The AGM battery is a superior battery, but whether their superior qualities are ever utilized is the real question. For many they are a bragging rights battery, or perhaps a little more peace of mind.
Take 2 of the same group size batteries, one flooded one AGM.
The AGM battery should have lower resistance, and as such Should recharge faster at the same voltage chosen by the vehicles voltage regulator.
The AGM battery should have higher CCA figures because of this lower resistance.
So Will it? c When shopping, compare the same size flooded battery with the AGM? how different are the CCA figures?
The AGM battery if discharged the same exact capacity as the flooded battery, should retain more of its CCA, say both batteries are depleted 80 of their 100 Amp hours of capacity, the AGM will likely be able to start the engine, the flooded battery might be right on the cusp of being able to or not.
The AGM battery should have significantly less self discharge, andywhere from 1$% to 5% per month, where the flooded battery can be 15% per month, at 77f aveage temperature.
All batteries as they age, will self discharge faster.
Not All AGMS are built the same, Lifeline, Northstar and Odyssey are in a different league than anything sold in an Autoparts store in CCA, cycleability and self discharge.
The Greater price of AGMS, compared to flooded, does NOT make them a super battery capable of surviving the abuse that would kill a flooded battery. No battery enjoys being left at partial state of charge, The AGm is not more resistant to partial state of charge living than the flooded battery, but since it can still start the engine with the same level of sulfation caused by chronic undercharging, will seem to last longer, as an engine starting battery.
The slightly different recharging parameters of AGM vs flooded are not really large enough to make a difference, in an engine starting application. The ideal parameters being met are much more important, when the battery is intentionally cycled deeply.
When the AGM battery is Cycled deeply, it should not be 'trickle' charged, it should be fed a higher amp rate as high as 30% of the amp hour rating on the lesser $$ AGM and no maximum upper amperage limits on the higher$$ AGMs, although voltage should be kept below 14.8v or so
the ONLY way to absolutely determine full charge on an AGM is by measuring how many amps the battery is accepting at absorption voltages( 14.2 to 14.7v). When a 100Ah AGM battery can only accept 0.5 amps at absorptoin, only then can it be considered fully charged. Full charge cannot be determined when charging at lesser voltages. Sorry.
Since almost NO smart charger is actually asking for the Amp hour capacity of the AGM battery, it cannot know for how long in which to hold the battery at absorption voltage. Any 'Smart' charger is therefore simply guessing, as to how long it should hold the battery at absorption voltage. The green light does not mean it is fully charged, only that the charger has decided to stop charging the battery. No 'smart' charger has magical circuitry which goes inside and determines the battery state of charge or the battery state of health.
While many Flooded batteries can have their electrolyte dipped to see its density and then determine how chrged the battery is, AGMS cannot have this done, but lets face it how many here have actually used a hydrometer, much less onw which measures electroyte temperature, and has a baseline maximum specific gravity to compare the cells to, when recharging.
AGM's are overkill in many vehicles.
The fact that there are more and more AGM bateries marketed, does not mean these AGMS are all superior, they are still built for maximum profit, as opposed to maximum potential.
Compare the potential AGM to the same group size flooded battery, Is the CCA significantly higher? If it is not, it might not have the significantly less self discharge, might not recharge any faster, and its only advantage might be in reduced corrosion of the battery terminals.
As far as a car which came with a flooded battery not being able to recharge an AGM properly, this is pretty much BS. No car out there can perfectly recharge a battery. The voltage regulation of most every vehicle is intended to not overcharge a battery, and lesser voltages insure this is extremely unlikely, and guess what, this is what most vehicles will do.
BUt the AGM placed in a car designed for a flooded batery is not going to have any issues, unless the battery is going to be intentionally deeeply cycled.
AGMS will only offgass when they are brought to voltages well above 14.8v, when well depleted on a hot day , by a charging source capable of well over the 30 amps per 100Ah of capacity, and will push this battery over 14.8v with that much amperage. This is overwhelmingly unlikely to ever occur in a vehicle. The exact opposite is the issue. Too little a voltage is allowed, and the well depleted battery never reaches full charge, and sulfates prematurely.
It is the Voltage that determines how many amps flow. The 140 amp alternator being told to seek 13.8v, will only produce enough amperage to get system voltage to 13.8v.
If the battery requires 2 amps to be held at 13.8v, thats all it will accept.
What I would warm buyers of higher quality AGM batteries about, is depleting the battery very low, and then simply Idling to recharge. This can be extremely hard on the alternator, as the AGM battery can accept very high amperages when well depleted, and can overheat the alternator. Underhood airflow at speed wil help greatly to keep the alternator cooler, but obviously this is very vehicle specific, and a hot alternator might have the voltage it is told to seek, lowered greatly, and the lesser the electrical pressure, the less amps that can flow into the battery, whether well depleted or closer to full.
Anyone wanting to achieve excellent battery life would have a charging source capacble ot holding absorption voltage until the battery is indeed full. This requires an Ammeter in the case of an AGM battery, and a hydrometer with a flooded battery with removeable caps.
If one were to actually check the state of charge of a battery fresh of the best marketed whizz bang 32 stage smart charger out there, they will likely find the battery well below fully charged, especially if it is an older battery, or a battery that was unintentionally deeply cycled. Will the smart chargers recharge the battery to a good enough level, certainly, but 98% charged, is only 1/2 as good as 100% charged, in terms of achieving maximum longevity of any Lead acid battery.
If someone wants to pay the extra for an AGM, by all means do so, but do not believe that it is either going to blow up, or blow up the vehicle, or give any appreciable benefit over the cheapest flooded battery that will fit.
The absolute best lead acid battery kept chronicaly undercharged, will not last as long as the worst battery kept at full charge.
Since basically No one actually knows the actual state of chrge of their battery, and either assumes the vehicle always keeps it fully charged, or that their charging source's green light is not a liar, the reports of longevity, good or bad, based on the sticker on the battery, is unwise.
If someone is really wanting the best Lead acid AGM battery to use as a starting , the Northstar and Odyssey are several levels above the auto parts stores best deals, but these batteries are not immune to improper treatment, and if they are inadvertently discharged, and not recharged to full properly and promptly, then they are a waste of money lasting little longer than the cheapest starting battery.
Lifeline battery made by Concorde, are military grade AGM, but are more indended for Deep cycle applications. they do have more than enough cranking amps to be used as starting batteries.
http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf
Rolls Surrette are teh kings of deep cycle flooded batteries
http://rollsbattery.com/public/docs/user_manual/Rolls_Battery_Manual.pdf
The two links above are well written and contain all one needs to know about lead acid batteries, and will dispell a lot of the myths promulgated on the internet by people with little clue.
Odyssey AGM also have good information:
http://www.odysseybattery.com/documents/US-ODY-TM.pdf
Autopart store AGMS made by Exide, or eastpenn/deka johnson controls or perhaps some asian manufacturer, cannot be compared to the Concorde or Enersys made AGMs, unless they are to be treated equally poorly.
