The "total amp hour" figure is not really important to the charger's rate of charge because the charger needs to determine when to change from bulk to absorption stages.
When charging a battery with a certain amount of current, the battery's voltage changes at a particular expected rate (regardless of the capacity, all types of lead acid batteries do this - wet cell, gel and AGM). When the voltage reaches a certain threshold, the charger can switch modes.
Specifying an upper limit of capacity is partly to do with the rate of charge required to properly charge the battery, and partly because you want the battery charged within a certain amount of time.
The lowest charge rate: gel batteries. Typically DC (Deep Cycle) Gel batteries can only be charged at C/10 (capacity divided by 10). A 90Ah Gel battery should be charged at a maximum of 9A per hour. The D250S is capable of charging at 20A per hour, so is less suitable (although it can determine that it's a gel battery and charge it appropriately). Gels are special: charging them too fast causes bubbles to form in the gel. That's doom for the battery.
Flooded cell DC batteries come next, and can usually be charged at around C/5. Thus an 80Ah battery can be charged at 16A per hour. The D250S may not see the difference between a flooded cell and an AGM (similar internal resistances) so it's worth being careful with it.
AGM batteries come in 2 primary types of their own - flat and spiral wound. Flat plate batteries (plates are just like gel/flooded) can be charged up to about C/3, so a 225Ah AGM can be charged at 75A per hour. Obviously well beyond the capability of the D250S, but this is a MAX charge rate. Beyond this rate, the AGM may vent gas - irrecoverable, since the units are sealed. Spiral wound AGMs can be charged at up to C/2 - since the largest I've seen of these is 75Ah, the most it can be charged at is 37.5A per hour. Still beyond the D250S's capability, but BOTH of these AGM types can be serviced well by the D250S.
To provide a real-world example: in my tub, I have an Optima Yellowtop D31A spiral wound AGM. I charge it mostly through solar (peak about 6A, but often between 4-5A/h). I have a battery charger in there "just in case" and it's capable of 6A or 16A charge rates. I leave it on 6A.
So the C-Tek's "max" capacity isn't really a big concern. Plug it in and enjoy.
I just checked Century's web site. The C105 is a 6V battery so you cannot connect them in parallel. Connect them in series ONLY. The C-Tek should refuse to charge them connected in parallel. So in reality, you have 225Ah of 12V power.
