Why ?
A "good" battery is only required to put out a reasonable voltage at a reasonable discharge rate for some period of time.
I don't care what the battery chemistry is, you can not do a reasonable test unless you know that the battery is fully charged. With AGM/spiral there is no electrolyte to check with a hydrometer, so the only way to know it is fully charged is to place it on an appropriate charger for as long as it takes. (The 48 hrs was referring to something like an 8D size battery.)
Measuring current is great for starting and charging system health.
The only way to test a battery is to 1) charge it (or know for 100% certain that it is fully charged which impossible if the charging system is in question) , 2) put an appropriate load on it, 3) observe the voltage as the battery discharges.