Gonzo, I'm curious, what were you trying to figure out here? Destructive battery testing, well beyond reasonable or practical use, seems like a costly academic exercise. The batteries you tested would be good for nothing but scrap as a core exchange?!
ANY lead acid battery run down to 1.66 Vpc is not long for this world. Anything below 1.70 Vpc is fatal, according to every reputable supplier. It has nothing to do with weather or not an appliance will still run, and everything to do with permanently degrading the battery. Anyone who owns a car knows that running a FLA battery flat, sometimes just 2 or 3 times, is enough to make cold cranking an unnecessary adventure.
It's seems misleading to me to characterize batteries on the basis of discharge rates, which are only one of the limiting factors in design, and not usually the critical factor anyway. For most of us, capacity data published by the manufacturer is not only more readily available (C1, C5, C10, C20 rather than 1C, 3C, 10C) but provides sufficient insight for making a good decision.
Consider a 100Ah FLA battery. If the rated capacity is C10, then it will reliably and safely discharge 10A per hour for 10 hours. But since we KNOW that running an FLA below 50% DoD will significantly degrade the number of cycles (lifetime), we also know that we should only design for a total capacity of 50Ah. So if our energy budget is 50Ah (or less), spread roughly evenly over a 10 hour duty cycle, we're in the clear. And our battery will likely last for the number of duty cycles the manufacturer publishes.
If our energy budget is greater than than 50Ah, then we need a bigger/better battery. If our duty cycle is less than 10 hours, then we need to go back to the published data to see how many total Ah we get for C1, C5, etc. Because we KNOW that a shorter duty cycle will reduce the total capacity of ANY battery. We iterate through our energy budget again, and it will be obvious whether we need a bigger/better battery.
The question of maximum continuous discharge only arises after iterating through the capacity process first. In our energy budget, we have a peak discharge current for all appliances running simultaneously. If our peak discharge is 10A, or less, we're still in the clear. If it's greater, then we also need a bigger battery, since we choose to design for the worst case in order to create a prudent safety margin. If the peak discharge is 20A, then we need a C10 rating of 200Ah, e.g. 20A per hour for 10 hours.
This process is appropriate for any battery chemistry. Even though AGM can run down to 80% DoD, or 90% DoD for lithium, it is also true that shallower discharge will extend the life of ANY battery. For instance, the 5% degradation in capacity you note after 50 cycles for an AGM battery is not good. AGMs typically last 1000-1200 cycles at 50% DoD, but you lost 5% in just 50 cycles running them flat. At that rate you'd lose 20% of capacity at 25% of service life, which practically represents 40% of usable capacity (at 50% DoD). Not a happy battery, not a good investment of $350+.
