The normal response goes something like this. Thereafter the voltage drops off quickly to below 1 volt in a matter of a minute or two. It is easy to see the early death of a cell or two by the notches in the graph. When repacking I use AA size cells of 1000mAh capacity. It is possible that they do not like the high discharge rates that nicads will tolerate, which can be up to 1.2 amps during transmit on some radios.
Video cameras are really vicious with their current demands.
Anyone who has a Garmin 38 will notice the voltage scale starts high with full batteries, and quickly drops to half scale, where it sits for hours until the batteries go flat suddenly. Incidentally, this scale on the Garmin is calibrated for alkaline batteries and that is why nicads read low even when fully charged. The only way I have found to test a pack is to do a full discharge test or two and study the graphs. It is quite safe to leave it on for a couple of days, and some of the newer high capacity batteries may require 15 to 20 hours at this rate to reach their full charge. If you are in doubt about the rate at which you charger actually charges your battery packs, it is a relatively simple matter to make a patch cord to break the circuit and insert an ammeter to monitor the charging current.