(You call THAT pedantic? Nah. Now THIS is pedantic!)
I haven't been around much, and it's been a while. When I first read this I was in a hurry and glossed over it, and besides - it's Diplo so there really isn't any need to double check his posts - he's generally right.
But...
What the hell - it's been a while since I reviewed the docs on these things, so I guess now is a good time to get caught up.
-- AGM batteries want a minimum charge rate of 25A per 100Ah of battery bank and can take a rate of up to 500A per 100 Ah. (Lifeline battery spec.)
As I recall, most AGM manufacturers spec C/4 as MAX charge rate. (for those that don't know; C means total battery or bank capacity; so that formula is "capacity divided by four"). Which would make 25a per 100ah the MAX.
I seem to remember Lifeline used to spec C/2 max (which made them different than the rest), but looking at their latest manual, I see they state their battery can handle C*5 "in-rush current". That might be a bit misleading, since "in-rush current" normally means a short spike (momentary startup surge load) and not a sustained current level and they don't specifically say that their battery can handle a sustained C*5. I wouldn't use that kind of current without calling them first and asking.
http://www.lifelinebatteries.com/manual.pdf (bottom of page 98)
On the next page they say: "For repetitive deep cycling applications (deeper than 50% DOD), chargers should have an output current of at least 0.2C (20 Amps for a 100 Ah battery)."
"should" have...
So that'd be C/5 (recommended, not absolute) minimum charge rate, C*5 (absolute) maximum. Presumably, if the DOD doesn't regularly exceed 50% a smaller charger (such as a small solar rig) could be used without ill effects.
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The Odyssey tech manual:
http://www.odysseybattery.com/documents/US-ODY-TM-001_0411_000.pdf
Says:
"To properly charge your premium ODYSSEY®battery, EnerSys has developed a special charge algorithm. It is designed to rapidly and safely charge these batteries. Called the IUU profile (a constant current mode followed by two stages of constant voltage charge)," (page 15)
and
"Table 4 shows the minimum charge currents for the full
range of ODYSSEY batteries when they are used in deep cycling application. When using a charger with the IUU profile, we suggest the following ratings for your ODYSSEY battery. Note the charger current in the bulk charge mode must be 0.4C10 or more." (page 15)
Their "IUU Profile" is just CC bulk to 14.7v, then CV absorb at 14.7v, then CV float at 13.6v. So that would be (0.4 * C10) or (.4 * 100a = 40a) minimum charger per 100ah for an Odyssey - FOR THE IUU PROFILE.
They do state LOWER amp numbers than that in their table of recommended minimums for various batteries on that same page. Their Table 5 on page 16 shows recommended charge times for I guess all their batteries - and that table ONLY shows charge times using 10a and 20a chargers.
Odyssey does define what they mean by in-rush at the bottom of page 17: "Inrush is defined in terms of the rated capacity (C10) of the battery. A 0.8C10 inrush on a 100Ah battery is 80A."
Now, all that's from the Odyssey Tech Manual. The Owner's Manual also has some interesting data:
http://www.odysseybattery.com/documents/US-ODY-OM-011_0213.pdf
In particular, on page 6: "If a standard automotive charger is used to boost charge a discharged battery because of an accessory left on, it is important to make sure the charging voltage does not exceed 15 volts during charge. A hand held voltmeter can be used to monitor this periodically. The following chart provides recharge times under this type of boost charging to an 80-95% recharge and then allows the vehicle charging system to complete the charge."
And then they show the same charge times chart that's in the Tech Manual.
So...even though they say 40a per 100ah is the minimum, it really isn't. It's the "preferred" or "recommended" minimum, but you can still get the job done with a smaller charger. In other words - there's more than one way to skin a cat.
Also, I know I read somewhere (or maybe calculated) that the Odyssey MAX rate is C*4, but now I can't find it. If someone could point out where that data is I'd appreciate it.
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According to the Trojan Users Guide, Diagram 6 on page 14, the recommended AGM charge profile does bulk stage at "20% C20".
http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf
[Whatever the hell that means. Actually, what these C10 and C20 numbers are *supposed* to mean, is the amp*hour capacity at different discharge rates. Most deep cycle batteries are rated in amp*hours at the 20-hour rate. I.e., C20. So, when the Lifeline and Odyssey guys say the charge rate should be a percentage of C10 what that means is that they are basing that on the 10-hour rate, which sucks since they usually list their products amp*hour capacity at the 20-hour rate. Ergo, 40% of the 10-hour rate sure ain't the same as 40% of the 20-hour rate.]
When Trojan says C20, they mean the capacity at the 20-hour rate. In which case, 20% of that would be 20a per 100ah (recommended) charge rate. In other words, C/5. I don't see anything in that manual about minimum or maximum charge rates.
On their web site battery maintainance page, they say this (Under the Heading Charger Selection):
"When selecting a charger, the charge rate should be between 10% and 13% of the battery’s 20-hour AH capacity. For example, a battery with a 20-hour capacity rating of 225 AH will use a charger rated between approximately 23 and 30 amps (for multiple battery charging use the AH rating of the entire bank). Chargers with lower ratings can be used but the charging time will be increased.
http://www.trojanbattery.com/tech-support/battery-maintenance/
They make no distinction between flooded and AGM there, but further down they do make a distinction when talking about recommended charge voltages.
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Optima charge specs are all over the place (thereby proving there is more than one way to skin a cat):
http://www.optimabatteries.com/en-us/support/battery-care/charging/
But basically - 15.6v max and no amp limit as long as you don't overheat the battery. That's max. For normal day-to-day use 14.7v and no amp limit as long as you don't overheat.
They don't specify minimum, but they recommend a 10a charger repeatedly on that page, and the biggest super-whoopty high-speed low-drag Optima branded charger is 12a.
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Rolls (Surette) manual says:
"AGM Bulk Stage
The charger should deliver the initial current B until the voltage limit A is reached. Absorption Stage–the charger should maintain the voltage A until the current tapers to B. The initial charge current is recommended to be set at B = 0.25 X C20 (Imax = 0.35 X C20) in order to fully charge the batteries within a reasonable amount of time. It can be set lower, however; please be aware that charge time will increase so make sure the batteries have enough time to fully charge before being put back into service."
http://rollsbattery.com/public/docs/user_manual/Rolls_Battery_Manual.pdf
So Rolls is recommending C/4 or 25a per 100ah as their recommended charge rate for AGMs, and 35a per 100ah as the MAX charge rate.
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Specs on the UPG 8D AGM:
http://upgi.com/Themes/leanandgreen/images/UPG/ProductDownloads/45964.pdf
Show 250ah @ 20hr rate, and 75a max charge current which would be about 33.3a per 100ah MAX but at the bottom they list Max Charge Current at 0.30C.
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So...Diplo's statement quoted above applies to Lifelines, and is close enough for Odysseys. But 25a per 100ah is a recommended minimum and you can still get the job done with a lot less than that. And whatever you do - DO NOT assume that just because a battery is AGM it can handle C*5 or 500a per 100ah capacity. Lifelines and Odysseys...yea, okay. But you pump that kinda current into most AGMs and it's probably gonna blow up in your face, kill your dog and make Baby Jesus cry.
[And that, boys and girls, is truly pedantic. 
)
