Ditto!
Yes, there is some literature and some basis for allowing a rapid charge at the start, especially when depleted.
AGM lead acid has a low enough impedance that it will allow a pretty substantial current inrush. In a range from, say 20-80% SOC, it can be done.
But here's the thing... Remember that the Voltage to charge the battery is:
Vcharge=Voc + I*R
Where Voc is the open circuit voltage rested and disconnected at chemical equilibrium, and I is the current, R is the battery DC impedance.
So rearrange algebraically, you get:
Vcharge-Voc=I*R
Or I = (Vcharge-Voc)/R = I = DV/R
Which is consistent with Ohm's law.
So what happens is that for a given R, the bigger that you want I to be, the higher DV (Delta V) needs to be.
So what happens is that you need to drive higher and higher voltage to get more current in.
Look at the Odyssey 65-PC1750. It claims to be a 2 mOhm (0.002 Ohm) battery. The short circuit voltage is 5000A, and if I=V/R and we assume a depleted conduction voltage of 10V then 10/0.002 = 5000. That's good, it means that we have a DC impedance value, not a 1kHz AC impedance (for which a DC resistance is 2-3x higher).
Lead acid batteries have a changing impedance with state of charge, with higher impedance at low SOC.
(source: battery university).
What does that mean? It really means that a higher impedance is found at low SOCs than the nameplate.
So let's do an example...
If we want to get 20A into that battery above with 4mOhm impedance when it is discharged to 11.5V, we need to get:
Vcharge = 11.5 + 20*0.004 = 11.58V
Not bad, huh?
Yet youll find that chargers do a few things... First, they will PWM the input. So if youre talking 20A in, it may well PWM a 100A circuit 1/5 of the time. What happens then? Your 0.08V charging force becomes 0.4V, and the 11.5V battery is seeing 11.9V for a short time, though their chemistry doesnt necessarily want that. Smaller chargers likely can't do that, but you have to see.
Beyond that, what kind of rectifier is used? Some DC waveforms can look like this:
(source electtonics-tutorials).
Now imagine where this puts us... PWM plus some really lumpy DC that has a peak that is much higher than RMS.
Where does that put us?
It creates the opportunity for some really high intermediate voltages, with some really high voltages on the double-layer capacitance inside the cell, because the chemical reaction cant occur fast enough, especially in electrolyte starved batteries like AGM, and especially if the electrolyte starts to be lost due to use. Remember that lead acid batteries can and will polarize which means that the electrolyte can be starved of reactive species. Continued loading or forcing of charge has to go to the electrolyte, because there isnt enough material in the reaction to proceed as desired. The practical and observable embodiment of this is exactly why lead acid batteries can have very high capacity in Ampere-hours at a 4-10 hour constant current discharge, but can have a small fraction of that capacity if discharged at a proportionally higher current for just a few minutes. For example, look at the Odyssey PC 310 battery. Over 20 hours, you can pull 0.4A for 5W, and get 8.6Ah. At a discharge rate of two hours, 10% as long, you can pull 3.2A for 41W and only remove 6.5Ah of the capacity. Over 15 minutes, roughly 1% of the duration of the 20 hour experiment, you can pull 19A for 236W, but you can only retrieve 4.8Ah!
That's real, and practical... See it here, page 4:
http://www.odysseybattery.com/documents/US-ODY-TM-002_1214.pdf
And the same is the case on charge. There is only so much material available for the chemical reaction. The system is not in equilibrium. But the issue is that under charge, you are at even higher overpotentials, which means that you move asymptotically up on the gassing rate for decomposition of water, and the low overpotential of the lead reactions arent occurring fast enough to compensate.
So consider that chargers will often put out a current-limited but higher voltage than is required. Remember that water will decompose at any voltage above approximately 1.22V, and these fixed current chargers, putting out >13V on a depleted battery, where the >13V is really RMS, and the peak is higher, corresponding to the PWM current, means that there is a lot of opportunity for corrosion and gas evolution. Especially when the charging overpotential required is really low, and so the lead can react and go back with a much lower voltage, and there is just a ton of excess voltage and current.
This is why AC ripple, clean, stable output, and other considerations are so critical from the power electronics side of a charging system.
A utility UPS that can get 15-20 years out of monoblock AGM batteries under climate controlled storage does so because there is negligible ripple from the 12+ pulse rectification (plus filtering) coming from three phase utility power off a zig-zag transformer).
Have you put your charger output through an oscilloscope to see how it behaves? Keep in mind, Im not talking pulsed charging of the battery, Im talking PWMing of the output current from a fixed source to achieve a certain output.
Keep in mind also, that these things are not magically free to do what you want. Most vendors recommend ensuring that a temperature rise of over 10C is avoided. It can be real tough to measure this, especially since the inside of the battery is where it really matters. Many also recommend bringing them back carefully, limiting charge to say, 5% of their 10 hr rate, and limiting temperature rise. example:
https://www.cdtechno.com/pdf/ref/41_2128_0212.pdf
So my point after all this rambling is that you may think youre doing the best possible thing for the battery, following guidance, etc., and actually not doing well for it at all, either due to ripple, dc waveforms, temperature rise, etc. This can cause gassing that burps from the battery, plate corrosion, dryout, and even thermal runaway if sustained. Not saying youre doing any of those things, but you do need to be a bit careful. Not all chargers or power electronics necessarily do the right thing or exactly what an RMS (or refresh rate) meter claims...
And did you register and code the AGM battery to the car???