We do have a Battery Beak Tester as well, we use for a spot check right before the battery goes in the robot.
http://www.andymark.com/product-p/am-0995.htm
But so far it seems a basic CCA rating from our "cheap" automotive tester seems to be a good gauge based on what we see on the field. I am mostly interested in "capacity" of the battery. How long can I run a robot on a fully charged robot before it dies completely.
At 500 CCA - we might get 6-7 minutes
At 800 CCA - we will get 10-15 minutes
That's about some rough numbers we've seen over the years.