Duracell AGMs are made by East Penn. They are a rebranded Deka Intimidator, which is one of the most rebranded AGMS in The US.https://remybattery.com/intimidator-8a31dtm-battery.html
Practical sailor magazine did a PSOC test with several different AGMs.
Partial state of charge cycling means starting the next discharge cycle before a true full charge is reached.
Of the AGMS tested the Intimidators did the worst, by far, in this regard.https://www.practical-sailor.com/systems-propulsion/fighting-sulfation-in-agms
I bring this up not to say your batteries are crap, but to let you know how important it is to actually achieve a true full charge with them, to a higher degree than some other AGM's.
All AGMS seem to behave poorer than their cheaper flooded/wet brethern in PSOC duty, making them the poorer choice when PSOC cycling is unavoidable. Many people seem to think their higher price makes them the better battery always, but I call them a princess battery, in that if they are not coddled they will take their ball stomp their feet and go home.
But 12v flooded Marine batteries, which proudly state deep cycle, are not deep cycle. They are usually quite poor at PSOC cycling and lose capacity quickly.
The most resilient and best bang for the buck lead acid battery, is the 6v golf cart battery, but they are 11 inches tall, where a group 31 is ~9.5 inches.
Anyhow, the East Penn Intimidator AGM lineup has different charging specs than some other AGMS.https://www.eastpennmanufacturing.c...able-Energy-Charging-Parameters-1913.pdf
The Guest marine charger is quite $$$. I've no experience with it, so no comment.
While Lifeline AGM says the higher charging current the better in deep cycle applications, and Odyssey TPPL AGMS say no less than 40 amps per 100Ah of capacity in deep cycle duty, the Deka Intimidators say no more than 30 amps per 100 ah of capacity.
Most lower end AGMS say no more than 30% charge rate.
I generally ignore this 'recommendation' at least while the battery is newer. the danger is excessive heating, and I've hit smaller Asian made 30% AGMS with 150% charge rates and usually settle in at 60% without issue, but the older one does heat a bit more than the newer one and I will soon restrict the amperage it gets fed and monitor it closer for excessive heating
I have a group 31 Northstar AGM which is similar to Odyssey, and when newer and well depleted it was able to easily suck up 134 amps of combined plug in charging sources. My previous Northstar-27 got deep cycled ~1200 times over 6 years , and a big part of that was not just insuring it reached true full charge often, but that after many low and slow charges I depleted it well and then blasted it with high amperage.
Anyway, Its hard to really quantify the remaining capacity of a battery without actually performing a 20 hour load test, which in itself is abusive to the battery, and quite difficult to perform accurately.
The General method to not overdischarge is to not go below a certain voltage. The voltage under load will be lower than when the load is removed though. so using the laptop and seeing 11.9v, then turning off the loads and voltage rebounds to 12.2v in 15 minutes means you are in the 50% range, and taking lead acid batteries below 50% is not recommended. When taken lower than 50% reaching true full charge afterwards becomes even more important and tickling them with 'trickle' currents, when drawn this low is not beneficial, it is detrimental, but not as detrimental as leaving them discharged or depleting them even further.
Upgrading to a higher rated alternator, can often be a fools errand. Often the higher rated alternator can only achieve this rating when spun at rpms that the engine, in normal use, will not allow. The higher rated alternator often sacrifices low rpm output for that higher total rating. There's a whole bunch of dishonest claims by high output alternator manufacturers.
Low rpm output is highly desirable, but also comes with its own can of worms, when one Idles to charge.
A depleted healthy battery can ask for a LOT of amperage, and the voltage regulator, which controls the alternator output, fully fields the rotor, and the alternator can generate tremendous heat.
I wanted to know just how much, so I set up temp sensors. The short version is that fully fielding the alternator under 25mph allows it to quickly shoot towards the 220F danger zone, where fully fielding it at 65mph and it struggles to get to 140f. Underhood airflow is that important, but of course varies depending on the vehicle.
If you need to recharge, when parked, do not do so for more than about 15 minutes, and don't really expect more than 50 amps from the stock alternator idling when hot, and expect 8 to 12 of these amps to be eaten up by fuel pump and ignition.
The Voltage regulation is also a huge consideration, and in most instances plays a much larger part than the alternator's rating. The VR varies the field current sent to the rotor to allow it to make just enough amperage to seek or maintain a certain voltage. There's all sorts of VR strategies, and almost NONE of them are actually good at recharging a deeply cycled battery. After startup they might goto 14.4, but once they get hot they will drop to 13.6v range.
A depleted battery with 14.7v reaching terminals will accept 3x more amperage than it would at 13.6.
Driving or idling to recharge can be frustrating, when the voltage regulator has decided that 13.7v is fine and dandy.
13.7v is likely fine and dandy, for a starting battery depleted 0.05% by starting the engine, but not a marine battery drawn to 50%$ by a laptop.
So voltage regulation is often far more important than the alternator's rating, in terms of charging depleted batteries. Obviously an alternator with excellent cooling and high output at low rpms is ideal, but if the voltage regulator controlling it is only seeking 13.6v, then a lower rated alternator whose voltage regulator is seeking 14.7v will charge more effectively, except perhaps at hot idle or low engine rpms.
There are DC to DC converters which step up that 13.6v to 14.4 or so for faster and more complete recharging of depletedhouse batteries, but these are $$$ and generally limited to 20 40 or 60 amps.
My personal strategy, aided by Dodge using exernally regulated alternators, was to bypass the VR inside the engine computer, and use an adjustable voltage regulator. I modified it to be adjustable via a knob on my dash, and I can and do get the full expected 50 amps at idle from my 50/120 chrysler alternator and 120 amps, but at 2350+ engine rpm, and at 65mph, my engine turns 1950 rpm.
Basically, when I drive, I can almost always instantly achieve 14.7v, even if the battery needs 80 amps to achieve that voltage instantly. Idling, I have 38 amps max for battery charging from the one chrysler alternator, and it gets so hot so fast that I avoid idling just to charge, or limit it to no more than about 10 minutes, which does not really accomplish all that much. A VR seeking only 13.7v would not heat up as much, but also charge 3x slower.
My system is under consistent modification. My 'lifetime warranty' chrysler alternator had failed on a cross country journey, and Oreilleys claimed to have no records of me ever having bought an alternator from them. I bought a new Nippondenso alternator from AZ, as I was told its dual internal fans would be better at hot idle speeds in terms of output. Perhaps an original ND alternator would have been better, but not my Made in Malaysia one. Its hot idle speed output was way worse, and it would max out at 109 amps. The rpms just over idle to that 109 limit were pretty good.
When I took apart the failed alternator I saw the previous rebuilders just screwed up the brush alignment, and remedied that and returned it to service.
Last december I replaced my useless AC compressor with teh ND alternator. I also found the original alternator location was 6mm out of parallel alignment with teh crank pulley, and spent considerable time and effort getting that alignment within 1mm.
I've still not activated that alternator, but soon.
I got some used Deka intimidator 6v GC-2 AGMS from a boat for free. I modified my dual 27 underbody battery tray with a taller hatch lid to accommodate these taller batteries.
Soon,( relatively) I will have the chrysler alternator feeding the G-31 Northstar under the hood, and the ND alternator feeding the Deka's under the floor behind drivers seat with the ability to combine their output into one or the other.
I don't really require this ability at the moment.