Ahh........ the important Absorption duration question is asked.
And there is no right answer, other than ' it depends'
The duration at which absorption voltage should be held depends on the state of charge the battery is recharging from, the amperage at which it reached absorption voltage, and battery temperature, battery health, quality, resistance, phase of jupiter's third moon, and the vincinity of the nearest locust.
If battery X is discharged 65% by morning, and it slowly gets brought upto absorption voltage by noon, then Absorption voltage might only require 2 more hours.
With 3 group 27 UPG agms in parallel, when they only accept 1.5 amps, at 14.5 to 14.9v, they can be considered fully charged.
Once absorption voltage is reached, amps required to hold absorption voltage will keep dropping. How long they take to taper to 1.5 amps is a question only YOU can answer. I could guess. I'd likely be off by an hour and likely more, as this is dependent on the 'personality' of the battery bank.
Also this time requirement, is longer when the batteries have gone a few days without quite getting there, partial state of charge cycling. This Longer absorption requirement also happens as the batteries age, and if they do not get it the pace at which their capacity degrades faster.
So lets say your 3 group 27s are brought down to exactly 50% state of charge(SOC) right before sunrise.
Disregarding the Ctek+ EZ pass, and the particular ability of your alternator/ vehicular voltage regulator.
Lets say you start the engine, and 100 amps is theoretically going into the 3 batteries and that 100 amps continues until the battery terminals reach 14.7v.
This will likely take about 35 minutes minimum for terminal voltage to get this high. 100 amps for 35 minutes, and here is the absorption stage. Amps will start tapering from 100. How long will it take for amps to taper to 1.5a at 14.7v from this point? About 6 more hours, with healthy batteries. Not so healthy 7 to 8 hours. Unhealthy 9 hours or more, and perhaps it will never taper to 1.5 amps, just dissipating it as heat.
As the batteries age it will take less and less time for this 100 theoretical amps to get your 3 batteries to 14.7v, and the tapering slope is also stretched out.
Oner thing to note, Your compressor fridge is not a constant load. When the compressor runs it will draw 4.x amps, or whatever speed truckfridge wanted the danfoss compressor to run at, and it will likely only run ~20 minutes per hour, unless you just stuffed it with warm stuff, or have been opening the door dozens of times.
When the fridge shuts off near or at absorption voltage , then instantly there is 4.x amps less required for the solar to make to hold absorption voltage. The controller might not drop the 4 extra amps fast enough and voltage quickly rises well above 14.7v, then the controller cuts back and waits for voltage to drop again. This cut back and waiting for voltage to drop, MIGHT, on some controllers in some situations, trigger float voltage, prematurely.
Really even with something that is supposed to be fully automatic, unless the solar controller knows how much juice is going to loads and how much is going into the batteries, Absorption duration or threshold is very hard to do precisely, automatically.
Say float voltage is prematurely triggered, Someties a big load might need to be turned on to drop voltage below the programmed float voltage, then the controller 'SHOULD' go back into bulk, seeking absorption voltage, and holding it again as it should.
But then the fridge cycles on again, and then off. It it going to trigger float voltage again prematurely?
This is an issue that can defeat 'automatic' and turn it into premature sulfated batteries.
What to do about it really depends on how the controller handles the fridge or any larger load cycled off when the battery is near absorption or at absorption voltage in the amp tapering, constant voltage stage.
I do not know what other solar controllers other than my own do in this situation. I have my amperage threshold set at 0.4, at 14.7v. I programmed it for 3.5 hours once, I might have bumped this way up, because if amps taper below 0.4 then float voltage is triggered anyway.
Just checked and I have it set for 5.5 hours at absorption. Hoping there is enough daylight and than tapering to 0.4 amps triggers float instead.
Ideally one wants to get the batteries to absorption voltage ASAP, so that one has enough time to hold absorption voltage each afternoon as required. Once the solar is making well more than the battery requires to be kept at 14.7v, then there is extra wattage available to charge other things, like laptops, cell phones, and perhaps crank the fridge up to take advantage of the solar surplus, just do not turn on so much stuff that it drops voltage below absorption. The goal is to hold absorption voltage. Early in absorption stage there is not too much extra to be able to use, the longer absorption is held, assuming high sun still, then the more extra power one can choose to make use of.
If high amperage necessities are used in the morning, or that cell phone and fridge is turned up in the morning, then they will delay the achievement of absorption voltage, and there then might not be enough time to hold it until amps taper to 1.5a @14.7v. However long that may take.
If one wants to use the high amperage appliances in the morning, then consider running the engine during that event. This can make one achieve absorption voltage much earlier than asking the batteries to provide the entire load, and have a much better chance of having enough time for amps to taper to 1.5, or 0.5% of battery capacity(20 hour rate).
I'd not idle longer than 10 minutes from overnight cold, when maxing out the alternator powering big loads and feeding well depleted batteries. 220 F could be achieved in 10 minutes of idling maxed out. One of those IR temp guns is good to aim at alternator casing. Maybe it takes 20 minutes to reach 220F maybee 5, Maybee never?
This does sound like a lot of things one needs to keep track of possibilities to be aware of and attempt to mitigate, but keep in mind one only really needs to be vigilant on this if it has been several days without a true full charge, then I'd say do what is necessary to achieve absorption voltage well before noon on a sunny day. Amps might taper to 1.5 by 3PM, then great, but if they only taper to 2.0 by sundown, then 98% is good, but only half as good as 100% in terms of reinflating the battery back to full potential capacity.
Sometimes you just got to hammer the batteries. Its OK, ther are there to serve you, but just at some point in the near future, you got to hold absorption voltage as long as required for amps to taper to 0.5% of capacity, and if the batteries are really abused and worked hard, this can take longer than the day is long.
The person who gets a feel for, and overrides anythig automatic prematurely allowing float voltage, will achieve good to excellent life from their batteries, but Mediocre to good can be achieved without really caring if absorption voltage was geld long enough. Some setups might yield poor battery life if Absorption voltage is Never high enough, or held long enough.
Prevent premature float voltage. Be aware that a lard load cycling off in absorption 'might; cause the solar controller to prematurely trigger float voltage, and this will sulfate batteries while reassuring all is well as the batteries are in float, as float SHOULD, mean the batteries are full. Too often it does not.
Those with flooded batteries can use the hydrometer to determine how long absorption voltage is to be held in their particular usage. If the usage is regular, similar night to night, then one can notice the amperage at wich the batteries reach 100% specific gravity, and then program that time/ Amperage into their solar controller's end of absorption stage trigger.
Spaceman Spiff monitored water usage and determined he could use lower voltage for less time in his particular usage. If her were regularly discharging the batteries deeper, then he or his controller, might have to bump up Absorption duration considerably, and add water more frequently, but his batteries would still be retaining their capacity well.
AS I am not familiar with the ooperation of solar controllers other than my own, I do not know how well they handle the loads in regards to holding absorption voltage as long as required. Premature float is to be avoided. Hang around your voltmeter/ammeter when the voltage is approaching 14.7v, and again after the amps have been tapering for an hour. If it goes to float soon after achieving absorption voltage, it is likely Houston has problems. The exceptions would be a shallowly discharged battery very slowly brought to absorption voltage by limited solar wattage.
Also While I have been using my solar controller for 10 years, I was not so on top of things those first several years. I am not always there to notice when it goes from Absorption to float, but I know when See float voltage happening earlier in the day than expected, that if I flick on my headlamps for 10 seconds it will force the controller back into Bulk mode, seeking absorption voltage.
Sometimes one can intentionally add and or or remove larger loads when the solar has the batteries in or ner absorption, and see how it behaves.
One other thing, is other charging sources, in regards to the solar controller triggers to end absorption voltage. Back before I could control my Alternators voltage, it would regularly choose 14.9v on start up. 14.9v being higher than my solar controllers 14.7v, means that the solar controller basically goes open circuit, sends 0 amps to battery, and 0 amps is less than 0.4a, and what happened? Premature float voltage held the rest of the day when 14.7v was required for several more hours.
Now that I can control my alternator voltage, I limit it to or below 14.7v, so the solar in bulk or absorption soes not shut off as another source is holding voltage higher. My particular battery monitor would go from showingt 78% tro 100% and 0AH from full every time the vehicles vltage regulator achieved 14.9v.
So here is another possible false trigger for the despicable premature float voltage scenario where 'automatic' comes back to sulfate lead acid batteries.
