98% charged is only half as good as 100% charged, in terms of achieving excellent/ maximum LEad acid battery longevity. AGMS are lead acid, so are flooded and even gel batteries which are quite rare in automotive applications, unlike the term gel cell which erroniously gets applied to any sealed battery by many.
An AGM battery can ONLY be determined to be fully charged, when it can accept 0.5% or less of its amp hour capacity, and only when it is held at absorption voltage of 14.4 to 14.7v, assuming a battery temperature of 77f. Colder batteries require higher voltages, hotter batteries require lesser.
Meaning when a 100AH AGM battery can accept only 0.5 amps or less, as 14.4v to 14.7volts, can it be considered fully charged.
FULL charge CANNOT be determined at lesser voltages, on AGMs.
The green light on an automatic smart charger, unless it actually asks for the AH capacity of the battery, cannot determine full charge, and the anthrope who believes the green light actually indicates the battery is truly 100% fully charged, is deluded, delusional, misguided, ignorant, and a bunch of other unsavory words, at least on this topic.
Does anyone check the voltage at the battery terminals, and see how many amps are flowing to maintain that voltage? Anybody? Anybody ever double check the accuracy of the ammeter provided on some of the chargers? can one really determina the difference between 0.9 and 0.5 amps on an analog ammeter?
My friends old school Schumacher indicated 8 amps, I measured 3.6, when it measured less than one amp I measured 3.2. Just one example.
Also AGMs, if they are regularly depleted well below 80% state of charge, will be tickled to a premature demise by the usual 'trickle charge it overnight' mentality that pervades the internet and any mechanic within 20 years of possessing gray hair.
AGMS, if cycled deeply, appreciate higher charging currents, upto 30 amps per 100Ah of capacity on lesser quality AGMs, and some of them have NO upper amperage limit, only a voltage limit. 100 amps might take a few minutes to get the high quality Odyssey/Northstar/Lifeline depleted AGM battery to 14.7v but they will be happier than a pork in feces to get this much amperage when well depleted, and even happier if the proper absorption voltage is held until amps taper to 0.5% of capacity, which might take 4 more hours, or it might take 14 more hours. No AMMETER= NO ability to know if the AGM is fully charged. No ability to bring the battery to absorption voltage and hold it there while checking the ammeter...well hope you are there monitoring voltage when our charger decides that absorption vltage has been held long enough, or can trick it into re achieving absorption voltage by restarting it, but one will likely have to load the battery to lower its voltage to below 12.8 before trying to force it to restart and reseek and hold absorption voltage. See how smart smart chargers are? you have to trick them into doing what they claim they can do automatically. Unless a green light is all that is required for warm and fuzzies to ensue.
Keep in mind the battery capacity is always diminishing and cars these days can still start when teh battery has lost a considerable portion of its capacity. Especially AGMs with their inherent lower resistance.
The rate of capacity loss increases the lower the average state of charge of the battery, increases with average temperature, and increases the longer the battery ls less than 100% charged, and increases the lower below 100% charged it is and the longer it remains there.
If you can keep an AGM cool, if you can feed it higher amperage when it is well depleted, IF you can hold absorption voltage until amps taper to 0.5% of capacity, such an AGM will have a phenomenal service life.
YOur alternator can never meet this ideal, your smart charger cannot either, no matter how much you believe the marketing BS it came with or how much faith you have in your purchase.
But deplete AGMs regularly to the 50% charged range, tickle them to death with low amp trickle chargers, keep them perpetually undercharged, well then you wasted money buying an AGM, as AGM are no more tolerant of this abuse than Flooded batteries and are likely impacted more so by chronic undercharging than flooded. The thing is that since AGM's have lower resistance they will still have acceptable CCA available to start the vehicle once the battery is in poor condition as it takes so little of the battery to actually start most fuel injected engines.
I'm usually urinating into the wind on this topic on this forum, I should likely quit bothering to dispel myths regarding Lead acid batteries and lead acid battery charging, as the power of marketing and the color and words on a sticker, can seemingly overpower physics and reality in this day and age, and alternators are apparently seen as magical instant battery rechargers despite the fact that their voltage regulation is designed around never coming close to overcharging a battery.
But that said they are only batteries, and only rented, so do as you wish with whatever gives you warm and fuzzies.
If you are hoping to achieve excellent/ maximum possible battery longevity, you need an ammeter, a voltmeter, and a charging source which can be forced to hold the correct absorption voltage for as long as it takes amperage required to maintain that electrical pressure a the battery terminals, to taper to 0.5 amps per 100Ah of battery capacity or less.
Or just ignore it nad buy a new one when you hear the dreaded click click click. I hope you get your moneys worth. It is likely cheaper than the ammeter and voltmeter and the charging source capable of holding a proper temperature compensated absorption voltage for as long as required to reach full charge.
Any charging source applied to a less than 100% charged Lead acid battery is better than no charging source, but do not be deluded by modern charger's marketing mumbo jumbo(lies and deceit) or Grandpa's 'trickle charge it overnight' recommenation as being the end all be all cure all for lead acid batteries, especially AGM, and especially AGMS that are well depleted, intentionally or otherwise.
