I opened the charger today. It's a stupid charger with a bridge rectifier arrangement, some resistors, diodes and an LED. The LED comes on when the pack is inserted n goes out when batteries are removed. My guess is a thermistor is not the part to use, it should be a thermal switch of some sort to disengage the batteries when it gets too hot.

I'd really like to know what the origianls came with, stupid me threw it out years ago to make an external battery pack using plugs in the cube n extension wires to a sealed acid batttery.