, for best longevity.
While with flooded batteries one can dip a temperature compensated hydrometer and tell when the battery is fully charged, with AGM one needs to know the current the battery is accepting when held the mid 14 volt range.
When a 100 amp hour AGM battery can accept 0.5 amps or less at 14.4 to 14.7v, only then it can be considered fully charged. This cannot be determined at 13.6v, but if the battery is accepting 0.0x at 13.6v after a prolonged time on the charger, it is likely the battery is fully charged or nearly so, but It would be much better and much more revealing if one could goose the battery to 14.7v, to see how much amperage it accepts at that electrical pressure.
AGMs are often touted as a much better battery, but they are not immune to abuse, and abuse can be considered anything less than being fully charged. The deeper the average state of charge the battery resides, and the longer it resides there, the faster it will lose capacity and CCA.
NO voltage regulator in any vehicle is designed to top up well depleted batteries to 100% state of charge. They are more concerned with not overcharging them and lower system voltage prematurely.
The longevity of any lead acid battery is detemined by its average state of charge, and the average heat it is subjected to. These two factors have way more to do with any inividual battery longevity than who made it or the sticker on its casing.
I've been deep cycling a group 27 Northstar AGM for 50+ months. It has over 700 deep cycles and several dozen of those are to 20% state of charge.
This particular AGM benefits from rather huge recharge rates when it is depleted to the 50% level or less. Deeply discharged AGMs can be, and are regularly, tickled to death with too little charging current, so the whole 'trickle charger overnight' recommendation, often spouted on Automotive forums, on a regularly deeply discharged AGM, is not a good recommendation.
Many lesser$$ AGMs will say to limit charge current to no more than 30% of battery capacity. The Thinplate pure lead AGMS by Ody$sey and North$tar, recommend no less than 40% rate when deeply discharged.
When the battery is not deeply discharged the charging amps and voltage is much less important, but the wise AGM battery owner seeking maximum longevity will recharge to 14.4 to 14.7v(at 77f battery temperature) until amps taper to 0.5% of capacity.
Even the AGMS designed purely around deep cycle duty, like Lifeline, have quite high CCA figures, and can easily replace a flooded starting battery. Flooded marine batteries give up some CCA for enhanced deep cycle ability, and true deep cycle batteries, in a 12v format, are quite rare, and most will not even list a CCA figure.
Do note that flooded marine batteries will proudly state 'deep cycle' on thier stickers, but these are not true deep cycle batteries. their plate thickness is much closer to a starting battery than to a true deep cycle battery, like the GC-2 6v batteries, or the GC15 12v deep cycle batteries, which are a very rare bird to spot. Thr Trojan T-1275 is one of the 3 or 4 true deep cycle flooded/wet 12v batteries available. I know USbattery makes a GC15 for interstate, as east penn also has a GC15 size available, if one seeks it out and, i think Batteries+ can order them from Eastpenn. These are a few inches taller than the group31 size format, which is roughly 13 inches long, 6.5 wide, and 9.25 tall. This is generally the largest size format for a vehicle.
True flooded/wet 12v Deep cycle GC15's, with thick plates and a large reservoirs under those plates for sheddings to accumulate, would indeed have quite low CCA figures, but this is not true of 12v AGM.
AGMs like to list a Pulse cranking amp figure, which is more representative of a fuel injected engine starting. AGMS do very well in this scenario(PCA rating) while their CCA figures might not be all that impressive.
When my Northstar AGM-27 was new, its ability to crank my starter was blisteringly impressive. Almost scary fast.
Now at 50+ months of age and 700+ deep cycles this battery can still easily start my engine when depleted 65 of its original 90 total Amp hour capacity, but that scary fast starter cranking ability is gone, even when fully charged. I am in coastal california which does not get very cold.
When depleted to ~30% state of charge, I can hook up 65 amps worth of chargers seeking 14.7v, and it now takes 22minutes for battery terminal voltage to raise to this point( when new this was just over 30 minutes), and another 4 to 5 hours at 14.7v for amps to taper to 0.45 or less.
If this battery has gone 7 or more deep cycles without getting back upto 100% state of charge, this time for amps to taper to 0.45 or less, increases to as high at 10 hours. This time for amps to taper at 14.7v, depends on just how many partial state of charge cycles it has accumulated since the last 100% full recharge, and also to a lesser extent the amount of cycles accumulated since it last saw the 40 amp plus recharge rate.
The huge recharging rates seem to recondition this specific battery to its maximum remaining potential as the next several overnight discarges, it holds higher voltage for the same amp hours/capacity removed.
Most will never notice the trends and tendencies of battery voltage held under load, but the Ammeter is the AGM's best friend. Voltage alone can be misleading, but how many amps are flowing at that voltage can be very revealing as to state of charge and the general health of the battery, as long as there is a data inspired basis for comparison.
I believe in engine starting duty, the only benefit of AGM is the general lack of terminal corrosion from offgassing. Those who need more Oomph to start an engine with a well depleted battery would benefit. Those intentionally using the battery as a dual purpose battery, both engine starting and auxiliary loads with engine off, can benefit. Those seeking bragging rights, can benefit.
But like all Lead acid batteries, the proper recharging of them, and this means recharging to full asap as often as possible, will have much much more effect on battery longevity than the manufacturer's label/ sticker.
A battery kept at an average of 85F will degrade twice as fast as one kept at an average of 75f too, all other factors being equal.
AGMS are a superior lead acid battery, but they are not immune to abuse, and many will not be able to realize their benefits in simple engine starting duty. But those deepy discharging them will likely be able to start their engines at a much lower state of charge than those with regular flooded/wet starting batteries.
Also, regular flooded/wet/sloshy starting batteries with their super thin fragile porous plates do not take well to deep cycle duty, whereas the AGM can tolerate it, and tolerate it well, as long as it is promptly returned to full charge, and at a good recharge rate of at least 20% of battery capacity and held at voltages in the mid 14's until amperage tapers to a very low level.
Throw out the 'low and slow' is best mentality when AGMS are deeply cycled. They enjoy higher recharge rates. Most flooded deep cycle batteries will recommend a 10 to 13% rate. AGMS are better served by 20 to 30% when deeply cycled, and some of them (odyssey) say no less than 40%.
Northstar is very similar to Odyssey. Lifeline AGM is military grade, and quite pricey, and do not have the super high CCA of Northstar and Odyssey, but can likely outlast them by a good percentage when deeply cycled and recharged properly. All three of these brands basically have no Upper amperage limit, just do not allow voltage at 77F batter temperature to exceed 14.4 for lifeline and 14.7v for Odyssey.
Now holding a battery at 14.4 to 14.7v until amps taper to a certain level, where they can be considered fully charged, is difficult with the smart chargers on the market. The chargers green light, does not mean the battery is indeed full and getting a smart charger with over 20 amps available is also quite rare.
I use an adjustable voltage power supply as a charger and it has 40 amps available to seek and hold 13.12 to 19.23 volts. With an Ammeter on the output I can easily seen when the battery is full, and drop voltage to 13.6v or remove the charging source completely, but this is not automatic.
Do not simply trust the green light on any 'smart' charger. It mocks the human who believes it, especially on a regularly deeply discharged battery half way through its service life, when it is most important to truly fully rechare the battery.
If I could not bring my AGM battery to, and hold it at 14.7 volts, until amps taper to 0.45 or less, this battery likely would have been recycled 2 years and 300 deep cycles ago and have been really weak for the 6 months leading upto that point.
Proper recharging to full is Key with any lead acid battery. The voltage regulator controlling the alternator's output is not ideal for any well depleted lead acid battery, and AGMS can be said to be even more finicky in this regard to their flooded counterparts, but even more so when regualrly deeply cycled.
Not All AGMs are created equal and the lesser$$ ones generally do not have the super low self discharge nor significantly higher CCA figures, and also recommend not to recharge them with more than 30% of capacity recharge rate when deeply discharged.
Those seeking the most CCA would do well to get the Odyssey or Northstar(X2power) AGM's, and fitting the largest one thay can into the original space for it. relocating thebattery out of the engine compartment, or insulating it from engine heat will go a long way towards longevity.
These two AGMS are 3 to 4 times more expensive than the same size flooded starting battery, and are unlikely to last 3 to 4 times as long in starting only duty, so they can be a very false economy, but it all depends on how they are used and how they are recharged.
If one cannot regularly recharge to full and is deeply cycling their battery, then I would recommend getting the cheapest battery, as price is not going to make it immune to abuse. The higher end battery would stand a better chance of having its capacity restored than the cheapo, but to do so requires an extended recharge at the correct voltages. meaning specialized equipment. Without the ability to seek 14.7v with 20 to 30% of capacty in charging amps available, and holding it until amps taper to 0.5% of capacity, the high end AGM is not going to be very happy.
With my Ah counter and ammeter and voltmeter, I can also determine just how long it takes the alternator to restore a fully charged battery, back to fully charged after engine starting.
While many like to say lots of short trips are hard on a battery, my meters call out [censored] on this proclamation. When my battery is accepting 0.0x amps at 14.7v it can be considered fully charged. after engine starting, when amps taper to this same level at 14.7v, it can again be considered fully charged.
How long does this take? Under 45 seconds. It would take longer at 13.6v, but the point is it does not take very long for the alternator to restore what is drawn from the battery when starting a fuel injected vehicle. Now if the battery is at 80% charged, it can still easily start the engine, but getting a battery from 80% charged to 100% charged, at ideal recharging voltags, takes 3 to 4 hours, no matter what. And keep in mind the vehicles voltage regulator is not going to seek and hold ideal absorption voltages. So short trip driving is not going to be able to return battery to full quickly if it started the engine while at 80% charged.
So the maintenance minded individual will use a grid powered charger to seek and hold 14.4 to 14.7v until amps taper to very low levels, on flooded or AGm batteries, though flooded will be more along the lines of 1 to 1.5% of total battery capacity where agms need to taper to 0.5% of battery capacity or less.
Smart chargers might need to be tricked into holdingthe battery in the mid 14v range. Trick them by loading battery until its terminal voltage drops to 12.6v or less and then rehook up the charger and restart it. this may or may not work on any given smart charger. Most of them will actually stop charging at absorption voltage( mid 14's) and drop to float voltage( low to mid 13's) when the battery is 92 to 95% charged, and those last 5 to 8% of charge can take severl more hours at absorption voltage and a dozen or more hours at float voltage, on a healty batteyr. An unhealthy battery will likely never get fully charged at float voltages no matter how long it is held there.
Knowing how many amps the battery is accepting is Key.
One can hook a product like this inline on the charger to see what the battery is accepting and what the charger is trying to do.
Most smart chargers with an AGM setting will drop to a higher float voltage, than on the flooded setting. Do not fear using the AGM setting on a flooded battery. It is likely to get the battery closer to a true full state of charge than the flooded setting, just do not leave it hooked up for weeks on it, as it will cause more water loss and positive plate shedding.
All charge voltages listed are for a battery temperature of 77F. Hotter batteries require less voltage, colder batteries require higher voltage. Ideal voltages are hard to achieve, but seeking ideal can be taken to extremes. Batteries are rented and ideal can maximize longevity, but ideal also costs $$ and effort, and it is often easier and cheaper to accept less than ideal and simply replace batteries more often.
But knowing what Ideal is, is important, even if one does not care to go through the effort to achieve it.
