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re. that 50% rule of thumb....
I have no idea what the number is for AGM's, but if you're only using half it doesn't strike me as a good investment, especially for such an expensive battery...unless you need it for a non-vented installation, or inverted, etc...
so I say get a trojan deep cycle (or similar true deep cycle I suppose)
...you can use 80% of their capacity
from Trojan's user guide...
"Do not discharge your battery to more than 80% depth of discharge. This safety factor will eliminate the chance of over-discharging and damaging your battery"
(means that you should be able to take it down to 20% of the capacity (100-80=20)
Now I realize these numbers are considered to be generalizations, and that folks like to add safety factors on top of safety factors, often for good reason....
Anyway I wish I could have found one that fit when I was on the road and needed a new house battery....so I'm stuck with a very limited usable fraction.
This whole thing got me curious so I went and looked it up.
excide (just used as a typical cheap battery example) says in their user guide for lead acid to recharge at 12.3V...which seems to be regarded as about 70% state of charge (30% useable)...but I'm guessing that's probably a generic number to cover their starting batteries as well as their marine batteries.
Incidentally, Excide says 12.4 volts for a minimum for their AGM's.... does that imply that they are no better at handling deep discharges than a lead acid?
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