The "full load" battery test is available free at Auto Zone to allow you to determine whether your lead-acid car battery is capable of cranking the engine. Monitoring the battery terminal voltage while heavily loading it does provide some indication of its capability, but IMO it is only a go/no-go test useful for deciding whether or not to replace the battery. The manufacturer determines the capability of a battery (or a power pack) during their design phase, and then they label it so the consumer knows which one is appropriate for their needs. After that it is all down hill as the chemistry eventually fails to maintain charge and deliver rated maximum current after recharging.
NiCd cells are prone to "memory" effects, refusing to accept a full charge after several charge/discharge cycles that do not "empty" the cell. It's almost as if there was a little guy in there monitoring the usage, and when he sees you are only using, say, ten percent of the rated capacity, then he makes sure that is all you get: ten percent of rated capacity. Some of the "smarter" chargers will discharge NiCd cells before attempting to recharge them, an effort to defeat the "memory" effect. Better to use a different battery chemistry and avoid NiCd altogether if you need a reliable power source. I like Li-ion technology, but there are some "gotchas" associated with that too. Caveat emptor.