they do not discharge the battery, but tell you how much charge went into the battery and unit, which is a fair estimate of the battery capacity. To do a capacity test on the battery you will need to have another unit which is a battery tester, which takes a full battery and discharges it at a constant current till it reaches the low voltage cut off point. Frankie also has those in his store.
Using both together ( providing you can take the battery out) will tell you battery capacity and charge efficiency, but in many cases the battery is not removable so you can only do a charge capacity and then estimate the usable capacity.