Mike,
Robot batteries are best tested for the manufacturer's specifications. Although our batteries are rated at 18 amp hour, that capacity is only valid when discharged at 0.1 C or about 1.8-2 amps. The AH rating falls rapidly when discharged at higher than that rate. Many team are now using the West Mountain Radio CBA-III battery analyzer. One it's features is the ability to record discharge curves and compare them over a period of time by overlay. You can find it here...
http://www.westmountainradio.com/CBA.htm
We have been using it at the 7.5 amp discharge setting and that is sufficient to give us approximate curves that relate to published discharge curves from the manufacturer. One of the greatest failures of our batteries is reduced cell capacity. This is shown on the curves by a two volt drop in terminal voltage during the test. Each of our batteries is serialized when received and we test at least bi-annually to track battery life. When capacity starts to fall of, we relegate the battery to practice only. I do not recommend high current discharge testers of the type normally found in automotive stores. These are designed to test CCA ratings of car batteries and can damage our batteries with repeated and/or unsupervised use.