Voltage = electrical pressure.
higher pressure, more amperage flow into a depleted battery, in most instances.
The battery itself, determines how much amperage it will accept at the voltage the charging source is able to deliver to its battery terminals.
The higher the state of charge, or the less healthy it is, the less amperage will be required to bring it to 'absorption voltage', generally 14.2 to 14.8v.
The bigger the battery, and the more depleted it is, and the more healthy it is, the more amperage it can accept, for longer, especially when the charging source is not only seeking to bring the battery to mid 14 volts, but hold it there for a few hours.
So one can put a 100 amp charger on a 98% charged battery and it will still only require 2 to 3 amps after 5 seconds to hold the battery at 14.5v, and the amperage required to hold the battery at 14.5v will keep declining, and at some point stop declining.
Lifeline batteries, basically top dog deep cycle AGM batteries, say when a 100 amp hour battery can accept no more than 0.5 amps when held at 14.4v, it can be considered fully charged.
Lifeline also says that in deep cycle duty, when discharging the battery to the 50% charged range, that the charging source needs to be able to deliver no less than 20 amps per 100 amp hours of capacity.
There is reports all over the web, on Rv and boating forums, where they use primarily solar to recharge their batteries, their hyper expensive top of the line Lifeline AGMS, only to have them obviously capacity compromised in far too short a period. They whine and scream and complain, not realizing that their solar is not only well below a 20% charge rate, even at noon, but that slow ramp up in potential wattage, as the sun rising towards solar noon, is inadequate to meet the requirement that the battery be hit with higher amperage at its most depleted state.
Odyssey AGM states no less than 40 amps per 100Ah of capacity when their batteries are deeply cycled.
The AGMS marketed to the automotive starting battery world are no anywhere near the quality of Lifeline or Odyssey, and their price reflects that. Their CCA and reserve capacity is often poorer than their flooded counterparts.
Most lower$$ AGMs will recommend limiting amperage to 27 to 33 amps per 100Ah of capacity. Somewhere in the 30% range.
Note that a healthy AGM unintentionally depleted and jumpstarted, will easily accept more than this from an alternator, yet no mushroom clouds.
The AGM battery never discharged much will never approach 30% acceptance for more than a second or 5 after engine start up.
Since I deep cycle both some Deka intimidators, and Northstar AGMS, and have amp hour counters, and ammeters that not only display charger output, but also how much of that amperage output is powering loads vs what the battery is accepting, and ALL my charging sources have adjustable voltage, a voltage I choose, I get to experience the performance of batteries on a deep level.
It is easy for me to see when a battery is performing less well.
Its voltage retention, for amp hours removed, under X amount of amperage load, is less than I am used to seeing, means the battery is less than happy.
At 28 amp hours from full, under a 10 amp load, I might see 12.34v instead of 12.55v. Removing all loads and voltage might rebound to 12.55 instead of 12.72, as expected.
This poorer voltage retention, always occurs when the deeply cycling battery is either not cycled at all, just had been held at float voltage by my adjustable voltage power supplies, or when deeply cycled and it is only returned to full by low and slow solar, many cycles in a row, and or when deep cycling and there is not enough time, at absorption voltage, to get the battery beyond 96% charged.
The biggest detractor in performance, is when the battery is cycled and only returned to 95% charged, time after time. When this happens that 10 amp load, even when 95% charged, will have voltage drop instantly to 12.2v, and just in general act like a much physically smaller battery.
At this point, simply plugging in and holding the battery at 14.5v until amps taper to very low levels does little to nothing but reset my battery monitor to full, but does nothing for voltage retention during load. Its kind of like expecting to run 11's and only getting 13.5's. Very obvious, to me.
In order to return the expected/ hoped for performance, I need to discharge the battery to ~ 1/2 of its amp hour rating, and then apply my high amperage charging sources.
In some instances, usually when My Northstar TPPL AGM discharged below 50%, 40 amps quickly raises voltage to 14.4v, but than at a constant 40 amps, the maximum output of my one power supply, the voltage will drop to 13.9v, and then again start climbing towards 14.4v over the next 25 to 35 minutes.
I then hold that 14.4v to 14.8v, for as long as it takes for amperage to taper to the 0.4 range, at which point I lower voltage.
With my Deka Intimidator AGMS, I've noticed similar, but once they get back upto 14.4v at 40 amps, lowering the voltage to 14.0, does not lower the amperage required to hold them at 14.0v much if any, where the Northstar will require ~ 25% less amperage to be held at 14.0 vs 14.4, and so there is little benefit to holding these particular Deka AGMS at 14.4v, vs 14.1v, but it is important to get them to 14.4v before lowering the pressure.
So in Deep cycle duty, one should have a high amperage charging source, to high amp recharge from the battery's most depleted state. If applies a high amp potential charger to a battery at a high state of charge, the battery will not be able to accept the higher amperage, even if the charging source is seeking and holding 14.8v. Higher voltage will allow only slightly more amperage to flow, and if the charging source is constant amperage with out voltage limitation occuring then the voltage will be pushed well higher than 14.8v, at higher states of charge.
The automotive starting AGM is far different than an AGM in deep cycle duty, and the SLI( starting lighting ignition) AGM marketed to the automotive crowd, is built far different than a Lifeline or Odyssey TPPL AGM.
Beware of marketing, as many AGMS and Marine flooded batteries claim to be deep cycle batteries, but really cant handle lots of deep cycling. The Deka intimidator AGM is not rated by Deka themselves, for may 'lab' cycles, which never occur in real life, yet the bitog reader might be led to believe this battery can walk on water.
I do feel that an AGM battery, kept in the 80% to 95% charged range, as the voltage regulation of many modern vehicles intentionally seek, will benefit from an intentional deeper discharge, drawn to the 11.8v range, in a few hours or less, then immediately hit with no less than 20 amps per 100Ah of capacity and then held at 14.4 to 14.8v until amperage tapers to very low levels. It might benefit from doing this twice intentionally. It will not benefit from this if done daily.
This low level of amperage threshold, when brought to and held at absorption voltage from the 50% charged range, will take no less than 5.5 hours, and might take 24 hours, or more. The charge logic many chargers apply, is largely just a one size fits all. They might hold absorption voltage for X amount of time, or they might hold absorption voltage until amperage required to maintain that voltage falls to x.x amount, or they might hold absorption voltage until the amount of amperage required to hold absorption voltage only declines x amount of X amount of time.
When one hooks a battery to a charger and the charger says 80%, this is just voltage based. One can unplug the charger 5 minutes after starting it, from AC and DC, then re apply it it will likely show 100% and instantly switch to float voltage. One must be far smarter than their smart charger.
Most chargers marketed to today's automotive consumer, know their customers are not going to monitor the charge profile, and really only care about the green light. The green light means the battery is charged enough to start the vehicle, but it does not mean the battery is truly fully charged. In most cases the green light only means the charger has switched from absorption voltage, to float voltage.
Many chargers get totally confused easily. The simple act of opening the vehicles doors triggering dome lights and other DC loads, when it is absorption stage, will have the amperage required to hold absorption voltage rise, but then fall once the doors are closed. This amperage fall triggers float voltage, and the charger owner sees the green light and assumes the battery is full when it is only actually 89% charged.
HOw any specific 'smart' charger responds to loads placed on the dc system, then removed, is going to vary wildly.
Ideal is not subjective, but 'good enough' certainly is.
Ideal is a battery returned to truly fully charged, and kept cool.
With AGMS and arguably, with flooded, in deep cycle duty, then the charge rate also plays a part in achieving ideal.
'trickle' charging a deeply cycling AGM will tickle it to a premature demise, despite how many times one reads abut how awesome trickle charging is, despite the term 'trickle' having no set value of ampergae, much less what amperage at what pressure. Its as if the word 'trickle' releases serotonin to the person quoting grandpa.
One can put a huge amperage charger on a sulfated AGM or one at relatively high states of charge, and it will never accept large amperages, even if the charging source allows 17 volts, which it never should.
I am not advocating for high amp recharging each and every deep cycle, nor am I advocating that the average auto enthusiast regularly and frequently deeply discharge their AGMS and high amp recharge them.
I am however advocating that the automotive AGM that is kept in the 85% to 95% charged range, will benefit from the owner draining it relatively quickly to the 50% charged range, then immediately applying no less than 20 amps per 100Ah of battery capacity, and then once the charger has gotten it to the 14.4 to 14.8v range, hold it there until amps stop tapering, or taper ot 0.5 per 100Ah of capacity, and only then lowering it to the usual AGm float voltage of 13.6v.
MOst smart chargers will not do this, most smart chargers do not have enough amperage to meet 20%, much less Odyssey's 40% recommendation, and most consumers will never ever be able to tell the difference in performance, whether their battery maintains 11.6v starting their overnight cold engine, or 10.2v.
Most will only know when it starts cranking slower on that cold morning start, likely only when it is falling below 9 volts, and only then start putting a plug in charging source on the battery. But by then it is far too late to return performance with an intentional deep discharge followed by a high amp recharge and then holding absorption voltage for as long as it takes for amperage to taper to low levels or stop tapering, at absorption voltage.
It might help, it might not, but it should buy some time.
Both of My NOrthstar AGMS, would not hold a 13.0v+ full charge resting voltage, until I cycled them and recharged them with 25+ amps from that well depleted state. The voltage they could hold when cranking my overnight cold engine also improved after the deep cycle high amp recharge. I've not tested other AGMS in this regard, but feel a good lung stretching is beneficial, just dont discharge much below 50% and leave them there, dont slowly drain them to below 50% over days, and dont think trickle charging from this low state of charge is beneficial to them.
Most readers here, their largest potential amperage source is their alternator, but the alternators amperage output is determined by the field curent sent to its rotor by the voltage regulator.
Vehicles voltage regulators are as imperfect as smart chargers, in achieving ideal lead acid battery recharging, though by reading comments here some seem to think that they are.
Those who want to be able to accurately gauge the performance of their battery, and/or their charger, need to see how much amperage is flowing to or from the battery, at the electrical pressure measured at the battery terminals.
For most, the only thing that matters is whether the vehicle starts, and for how long.
That so many batteries last for as long as they do is not a testament to the vehicles charging system, but more that modern fuel injected engines start so easily, the battery can be severely degraded, and still easily start the cold engine, in most of the lower 48 anyway.
Cold reduces available capacity and cranking amps of a battery, as well as increasing the amperage it must provide to the starter, but cold is not the battery killer. Heat and chronic undercharging/ living at well less than 100% charged is.
With AGMS, deep discharges followed by low amperage rate recharging, is also detrimental, more so than their cheaper flooded counterparts.
Unfortunately most people seem to believe AGMS are super batteries, immune to abuse and far superior, when in reality they are finicky princesses apt to pack up all their shoes and stomp off home, if not treated like a princess.