Hi Frank,
Yes, a much longer answer is forthcoming ... but,
1) different constructions/capacities/brands/ etc will "normally" show difference cell resistances.
2) cell resistances drop during the battery break-in period ...
3) assume a relatively stable or slowly rising value with number of charge-discharge cycles ...
4) and start to rise more rapidly at life's end.
5) high temps lower cell resistance and vice-versa. The resistance vs temp relationship is a strong one.
6) comparisons must be done at the same state-of-charge. i suggest the storage voltage.
7) Record keeping is the answer.

later,
 Dean

Hi Dean,
Thanks for sharing your insights  and knowledge regarding  the mysteries of  I.R. .

Hi Bob,
The better chargers do this by pulsing some current either into and then out of the battery (that is back and forth like a small AC current) and then measuring the voltage difference that results.
That AC current is imposed on top of the DC charging current, or in some cases, the test happens after the storage charge is achieved.
Comparing the fully charged resistances is also meaningful, but most often it's done at either half-charge or storage charge.

Yeah, Tim ... it's a drag if you want to do it to the nines.
Fortunately, we don't need to be anal: checking the resistance at room temp, at home after the day's flying, at storage charge, every so often will provide the battery health info we need.
You might also develop a good sense of what to expect for a known, often encountered, condition like reading whatever the charger gives you, after a charge that was started soon after a flight on a nice Summer day.
Comparing those readings will tell a story.

later, Friends ...
Dean