With regard to automotive style cyclable batteries (Telecomm batteries have entirely distinctive characteristics), here is how I would rate an AGM battery.
Ratio of case size to gross weight (meaning BCI group size to weight)
BCI Cold Crank Test Amperage. The lower the better weight versus amperage.
Advertised 20 hour rate Ampere Hour rating. Again the lowest ampere rating in relationship to weight is an excellent clue.
However let's use the Lifeline 105-amp group 31T versus Lifeline's 31XT. The XT is a 120 ampere hour group 31. 15 amp hours more than the "lighter weight" battery. The 31XT will have a higher CCA rating, but the construction components are just the same - one more plate pair per cell.
So, an equally heavy other brand of battery comes along at near the same weight but with a significantly higher ampere hour rating, and corresponding CCA increase over the Lifeline, plate area has to be increased. More and thinner plates.
For a useful test of an unknown accumulator, the first thing to do is try and find it's BCI group number by measurement of the case.
Find a corresponding "known" thick plate battery i.e. the Lifeline.
An adjustable carbon pile load test is performed. Whatever amperage it takes to slump voltage to 9.6 volts for 15 seconds will be the battery's existing CCA rating. I say existing because an undercharged or mistreated battery would naturally underperform. Recommended, is a full charge then a conditioning charge, wait one day then perform the CCA test.
It would be a waste of time to do this with an old battery. The history of the DUT has to be known in advance. The Lifeline method of capacity testing is most accurate to determine the viability of an old AGM CCA test.
Why am I so fixated on cycle life and overall lifespan? I project a scenario in which I estimate what an unexpected failure is *really* going to cost me. Someone who lives in a major USA metropolitan area will have a far less intensity of hassle than I would. By the same token, I can substitute an emergency flooded 12 volt battery out of doors, then hassle about how to pass wire through concrete and reconfigure the charging protocol. Someone who is relying on a precious boondocking vacation in a remote area is similarly at risk of having a disrupted or destroyed (expensive) vacation by a premature battery failure. If the object is no money, preventative replacement can be a problem for those folks not well endowed with surplus income. Money is no object on the other hand can well afford preventative replacement management. One class of individual -must not- judge the other. It would be irrational. I fall into the obviously vulnerable first of the two categories. To have a individual possessing an abundance of disposable income along with minimum risk of inconvenience ridicule my doctrine would show either a lack of conscious awareness, or a lack of intelligence. That's pretty abrupt even rude but it's fact not opinion.
