Well, the thermistor would have been set to feed a temperature variable resisitance to a regulator circuit. The regulator circuit would have been calibrated to limit the charge current to not overheat based on the nicad charge characteristic, which is likely not the same charge characteristic as NiMH. NI-MH generally can be charged faster thatn Ni-Cad, as I recall. Look up the technical charactersitics of the cells.
I am old school. My packs are generally on the end of a coiled cable, and i make them up from gelled lead acid batteries. I charge them fully winthin two days after using them, and sometimes months will go by before I ned to use it again. It will be still fully charged, and ready to go.
My work digicam is fed Ni-MH AA cells. If you don't charge them before you need them you are screwed, because high energy density or not, they are flat within 3 days of charging them.